DS 1-54 Roof Loads for New Construction (Data Sheet) - FM Global

FM Global 

Property Loss Prevention Data Sheets 1-54 

ROOF LOADS FOR NEW CONSTRUCTION 

Table of Contents 

October 2012 

Page 1 of 80 

1.0 SCOPE ................................................................................................................................................... 4 

1.1 Changes .......................................................................................................................................... 4 

2.0 LOSS PREVENTION RECOMMENDATIONS ....................................................................................... 4 

2.1 Use of Other Codes and Standards .................................................................................................. 4 

2.2 Roof Loads and Load Combinations ............................................................................................... 4 

2.2.1 Roof Live Load Reduction ................................................................................................... 4 

2.3 Snow Loads ..................................................................................................................................... 5 

2.3.1 General .................................................................................................................................. 5 

2.3.2 Snow Load Notation .............................................................................................................. 5 

2.3.3 Ground Snow Loads ............................................................................................................. 6 

2.3.4 Snow Density ........................................................................................................................ 7 

2.3.5 Flat-Roof Snow Loads ........................................................................................................... 8 

2.3.6 Minimum Snow Loads for Low-Sloped Roofs ....................................................................... 8 

2.3.7 Sloped-Roof Snow Loads ..................................................................................................... 8 

2.3.8 Unbalanced Roof Snow Loads ............................................................................................. 9 

2.3.9 Hip and Gable Roofs .............................................................................................................. 9 

2.3.10 Curved and Domed Roofs ................................................................................................... 9 

2.3.11 Valley Roofs ....................................................................................................................... 10 

2.3.12 Drifts on Lower Roofs — Snow Loads .............................................................................. 12 

2.4 Rain-on-Snow Surcharge ............................................................................................................... 19 

2.5 Rain Loads and Roof Drainage ..................................................................................................... 19 

2.5.1 General ................................................................................................................................ 19 

2.5.2 Bases for Design Rain Loads ............................................................................................. 19 

2.5.3 Designing for Stability Against Ponding ............................................................................... 22 

2.5.4 Roof Drainage ...................................................................................................................... 23 

2.6 Other Roof Loads and Roof Overloading ..................................................................................... 47 

2.7 Use of Eurocode ............................................................................................................................ 47 

2.7.1 Eurocode for Snow Loads ................................................................................................... 47 

2.7.2 Eurocode for Roof Live Load (Imposed Load) .................................................................... 48 

2.7.3 Eurocode for Rain Loads ..................................................................................................... 48 

2.7.4 Different Partial Safety Factors for CEN Member Nations ................................................... 49 

2.8 Use of ASCE 7 for Snow Loads ..................................................................................................... 50 

2.8.1 Factors .................................................................................................................................. 50 

2.8.2 Hip and Gable Roofs ............................................................................................................ 50 

2.9 Plan Review and Submissions ....................................................................................................... 50 

2.9.1 General ................................................................................................................................. 50 

2.9.2 Other Codes and Standards ................................................................................................ 51 

3.0 SUPPORT FOR RECOMMENDATIONS ............................................................................................. 52 

3.1 General ........................................................................................................................................... 52 

3.1.1 Use of Other Codes and Standards .................................................................................... 52 

3.1.2 Rainfall Intensity, Duration, and Frequency Used for Roof Drainage ................................. 52 

3.1.3 Siphonic Drainage ................................................................................................................ 53 

4.0 REFERENCES ..................................................................................................................................... 53 

4.1 FM Global ...................................................................................................................................... 53 

4.2 Others ............................................................................................................................................ 53 

APPENDIX A GLOSSARY OF TERMS ..................................................................................................... 54 

A.1 Roof Loads and Drainage ............................................................................................................. 54 

©2008-2012 Factory Mutual Insurance Company. All rights reserved. No part of this document may be reproduced, 

stored in a retrieval system, or transmitted, in whole or in part, in any form or by any means, electronic, mechanical, 

photocopying, recording, or otherwise, without written permission of Factory Mutual Insurance Company. 

Page

1-54 Roof Loads for New Construction 

Page 2 FM Global Property Loss Prevention Data Sheets 

A.1.1 Controlled Roof Drains ........................................................................................................ 54 

A.1.2 Design Roof Line ................................................................................................................ 54 

A.1.3 Ponding and Ponding Cycle ............................................................................................... 54 

A.1.4 Dead Load .......................................................................................................................... 54 

A.1.5 Live Load ............................................................................................................................ 54 

A.1.6 Total Load ........................................................................................................................... 55 

A.1.7 Tributary Loaded Area (TA) ................................................................................................ 55 

A.1.8 Roof Strength ...................................................................................................................... 56 

A.1.9 Safety Factor ....................................................................................................................... 56 

APPENDIX B DOCUMENT REVISION HISTORY ...................................................................................... 57 

APPENDIX C SUPPLEMENTARY INFORMATION .................................................................................... 58 

APPENDIX D RESERVED FOR FUTURE PROVISIONS ........................................................................... 73 

APPENDIX E ILLUSTRATIVE EXAMPLES AND JOB AIDS ................................................................... 73 

E.1 Snow Loading Illustrative Examples ............................................................................................. 73 

E.2 Roof Drainage and Rain Loading Illustrative Examples ............................................................... 76 

List of Figures 

Fig. 1. Snow loads for hip and gable roofs .................................................................................................. 10 

Fig. 2a. Snow loads for curved and dome roofs .......................................................................................... 11 

Fig. 2b. Unbalanced snow load distribution on dome roofs ........................................................................ 11 

Fig. 3. Snow loads for valley roofs .............................................................................................................. 12 

Fig. 4a. (To be used with Table 3) Snow loads for lower roofs .................................................................. 13 

Fig. 4b. Snow drift intersection at lower roofs ............................................................................................. 13 

Fig. 5. Snow loads for lower roof of adjacent structures ............................................................................ 17 

Fig. 6. Sliding snow load for lower roofs (upper roof snow load not shown) ............................................. 18 

Fig. 7. Snow load at roof projections ........................................................................................................... 18 

Fig. 8a. Typical primary and overflow systems for pitched roofs ................................................................ 20 

Fig. 8b. Typical primary and overflow drainage systems for flat roofs ....................................................... 21 

Fig. 8c. Primary roof drain and hydraulic head ............................................................................................ 28 

Fig. 8d. Overflow (secondary) roof drain with hydraulic head and total head ............................................ 28 

Fig. 9. Flat and sloped roofs with interior roof drains ................................................................................. 36 

Fig. 10. Sloped roof with roof edge drainage .............................................................................................. 36 

Fig. 11. Diagram of siphonic roof drain system ............................................................................................ 44 

Fig. 12. Elevation view of siphonic system and disposable (available) head ............................................. 44 

Fig. 13. Siphonic roof drain (photo courtesy of Jay R. Smith Mfg. Co.) ..................................................... 45 

Fig. 14. Siphonic roof drain for gutters (without dome strainer or debris guard) 

(photo courtesy of Jay R. Smith Mfg. Co.) .................................................................................................. 45 

Fig. 15. Typical tributary loaded areas for primary and secondary members ............................................ 56 

Fig. 16a. Ground snow load (P g) in psf for Western United States ............................................................ 59 

Fig. 16b. Ground snow load (P g) in psf for Eastern United States (To obtain kN/m 2 , multiply by 0.048) . 60 

Fig. 17a. Ground snow load (P g) in kN/fm 2 for Western China ................................................................... 65 

Fig. 17b. Ground snow load (P g) in kN/m 2 for Eastern China ..................................................................... 66 

Fig. 18. Roof live load reduction flow chart/decision tree ........................................................................... 67 

Fig. 19. Rainfall intensity (i) in inches per hour for the western United States .......................................... 68 

Fig. 20. Rainfall intensity (i) in inches per hour for the central and eastern United States ........................ 69 

Fig. 21a. Rainfall intensity (i) in inches per hour for Puerto Rico ............................................................... 70 

Fig. 21b. Rainfall intensity (i) in inches per hour for Hawaiian Islands . .................................................... 71 

Fig. 22. Rainfall intensity (i) in inches per hour for Alaska ......................................................................... 72 

Fig. E1.1. Design snow loads for Example 1 .............................................................................................. 73 

Fig. E1.2. Design snow loads for Example 2 .............................................................................................. 74 

Fig. E1.3. Design snow loads for Example 3 (Leeward Drifting) ................................................................ 75 

Fig. E1.4. Design snow loads for Example 4 (Windward Drifting) .............................................................. 76 

Fig. E1.5.1 Flat roof plan for Example 5 ..................................................................................................... 77 

Fig. E1.5.2 Sloped roof plan for Example 5 ................................................................................................ 78 

Fig. E1.5.3 Sloped roof section for Example 5 ........................................................................................... 79 

Fig. E1.6. Roof plan for Example 6 ............................................................................................................. 80 

©2008-2012 Factory Mutual Insurance Company. All rights reserved.

Roof Loads for New Construction 1-54 

FM Global Property Loss Prevention Data Sheets Page 3 

List of Tables 

Table 1. Ground Snow Load (P g) versus Balanced Flat-Roof Snow Load (P f), Density (D), and 

Height of Balanced Snow Load (h b) for Flat and Low-sloped Roofs ............................................. 8 

Table 2. Roof Slope Factor C s ...................................................................................................................... 9 

Table 3. (To be used with Figure 4a) Ground Snow Load (P g) versus Balanced Snow Load (P f), 

Density (D), Balance Snow Load Height (h b), Drift Height (h d), Max Drift Load (P d) and 

Max Load (P d+P f) ......................................................................................................................... 15 

Table 3. Continued. ..................................................................................................................................... 16 

Table 4. Rain-on-Snow Surcharge Load ..................................................................................................... 19 

Table 5. Flow Capacity for Roof Drains ...................................................................................................... 27 

Table 6a. Hydraulic Head Versus Flow Capacity for Rectangular Roof Scuppers 

(Depth of Water Over Invert Versus Flow of Water Through Scupper) ....................................................... 29 

Table 6b. Hydraulic Head and Flow Capacity for Circular Roof Scuppers .................................................. 30 

Table 7. Conversion of Rainfall Intensity to Flow Rate and Rain Load per Unit Area ............................... 31 

Table 8a. Hydraulic Head and Corresponding Drain Flow for Primary Roof Drains (US units) ................. 32 

Table 8b. Hydraulic Head and Corresponding Drain Flow for Primary Roof Drains (SI units) ................... 33 

Table 8c. Hydraulic Head and Corresponding Drain Flow for Secondary (Overflow) Roof Drains (US units) . 34 

Table 8d. Total Head and Corresponding Drain Flow for Secondary (Overflow) Roof Drains (SI units) ... 35 

Table 9. Rainfall Intensity Conversion Rates ............................................................................................... 46 

Table 10. Schedule 40 Pipe Dimensions and Geometric Properties .......................................................... 46 

Table 11. Standard Atmospheric Pressure at Various Elevations ................................................................ 47 

Table 12. Ground Snow Load (P g) for Alaskan Locations, psf (kN/m 2 ) ....................................................... 61 

Table 13. Ground Snow Load (P g) for Locations in Korea, psf and kPa .................................................... 61 

Table 14. Ground Snow Load (P g) for Locations in Japan, psf and kPa ..................................................... 62 

Table 15. Ground Snow Load (P g) for Locations in China* ........................................................................ 63 

Table 15. Continued ..................................................................................................................................... 64 




1.0 SCOPE 

This loss prevention data sheet presents recommendations principally for snow and rain loadings and 

drainage for the design of new roofs of buildings and other structures. 

In general, it is the function of this data sheet to present background details and guidelines for building 

designers to use in carrying out the requirements or intent of typical building and plumbing codes regarding 

design roof loads and roof drainage. 

It should be noted that the various recommendations presented here are not based on the worst conditions 

possible, or even the worst conditions recorded. A probabilistic approach is used to establish design values 

that reduce the risk of a snow-load-induced or rain-load-induced roof collapse to an acceptably low level. 

1.1 Changes 

October 2012. The following changes were made: 

A. Revised and expanded the tables (flow rate versus corresponding hydraulic head) for primary and 

secondary roof drains. 

B. Added new table for flow rate and corresponding hydraulic head for circular roof scuppers. 

C. Added new recommendations for ground snow studies where ground loads are not mapped. 

D. Added new recommendations regarding partial safety factors (load factors) for Eurocode harmonization. 

E. Added new recommendations for ground snow loads in Russia. 

F. Revised rainfall intensity maps for the US and Puerto Rico. 

G. Revised the roof drainage illustrative problems. 

2.0 LOSS PREVENTION RECOMMENDATIONS 

2.1 Use of Other Codes and Standards 

Refer to Sections 2.7 and 2.8 for the use of the Eurocode and ASCE 7, respectively. 

2.2 Roof Loads and Load Combinations 

Design the roof to resist the effects of dead loads in combination with the more demanding of the following 

roof live or environmental (e.g., rain or snow) loads: 

a) The balanced (uniform) or unbalanced snow loads, including snow drift surcharge and rain-on-snow 

surcharge where applicable, in accordance with Section 2.3 

b) The rain loads in accordance with Section 2.5 and precluding (i.e., ruling out in advance) instability 

from ponding 

c) Superimposed roof live loads, as specified, to account for the use and maintenance of the roof and 

the occupancy of the building/structure 

d) A minimum roof live load of 20 psf (1.0 kN/m 2 ) for flat roofs, sloped roofs less than 4 in./ft (18.4 degrees) 

and curved roofs with rise less than 1 ⁄ 8 of span, except when a reduction in the minimum roof live load 

is appropriate, as described in Section 2.2.1. 

2.2.1 Roof Live Load Reduction 

2.2.1.1 Reductions in the minimum unfactored (characteristic) roof live load of 20 psf (1.0 kN/m 2 ) for 

lightweight roof constructions (lightweight roof constructions include metal roofs, steel deck, boards-on-joists, 

plywood sheathing, and similar constructions), when permitted by applicable building codes and standards, 

are only allowed whenever both of the following conditions are met: 

a) The roof slopes at least 1 ⁄ 4 in./ft (1.2 degrees or 2%), and 

b) The roof snow load is zero, or the supported combined unfactored (characteristic) dead load plus 

resultant roof live load (reduced) is at least 28 psf (1.4 kN/m 2 ). 




See Figure 18 for a roof live load reduction flow chart. 

Note that for purposes of foundation design only (e.g., footings, grade beams, piles, and caissons), the use 

of roof live loads and live reduction techniques as permitted by applicable building codes and standards are 

acceptable without revision or exception. 

Do not use roof live load reduction for the following: 

1) Roofs that can have an occupancy function such as roofs on which an assembly or congregation of 

people is allowed or intended (e.g., some roof gardens (vegetated green roofs); or roofs that function as 

a balcony, elevated terrace, or viewing platform). 

2) Roof used for storage, including car parking garage roofs. 

3) Roofs where the code-required unfactored (characteristic) live load is greater than 75 psf (3.6 kPa). 

2.2.1.2 Where roof live load reduction is permissible under Section 2.2.1.1, use the following roof live load 

reduction procedure (where TA = Tributary Area): 

1) TA ≤ 200 ft 2 (19 m 2 ): No roof live load reduction allowed; use 20 psf (1.0 kPa). 

2) 200 ft 2 (19 m 2 ) < TA = < 600 ft 2 (56 m 2 ): 

Roof live load (psf) = (1.2 - 0.001(TA))(20 psf) 

or 

Roof live load (kPa) = (1.2 - 0.0108(TA)) (1.0 kPa) 

3) TA > 600 ft 2 (56 m 2 ): Roof live load = 12 psf (0.6 kPa) 

For example, if TA = 400 ft 2 (37.5 m 2 ), then the minimum reduced roof live load is 0.8 x 20 psf (1.0 kPa) = 

16 psf (0.8 kPa). 

For a continuous structural roof system, such as a concrete slab, use a tributary length equal to the span 

(use the lesser span for a two-way slab system), and use a tributary width not greater than 1.5 x tributary span; 

in other words: TA = 1.5 (tributary span) 2 . The same technique can be used for one-way systems such as 

metal roof deck, standing seam roofs, of lap seam roofs; however, based on typical spans, the TA will generally 

be less than 200 ft 2 (19 m 2 ) and therefore will not be eligible for roof live load reduction. 

2.3 Snow Loads 

2.3.1 General 

Determine roof snow loads in accordance with the guidelines of this section; however, ensure the roof loads 

are not less than the minimum live loads or snow loads designated by the applicable building code, nor less 

than the rain loads covered in Section 2.5. For roofs of unusual shape or configuration, use wind-tunnel or 

analytical modeling techniques to help establish design snow loads. 

2.3.2 Snow Load Notation 

C e = exposure factor 

C s = slope factor 

C t = thermal factor 

D = snow weight density (pcf [kN/m 3 ]) of drifted snow 

h b = height of balanced uniform snow load (ft [m]) (i.e., balanced snow load P f or P s divided by D) 

h c = clear height from top of balanced snow load (ft [m]) to the closest point(s) on adjacent upper roof; to 

the top of parapet; or to the top of a roof projection 

h d = maximum height of snow drift surcharge above balanced snow load (ft [m]) 

h r = difference in height between the upper roof (including parapets) and lower roof or height of roof 

projection (ft [m]) 

P d = maximum intensity of drift surcharge load (psf [kN/m 2 or kPa]) 

P f = flat-roof snow load (psf [kN/m 2 or kPa]) 




P g = ground snow load (psf [kN/m 2 or kPa]) 

P s = sloped-roof snow load (psf [kN/m 2 or kPa]) 

S = separation distance between buildings (ft [m]) 

W b = horizontal distance of roof upwind of drift (ft [m]), but not less than 25 ft (7.6 m). W b equals the entire 

upwind distance of roofs with multiple elevation differences, provided the predicted drift height at each 

elevation difference exceeds h c 

W d = width of snow drift surcharge (ft [m]) 

W p = width of rooftop projection (ft [m]) 

W s = width of sloped upper roof, from ridge to eave (ft [m]) 

Θ = roof slope from horizontal (degrees, rise: run, in./ft) 

2.3.3 Ground Snow Loads 

Use ground snow loads based on a 50-year mean recurrence interval (MRI) and from a nationally recognized 

building code or standard. Approximate multiplication factors for converting from lesser MRI ground snow 

loads to 50-year MRI ground snow loads are: 

50-year = 2.25 x 5-year 

50-year = 1.82 x 10-year 

50-year = 1.20 x 25-year 

50-year = 1.15 x 30-year 

At locations where the elevation exceeds the limits indicated on the ground snow load maps, and in areas 

where local variation in ground snow loads is substantial enough to preclude meaningful mapping (such as 

areas designated as CS [case studies] in the United States), refer to Section 2.3.3.6. 

2.3.3.1 Ground Snow Loads in the US 

Ground snow loads (P g) used in determining design snow loads for roofs are given in the two-part map for 

the contiguous United States in Figures 16a and 16b. The maps are provided in the publication Minimum 

Design Loads for Buildings and Other Structures by the American Society of Civil Engineers/Structural 

Engineering Institute (ASCE/SEI Standard 7-05 and 7-10). The maps present snow-load zones with estimated 

ground snow loads based on a 50-year MRI and provide the upper elevation limit for the presented ground 

snow loads. 

Ground snow loads are zero for Puerto Rico and most of Hawaii, although for mountainous regions in Hawaii, 

consult local building officials to verify ground snow load conditions. 

Ground snow loads (P g) for Alaska are presented in Table 12 for specific locations only and generally do 

not represent appropriate design values for other nearby locations. In Alaska, extreme local variations 

preclude statewide mapping of ground snow loads. 

2.3.3.2 Ground Snow Loads in China 

Use a snow load Importance Factor (l) of 1.2 for ground snow loads in China. Apply the 1.2 Importance Factor 

to the ground snow load values (P g) from Figures 17a and 17b China Ground Snow Load. Figures 17a and 

17b represent the 50-year ground snow loads. Note that the P g values for the various cities in Table 15 

Ground Snow Load (P g) for Locations in China include the 1.2 Importance Factor. For example, for Dalian: 

From Figure 17b, P g = 0.53 kN/m 2 ; and from Table 15, P g = 0.64 kN/m 2 . 

2.3.3.3 Ground Snow Loads in Europe 

Use ground snow loads that are based on a 50-year MRI, including any National Annex for CEN member 

nations. 

Refer to Section 2.7 for recommendations regarding the use of the Eurocode. 




2.3.3.4 Ground Snow Loads in Canada 

Use the 50-year MRI ground snow loads from 2005 or 2010 editions of the National Building Code of Canada 

(NBCC). 

2.3.3.5 Ground Snow Loads in Russia 

Ground snow loads are provided in the Russian Industry Standards- Loads and Effects (SNiP 2.01.07), and 

are based on a 25-year MRI. 

Adjust the ground snow loads by using both of the following: 

a) For locations where the 25-year ground snow load is less than 0.75 kN/m 2 , increase the ground snow 

load by 35%; where the 25-year ground snow load is 0.75 kN/m 2 or greater, increase the ground snow 

load by 20%. 

b) Increase the ground snow loads to represent the 50-year MRI. 

2.3.3.6 Ground Snow Loads where Ground Snow Mapping is Inadequate 

For some regions, the localized variation in ground snow conditions is substantial enough to preclude 

meaningful snow load mapping; these regions can include mountainous locations, or “lake effect” snow belts 

near large bodies of water. 

For regions where an acceptable ground snow map is not available; for regions on a ground snow map where 

snow loads are not provided (e.g., “CS” case studies regions as designated in Figures 16a and 16b Ground 

snow load (P g) for the United States); or for regions where the elevation exceeds the limits on the ground 

snow map; consult the local building authority or code official having jurisdiction (authority having jurisdiction 

[AHJ]) to obtain a regional or site-specific snow study. Use the following guidelines to provide an acceptable 

level of assurance that the design snow load values in the regional snow study are accurate and reliable: 

A. The snow load values are based on one of the following: 

1. A study by a professional engineering (PE) organization, such as the Structural Engineer’s 

Association (SEA), of the particular state in question. 

2. A study from a local university with credentialed (PhD or PE) authors. 

3. A study by a federal, state, or county public safety/building agency with credentialed (PhD or PE) 

authors or reviewers. 

B. The study uses a reputable source of raw data. For instance, U.S. National Weather Service (NWS) 

station data or similar for snow depths, and other national or state agency records, such as the U.S. 

Geological Survey (USGS), for topographical data. 

C. The study indicates how snow loads are derived from snow depths (i.e., how the snow density is 

determined). The snow density should be reasonably close to the density provided in this data sheet. 

D. The study indicates if the snow loads are for ground or roof. 

E. The study indicates that the intent is for structural loads. 

F. The study is based on at least 25 years of data collection on average for the various locations, although 

a few scattered locations with roughly 15 years of data would be acceptable as a bare minimum. 

G. The study indicates the snow load MRI (e.g., 50-year), as well as the statistical method (e.g., Log 

Pearson) used to derive the MRI snow loads. 

2.3.4 Snow Density 

Determine bulk snow (weight) density (to evaluate the heights of roof snow loads) (D) as a function of the 

ground snow load (P g) according to Table 1 or the following formulas: 

English Units: 

D (pcf) = 0.13 P g + 14 ≤ 30 where P g in psf 

Metric Units: 




D (kN/m 3 ) = 0.43 P g + 2.2 ≤ 4.7 where P g in kN/m 

2.3.5 Flat-Roof Snow Loads 

To determine the balanced (uniform) snow load (P f) on an unobstructed flat roof, including any roof with a 

slope less than 5° (1 in./ft or 8%), use Table 1 or the following formulas: 

• Where P g ≤ 20 psf(1.0 kN/m 2 ): P f = P g 

• Where 20 psf 

• Where P g > 40 psf (1.9 kN/m 2 ): P f = 0.8 P g, but not less than 36 psf (1.7 kN/m 2 ) 

2.3.6 Minimum Snow Loads for Low-Sloped Roofs 

The minimum allowable snow loads are the balanced snow loads (P f) of Section 2.3.5 or Table 1 and applied 

to shed, hip and gable roofs with slopes less than 15°, and curved roofs where the vertical angle (see Fig. 

2a) from the eave to the crown is less than 10°. The formulas in Section 2.3.5 satisfy the following minimum 

snow load guidelines: for locations where the ground snow load (P g) is 20 psf (1.0 kN/m 2 ) or less, the flat 

roof snow load (P f) for such roofs is not less than the ground snow load (P g); in locations where the ground 

snow load (P g) exceeds 20 psf (1.0 kN/m 2 ), the flat-roof snow load (P f) for such roofs is not less than 20 

psf (1.0 kN/m 2 ). 

In building codes, minimum roof live loads and live load reductions do not apply to snow loads. Snow loads 

greater than such live loads govern the determination of design loads. 

Table 1. Ground Snow Load (P g) versus Balanced Flat-Roof Snow Load (P f), Density (D), and 

Height of Balanced Snow Load (h b) for Flat and Low-sloped Roofs 

English Units 

Ground Snow Load, P g (psf) Balanced Flat-Roof Snow Load, P f (psf) 

P g 5 10 20 25 30 35 40 50 60 70 80 90 100 

P f 5 10 20 23 27 32 36 40 48 56 64 72 80 

Density D, (pcf) Balanced Flat-Roof Snow Load Height, h b, (ft) 

D 14.7 15.3 16.6 17.3 17.9 18.6 19.2 20.5 21.8 23.1 24.4 25.7 27.0 

h b 0.3 0.7 1.2 1.3 1.5 1.7 1.9 2.0 2.2 2.4 2.7 2.8 3.0 

Metric Units 

Ground Snow Load, P g (kN/m 2 ) Balanced Flat-Roof Snow Load, P f (kN/m 2 ) 

P g 0.25 0.5 0.6 0.9 1.0 1.4 1.9 2.0 3.0 4.0 5.0 

P f 0.25 0.5 0.6 0.9 1.0 1.3 1.7 1.7 2.4 3.2 4.0 

Density D, (kN/m 3 ) Balanced Flat-Roof Snow Load Height h b (m) 

D 2.3 2.4 2.5 2.6 2.6 2.8 3.0 3.1 3.5 3.9 4.4 

h b 0.1 0.2 0.2 0.3 0.4 0.4 0.6 0.6 0.7 0.8 0.9 

Note: Linear interpolation is appropriate. 

2.3.7 Sloped-Roof Snow Loads 

Determine balanced (uniform) snow load (P s) on sloped roofs, such as shed, hip, gable, and curved roofs, 

by multiplying the flat-roof load (P f) by the roof slope factor (C s): 

P s = C s x P f 

Values of C s are given in Table 2. Use cold roof values. The exception is warm roof values that apply for 

un-insulated glass or metal panel, plastic (e.g., acrylic or reinforced plastic panels), and fabric roofs with 

R-value less than 2.0 ft 2 •hr.•°F/Btu (0.4 m 2 -°K/W) of buildings continuously heated above 50°F (10°C); note 

that to take advantage of warm roof slope factor values, ensure the building has a maintenance technician 

on duty at all times and a temperature alarm system battery back-up is in place to warn of heating failures. 

Use ‘‘slippery surface’’ values only where the sliding surface is metal (aluminum, copper, galvanized or 

enameled steel panels, such as on all-metal buildings) and is unobstructed with sufficient space below the 

eaves to accept all sliding snow; if it is reasonable to assume snow guards could be installed (e.g., where a 




sloped roof overhangs a sidewalk) consider the roof obstructed. Note that for curved and dome roofs the 

”vertical angle” (see Fig. 2a) is measured from the eave to the crown. 

Roof Slope, degrees 

Table 2. Roof Slope Factor Cs Cs Values 

(Rise:Run) 

1,2 

Unobstructed Slippery Surfaces All Other Surfaces 

Cold Roof Warm Roof Cold Roof Warm Roof 

≤ 5° (1:12) 1.0 1.0 1.0 1.0 

14° (3:12) 1.0 0.8 1.0 1.0 

18.4° (4:12) 0.94 0.74 1.0 1.0 

26.6° (6:12) 0.79 0.62 1.0 1.0 

30° (7:12) 0.73 0.57 1.0 1.0 

33.7° (8:12) 0.66 0.52 1.0 0.91 

45° (12:12) 0.46 0.36 1.0 0.63 

60° 0.19 0.14 0.4 0.25 

70° 0 0 0 0 

1. Use “cold roof” and “all other surfaces” values unless conditions in Section 2.3.7 apply. 

2. Linear interpolation is appropriate within any column. 

2.3.8 Unbalanced Roof Snow Loads 

Consider balanced and unbalanced snow loads as separate load cases. Consider winds from all directions 

when establishing unbalanced snow loads. For design purposes, unbalanced and drifting snow due to 

orthogonal wind directions (90° to each other) are considered to occur simultaneously; however, winds from 

opposite directions, 180°, are not considered to occur simultaneously. 

2.3.9 Hip and Gable Roofs 

2.3.9.1 Unbalanced Snow Load 

Consider the balanced snow load case for all roof slopes. The unbalanced snow load case need only be 

considered for roof slopes between 5° and 70° (1 on 12, and 33 on 12 slopes) inclusive. Balanced and 

unbalanced snow loading diagrams appear in Figure 1. Apply no reduction in snow load for roof slopes up 

to and including 15° (i.e., C s = 1.0 and therefore P s = P f) and the snow surface above the eave need not be 

at a higher elevation than the snow surface above the ridge. Determine snow depths by dividing the snow 

loads by the appropriate snow density (D) from Table 1. 

2.3.9.2 Ice Dam Load 

For typical heated building structures that drain water over their overhanging roof eaves, and where the roof 

assembly has an R-value of less than 25 ft 2 •hr.•°F/Btu (4.4 m 2 •°K/W), apply a uniform snow load of 2P f 

to the overhanging roof eaves; if the R-value of the roof assembly cannot be verified, assume that the load 

2P f is applicable. The load 2P f is intended to account for the effects of ice dams along the overhanging roof 

eave, and need not be combined with any design load other than the dead load of the roof. 

2.3.10 Curved and Domed Roofs 

Consider unbalanced snow loads for slopes where the ‘‘vertical angle’’ from the eave to the crown is between 

10° and 60°. Consider portions of curved roofs having a roof slope exceeding 70° free of snow; consider 

the point at which the roof slope exceeds 70° the ‘‘eave’’ for such roofs. Unbalanced loading diagrams, Cases 

I, II, and III, for curved roofs with roof slopes at the eave of less than 30°, 30° to 70°, and greater than 70°, 

appear in Figure 2a. If another roof or the ground surface abuts a Case II or III curved roof at or within 

3 ft (0.9 m) of the eave, ensure the snow load is not decreased between the 30° roof slope point and the 

eave, but remains constant at 2.0 P s as shown by the dashed line. 

For domed roofs, see Fig. 2b for unbalanced snow load distribution with a single wind orientation. Note that 

since unbalanced snow loads due to orthogonal (90 degree) wind directions are assumed to act concurrently, 

consider also the load distribution with the unbalanced snow load on one-half the roof (180 degrees), two 




linearly decreasing to zero snow zones of 22.5 degrees each, and the remaining area (135 degrees) free 

of snow. Determine the governing orientation of the unbalanced snow load based on the maximum demand 

on the structure. 

2.3.11 Valley Roofs 

Valleys are formed by multiples of folded plate, gable, saw-tooth, and barrel vault roofs. No reduction in 

balanced or unbalanced snow load is allowed for any roof slope (i.e., C s = 1.0 and P s = P f). For valleys formed 

by roof slopes of 5° (1 on 12) and greater, consider unbalanced snow loads. The unbalanced snow load 

should increase from one-half the balanced load (0.5 P f) at the ridge (or crown) to two times the balanced 

load at the valley (2.0 P f) (see Fig. 3). The snow surface above the valley, however, need not be at a higher 

elevation than the snow surface at the ridge (or crown). Determine snow depths by dividing the snow loads 

by the appropriate snow density (D) in Table 2. The above snow load methodology is also applicable to 

multiple gable and barrel vault roofs. 

Balanced Load Case 

Unbalanced Load Case * 

Wind 

0.3 P S 

Eave 

Ridge 

W S 

* Note: Unbalanced load need not be considered 

when < 5º, or when > 70º. 

Fig. 1. Snow loads for hip and gable roofs 

©2008-2012 Factory Mutual Insurance Company. All rights reserved. 

P S 

1.5 P S 

Eave



Balanced Load 

Eave 

Unbalanced Load Cases: 

Case I Slope At Eave < 30° 

Eave 

Case II Slope At Eave 30° To 70° 

Eave 

Case III Slope At Eave > 70° 

Wind 

0 

0 

0 

Eave 

0.5 P S 

0.5 P S 

0.5 P S 

Crown 

Crown 

Crown 

Crown 

Case III Shown 

Eave 

Eave 

Eave 

2 P S 

2 P S 

2 P S 

2 P S 

Eave 

30° 70° 

Point Point 

*Allternate Distribution If Another Roof Or Obstruction Abuts 

Wind 

Crown 

30° 

Point 

Vertical Eave 

Eave Angle Slope Eave 

Fig. 2a. Snow loads for curved and dome roofs 

Dome 

Plan View 

22½° 

22½° 

* 

30° Pt. 

Zero 

Snow 

70° Pt. 

P S 

(( 

1 - slope -30° 

40° 

Decrease 

Linearly 

To Zero 

Snow 

Unbalanced 

Snow Load 

Fig. 2b. Unbalanced snow load distribution on dome roofs 


45° 

45°



Balanced Load 

Unbalanced Load 

2.3.12 Drifts on Lower Roofs — Snow Loads 

In areas where the ground snow load (P g) is less than 5 psf (0.25 kN/m 2 ) or the ratio h c/h b is less than 0.2, 

drift loads on lower roofs need not be considered. Otherwise, design lower levels of multilevel roofs to sustain 

localized loads from snow drifts caused by wind over upper roofs of the same structure, adjacent structures, 

or terrain features within 20 ft (6 m) (leeward drifting); sliding snow; or snow drifts formed on lower roofs by 

windblown snow across the lower roof (windward drift). 

Examine the following three load cases when determining the maximum demand placed on the supporting 

structure of the lower roof: 

a) Balanced Snow + Leeward Drift 

b) Balanced Snow + Windward Drift 

c) Balanced Snow + Sliding 

Note that drift load need not be combined with sliding snow load. 

Note that more than one load case may govern the structural design. For example, for a low roof joist spanning 

perpendicular to the line of the roof step (i.e., parallel to the worst-case wind direction for drifting), load case 

(a) may produce maximum shear, but load case (c) may produce maximum bending. 

2.3.12.1 Drift Load 

0.5 P f 

Roof 

Slopes (deg.) 

Fig. 3. Snow loads for valley roofs 

Ridge 

Take the drift load on lower roofs as a triangular surcharge loading superimposed on the balanced roof snow 

load (P f), as shown in Figure 4a. Note that the upper roof may be flat or sloped. For upper roof slopes less 

than 30-degrees, use an upwind distance (W b) equal to the upper roof width parallel to the wind direction 

(e.g., eave to eave distance for a sloped roof). For upper roof slopes of 30-degrees or greater, use an upwind 

distance (W b) equal to 85% of the upper roof width. 

Where intersecting snow drifts of lower roofs due to perpendicular wind directions are possible, at the 

theoretical drift intersection the larger snow drift governs; the two drift loads need not be superimposed to 

create a combined (additive) drift load. See Figure 4b. 

Note that parapet walls on high roofs will not substantially reduce leeward drifting on adjacent low roofs; 

therefore, do not credit high roof parapets as a method of reducing low roof leeward drifting. 


P f 

Valley 

2 P max. 

f 

Valley 

Depth



Wind 

Determine maximum drift height (h d) in ft (m) from Table 3 or the following formulas: 


hd (ft) = 0.43 √ 3 

Wb √ 4 

Pg+10 − 1.5 ≤ hc where P g in psf; W b and h c in ft 

Metric Units: 

Upper Roof 

hd (m) = 0.42 √ 3 Wb √ 

4 Pg+0.48 −0.457 ≤ hc where P g in kN/m 2 ; W b and h c in meters 

h r 

Drift surcharge load (maximum intensity), P d = (h d × D) ≤ (h c × D) 

Maximum snow load (at wall) = (P d + P f) ≤ (h r × D) 

h r 

W b 

h c 

h b 

h d 

P d 

P f 

Surcharge Load 

Due to Leeward Drifting 

Fig. 4a. (To be used with Table 3) Snow loads for lower roofs 

h d2 

The drift surcharge load (P d) and the maximum snow load at the wall (see Fig. 4a) may also be determined 

by Table 3, provided the product of the density (D) and h c or h r does not govern. 

Wall 

W d 

h b h b 

W d2 

W d1 

Uniform Snow Loads on Upper Roof not Shown 

Fig. 4b. Snow drift intersection at lower roofs 


Balanced Snow Load, P f 

Lower Roof 

h d1



Drift width (Wd) is equal to 4 hd except for rare cases when the calculated hd exceeds hc . For these cases, 

the minimum Wd is established so that the cross-sectional area of the drift (0.5 Wd × hc) is equal to the 

2 

cross-sectional area of the hypothetical drift (0.5hd × 4hd = 2hd ) that would be computed if hd were less 

than hc; however, Wd cannot be less than 6 hc and need not be greater than 8 hc. Thus, 

Wd = 4 hd, except when hd > hc , then Wd = 4 h 2 

d 

h c 

(but 8h c ≥ W d ≥ 6h c) 

If W d exceeds the width of the lower roof (this occurs frequently with canopy roofs), truncate the drift at the 

far edge of the roof and do not reduce it to zero. 




Table 3. (To be used with Figure 4a) Ground Snow Load (P g) versus Balanced Snow Load (P f), Density (D), Balance 

Snow Load Height (h b), Drift Height (h d), Max Drift Load (P d) and Max Load (P d+P f) 


Ground Snow Load, P g (psf) 

Balanced Snow Load, P f (psf) 

P g 5 10 15 20 25 30 35 40 50 60 70 80 90 100 

P f 5 10 15 20 23 27 32 36 40 48 56 64 72 80 

Density, D (pcf) 

Balanced Snow Load Height, h b (ft) 

D 14.7 15.3 16.0 16.6 17.3 17.9 18.6 19.2 20.5 21.8 23.1 24.4 25.7 27.0 

h b 0.3 0.7 0.9 1.2 1.3 1.5 1.7 1.9 2.0 2.2 2.4 2.7 2.8 3.0 

Upwind 

Distance 

W b (ft) 

Drift Height, h d (ft) a 

Max. Drift Load, P d (psf) a 

Max. Load at Wall, P d + P f (psf) a 

25 h d 0.97 1.16 1.31 1.44 1.56 1.66 1.76 1.84 2.00 2.14 2.26 2.37 2.47 2.57 

P d 14 18 21 24 27 30 33 35 41 47 52 58 63 64 

P d+P f 19 28 36 44 50 57 65 71 81 95 108 122 135 144 

50 h d 1.61 1.85 2.04 2.21 2.35 2.48 2.60 2.71 2.91 3.08 3.24 3.38 3.51 3.62 

P d 24 28 33 37 41 44 48 52 60 67 75 82 90 98 

P d+P f 29 38 48 57 64 71 80 88 100 115 131 146 162 178 

100 h d 2.42 2.72 2.96 3.17 3.35 3.52 3.67 3.81 4.05 4.27 4.47 4.65 4.81 4.96 

P d 36 42 47 53 58 63 68 73 83 93 103 113 124 134 

P d+P f 41 52 62 73 81 90 100 109 123 141 159 177 196 214 

200 h d 3.44 3.82 4.12 4.39 4.62 4.83 5.01 5.19 5.50 5.78 6.02 6.25 6.45 6.64 

P d 51 58 66 73 80 86 93 100 113 126 139 153 166 179 

P d+P f 56 68 81 93 103 113 125 136 153 174 195 217 238 259 

300 h d 4.15 4.59 4.94 5.24 5.50 5.74 5.96 6.16 6.51 6.83 7.11 7.37 7.60 7.82 

P d 61 70 79 87 95 103 111 118 133 149 164 180 195 211 

P d+P f 66 80 94 107 118 130 143 154 173 197 220 244 267 291 

400 h d 4.72 5.20 5.58 5.91 6.20 6.46 6.71 6.92 7.32 7.67 7.97 8.26 8.52 8.76 

P d 69 80 89 98 107 116 125 133 150 167 184 202 219 237 

P d+P f 74 90 104 118 131 143 157 169 190 215 240 266 291 317 

500 h d 5.20 5.72 6.13 6.48 6.80 7.08 7.34 7.58 8.00 8.37 8.70 9.01 9.29 9.55 

P d 76 88 98 108 118 127 137 146 164 182 201 220 239 258 

P d+P f 81 98 113 128 141 154 169 182 204 230 257 284 311 338 

600 h d 5.62 6.17 6.61 6.99 7.32 7.62 7.89 8.14 8.59 8.99 9.34 9.67 9.97 10.3 

P d 83 94 106 116 127 136 147 156 176 196 216 236 256 278 

P d+P f 88 104 121 136 150 163 179 192 216 244 272 300 328 358 

800 h d 6.34 6.94 7.43 7.84 8.21 8.54 8.84 9.11 9.61 10.0 10.4 10.8 11.1 11.4 

P d 93 106 119 130 142 153 164 175 197 219 241 264 286 308 

P d+P f 98 116 134 150 165 180 196 211 237 267 297 328 358 388 

1000 h d 6.94 7.59 8.11 8.56 8.98 9.31 9.64 9.93 10.5 10.9 11.4 11.7 12.1 12.4 

P d 102 116 130 142 155 167 179 191 215 238 262 286 311 335 

P d+P f 107 126 145 162 178 194 211 227 255 286 318 350 383 415 


a The drift height (hd), maximum drift load (P d), and maximum load at wall (P d + P f) are limited to h c, (h c × D), and (h r × D) respectively. 




Table 3. Continued. (To be used with Figure 4a) Ground Snow Load (P g) versus Balanced Snow Load (P f), Density (D), 

Balance Snow Load Height (h b), Drift Height (h d), Max Drift Load (P d) and Max Load (P d+P f) 

Metric Units: 

Ground Snow Load, Pg (kN/m 2 ) 

Balanced Snow Load, Pf (kN/m 2 ) 

0.25 0.5 0.6 0.9 1.0 1.4 1.9 2.0 3.0 4.0 5.0 

0.25 0.5 0.6 0.9 1.0 1.3 1.7 1.7 2.4 3.2 4.0 

Density, D (kN/ cu m) 

Balanced Snow Load Height, hb (m) 

2.3 2.4 2.5 2.6 2.6 2.8 3.0 3.1 3.5 3.9 4.4 

0.1 0.2 0.2 0.3 0.4 0.4 0.6 0.6 0.7 0.8 0.9 

Upwind 

Distance Wb (m) 

Drift Height, hd (m) a 

Max. Drift Load, Pd (kN/m 2 ) a 

Max. Load at Wall, Pd + Pf (kN/m 2 ) a 

10 hd .37 .43 .46 .51 .53 .59 .66 .67 .77 .85 .91 

Pd .85 1.04 1.14 1.34 1.38 1.66 1.97 2.07 2.68 3.30 4.02 

Pd+Pf 1.10 1.54 1.74 2.24 2.38 2.92 3.67 3.77 5.08 6.50 8.02 

15 hd .49 .56 .59 .65 .67 .74 .82 .83 .91 1.03 1.11 

Pd 1.13 1.35 1.47 1.70 1.75 2.08 2.45 2.53 3.18 4.03 4.89 

Pd+Pf 1.38 1.85 2.07 2.60 2.75 3.34 4.15 4.23 5.58 7.23 8.89 

30 hd .74 .83 .86 .94 .97 1.06 1.15 1.16 1.31 1.42 1.52 

Pd 1.69 1.99 2.15 2.45 2.52 2.96 3.44 3.61 4.58 5.55 6.69 

Pd+Pf 1.94 2.49 2.75 3.35 3.52 4.22 5.14 5.31 6.98 8.75 10.69 

50 hd .96 1.07 1.10 1.2 1.23 1.34 1.44 1.46 1.63 1.77 1.89 

Pd 2.20 2.56 2.76 3.13 3.20 3.74 4.33 4.54 5.72 6.91 8.30 

Pd+Pf 2.45 3.06 3.36 4.03 4.20 5.00 6.03 6.24 8.12 10.11 12.30 

100 hd 1.32 1.46 1.51 1.63 1.67 1.8 1.94 1.96 2.18 2.35 2.49 

Pd 3.05 3.51 3.77 4.25 4.34 5.04 5.81 6.08 7.62 9.16 10.97 

Pd+Pf 3.30 4.01 4.37 5.15 5.34 6.30 7.51 7.78 10.02 12.36 14.97 

120 hd 1.44 1.58 1.63 1.76 1.80 1.94 2.09 2.11 2.34 2.52 2.68 

Pd 3.30 3.80 4.08 4.59 4.69 5.44 6.26 6.55 8.17 9.84 11.78 

Pd+Pf 3.55 4.30 4.68 5.49 5.69 6.70 7.96 8.25 10.57 13.04 15.78 

150 hd 1.58 1.74 1.79 1.94 1.98 2.13 2.29 2.31 2.56 2.76 2.92 

Pd 3.64 4.18 4.48 5.03 5.14 5.96 6.86 7.17 8.96 10.75 12.85 

Pd+Pf 3.89 4.68 5.08 5.93 6.14 7.22 8.56 8.87 11.36 13.95 16.85 

180 hd 1.71 1.88 1.93 2.09 2.13 2.29 2.46 2.49 2.75 2.96 3.13 

Pd 3.93 4.51 4.83 5.42 5.54 6.41 7.37 7.71 9.62 11.53 13.78 

Pd+Pf 4.18 5.01 5.43 6.32 6.54 7.67 9.07 9.41 12.02 14.73 17.78 

200 hd 1.79 1.96 2.02 2.18 2.22 2.39 2.56 2.59 2.86 3.08 3.26 

Pd 4.11 4.70 5.05 5.66 5.78 6.68 7.58 8.03 10.01 12.00 14.34 

Pd+Pf 4.36 5.20 5.65 6.56 6.78 7.94 9.38 9.73 12.41 15.20 18.34 

300 hd 2.11 2.31 2.38 2.56 2.61 2.80 3.00 3.03 3.34 3.59 3.8 

Pd 4.86 5.54 5.94 6.65 6.79 7.84 8.99 9.40 11.70 14.00 16.71 

Pd+Pf 5.11 6.04 6.54 7.55 7.79 9.10 10.69 11.10 14.10 17.20 20.71 


a The drift height (hd), maximum drift load (P d), and maximum load at wall (P d + P f) are limited to h c, (h c × D), and (h r × D) respectively. 




2.3.12.2 Adjacent Structures and Terrain Features 

Apply a drift load to lower roofs or structures sited within 20 ft (6 m) of a higher structure or terrain feature 

(e.g., tanks, hills) as shown in Figure 5. Determine the drift load using the methodology of Section 2.3.12.1; 

apply the factor 1-(S/20) with S in ft (1-[S/6] with S in meters) to the maximum intensity of the drift P d to 

account for the horizontal separation between structure S, expressed in ft (m). Drift loads need not be 

considered for separations greater than 20 ft (6 m). 

Wind 

2.3.12.3 Sliding Snow 


W b 

h or P 

d d 

1 1 

(Adjacent Structure 

or Terrain Feature) 

h (1- S/20)* 

d 

Fig. 5. Snow loads for lower roof of adjacent structures 

For lower roofs located below slippery roofs having a slope greater than 1.2° ( 1 ⁄ 4 on 12), or below other 

(non-slippery) roofs having a slope greater than 9.5° (2 on 12), consider a sliding snow surcharge load (psf) 

of 0.4P fW s/15 where P f is psf, and W s is feet (sliding surcharge load [kN/m 2 ] of 0.4P fW s/4.6 where P f is kN/m 2 , 

and W s is meters); except that h s needs to exceed h c. Determine h s by dividing the snow surcharge load 

by the appropriate snow density (D). Note that W s is the horizontal distance from the ridge to the eave of the 

upper roof. See Figure 6. 

Apply sliding snow surcharge load to the balanced snow load (P f) of the lower roof. 

S 

Drift 


Balanced Snow Load, P f 

For consideration of the sliding snow surcharge, “slippery” roof surfaces are defined as metal (aluminum, 

copper, galvanized or enameled steel panels such as are used on all-metal buildings); rubber or plastic 

membranes; bituminous or asphalt without granular surfacing; or slate, concrete, clay tile, composite, or 

similar shingles without granular surfacing. Other (“non-slippery”) roof surfaces are defined as all surfaces not 

defined here as “slippery”. 

Sliding snow need not be considered if the lower roof is separated a distance S greater than h r, or 20 ft (6 m), 

whichever is less. 

W d 

hd Pd hr 


* Using English units, for metric units use h (1-S/6) 

d 


P f 

h b



Ridge Line 


2.3.12.4 Roof Projections and Parapets 

h r 

Slope* 

h c 

W s 

h s 

h b 

Eave 

15 ft. 

(4.6 m) 

* Consider sliding snow surcharge loads only when upper 

roof slope is greater than 1.2º (1/4 on 12) for slippery 

upper roofs or when upper roof slope is greater 

than 9.5º (2 on 12) for other (non-slippery) upper roofs. 

Sliding Snow 


Balanced Snow 

Load, P f 


Sliding snow surcharge load (psf) = 0.4 P W /15 with P in psf and W in feet. 

2 

f s f s 

2 

f s f s 

Sliding snow surcharge load (kN/m ) = 0.4 P W /4.6 with P in kN/m and W in meters. 

Fig. 6. Sliding snow load for lower roofs (upper roof snow load not shown) 

Projections above lower roofs, such as high bays or higher roofs of the same building, or penthouses and 

mechanical equipment, can produce windward drifting on the lower roof as depicted in Figure 7. Calculate 

such drift loads on all sides of projections having horizontal dimensions (perpendicular to wind direction) 

exceeding 15 ft (4.6 m) using the methodology described in this section, even though the surcharge loading 

shape may be quadrilateral rather than triangular. To compensate for a probable lower drift height, 75% of 

the drift height (h d) is used, based on a value of W b taken as the maximum distance upwind from the projection 

to the edge of the roof. 

Compute drift loads created at the perimeter of the roof by a parapet wall using 75% of the drift height (h d), 

with W b equal to the length of the roof upwind of the parapet. 

Where the width of the roof projection (W p) is 10 ft (3.0 m) or greater, consider leeward drift on the low roof 

adjacent to the roof projection in accordance with Section 2.3.12.1; however, if the length of the projection 

(perpendicular to W p) is less than 1 ⁄ 3 of h c , leeward drift need not be considered. Leeward drift load is 

superimposed on balanced snow load; it need not to be added to the windward drift load. 

Wind 

¾ h d 

h b 

Edge of Roof (typ.) 

W b (drift this side) 

Wd Wall 

Roof Projection 

Balanced Load, P f 

W p 

Fig. 7. Snow load at roof projections 

W (drift this side) 

b 

W d 


Windward Drift 


¾ h d 

h b 

h c 

h r



2.4 Rain-on-Snow Surcharge 

For locations where the ground snow load (P g) is 20 psf (0.96 kN/m 2 ) or less, but not equal to zero, use a 

uniform rain load surcharge of 5 psf (0.24 kN/m 2 ) in combination with the balanced snow load, depending on 

the roof slope (see Table 4). Note that the rain-on-snow surcharge load need not be used in combination 

with unbalanced, drifting, or sliding snow loads. 

W 

(ft) 

Table 4. Rain-on-Snow Surcharge Load 

Rain-on-Snow Surcharge 

Roof Slope W 

in./ft Rise: Run Degrees % (m) 

30 0.125 1 ⁄ 8: 12 0.6 1.0 9 

45 0.1875 3/16: 12 0.9 1.6 14 

60 0.25 1 ⁄ 4: 12 1.2 2.1 18 

90 0.375 3 ⁄ 8: 12 1.8 3.1 27 

120 0.5 1 ⁄ 2: 12 2.4 4.2 37 

150 0.625 5 ⁄ 8: 12 3 5.2 46 

180 0.75 3 ⁄ 4: 12 3.6 6.3 55 

240 1.0 1: 12 4.8 8.4 73 

300 1.25 1 ¼: 12 6 10.5 91 

360 1.5 1 1 ⁄ 2: 12 7.2 12.6 110 

Notes: 

1. For roof slopes less than those shown in the table, add a uniform design surcharge load of 5 psf (0.24 kN/m 2 )to the uniform design snow 

load. 

2. The 5 psf surcharge load need not be applied where the 50-year ground snow load is greater than 20 psf (0.96 kN/m 2 ). 

3. The 5 psf surcharge load need not be applied where the 50-year ground snow load is zero. 

4. The 5 psf surcharge load is applicable to balanced snow load cases only, and need not be used in combination with drift, sliding, or 

unbalanced snow load. 

5. W = the horizontal distance from the roof ridge or valley to the eave. 

2.5 Rain Loads and Roof Drainage 

2.5.1 General 

Determine design rain loads in accordance with the recommendations in this section; however, ensure the 

governing design roof loads are not less than the minimum live loads or snow loads designated by the 

applicable building code, nor less than the minimum roof live loads and snow loads covered in Sections 2.2, 

2.3, and 2.4 of this data sheet. Rain loads cannot be determined until the primary and secondary roof drainage 

systems have been designed. 

2.5.2 Bases for Design Rain Loads 

2.5.2.1 Design rain loads: Design each section of the roof structure to sustain the load from the maximum 

depth of water that could accumulate if the primary drainage system is blocked, including the depth of water 

ABOVE the inlet of the secondary drainage system at its design flow. 

Determine this design rain load (load due to the depth of water [total head]) by the relative levels of the roof 

surface (design roof line) and overflow relief provisions, such as flow over roof edges or through overflow 

drains or scuppers. If the secondary drainage system contains drain lines, ensure they are independent of 

any primary drain lines. (See Figures 8a and 8b.) 

2.5.2.2 The general expression given below for the design rain load for roof supporting members is the total 

head times the weight of the water. Total head is measured from the design roof line to the maximum water 

level (overflow discharge), as illustrated in Figures 8a and 8b. The total head includes the depths of water 

from the design roof line to the overflow provision plus the hydraulic head corresponding to either an overflow 

drain or scupper. In addition, have the roof framing designer prepare calculations substantiating that the roof 

design precludes roof instability due to ponding. 

Total head = maximum water depth from design roof line to overflow discharge level, including any hydraulic 

head. 





Design rain load (psf) = total head (in.) × 5.2 ≥ 15 psf for dead-flat roofs, and 30 psf at low-point of sloped 

roofs. 

Metric Units: 

Design rain load (kN/m 2 ) = total head (mm) × 0.01 ≥ 0.7 kN/m 2 for dead-flat roofs, and 1.4 kN/m 2 at 

low-point of sloped roofs. 

2.5.2.3 Minimum design rain loads: Design structural roof support members to support at least a 3 in. (75 

mm) depth of water on dead-flat roofs, or at least a 6 in. (150 mm) depth of water at the low point of drains 

and scuppers on sloped roofs, but not less than the total head. The actual rain load distribution to the 

structural members will depend on any roof slope and the overflow relief provisions. These minimum rain 

loads are included in the above equation. 

2.5.2.4 Ponding instability: Design roofs with a slope less than ¼ in./ft (1.2 degrees) to preclude (i.e., ruling 

out in advance) instability from ponding with the primary drainage system blocked. Use the larger of the rain 

or snow loads. 

2.5.2.5 Controlled drainage provisions: 

• Provide roofs with controlled flow drains with an overflow drainage system at a higher elevation that 

prevents rainwater buildup on the roof above that elevation, except for the resulting hydraulic head (see 

‘‘typical roof drains’’ in Fig. 8a). 

• Design such roofs to support the load of the maximum possible depth of water to the elevation of the 

overflow drainage system, plus any load due to the depth of water (hydraulic head) needed to cause flow 

from the overflow drainage system. 

• Design the controlled drainage system to meet the same maximum head restriction for conventional roof 

drainage (see Section 2.5.2.6). 

• Consider roof instability due to ponding in this situation. Likewise, ensure the overflow drainage system 

is independent of any primary drain lines. 

Overflow 

Scupper 

Actual Roof Slope 

(deflected) 

Primary Drain/ 

Pipe (blocked) 

Design Roof Line 

Water Buildup 

Primary Scupper - Beyond 

(blocked) 

Hydraulic Head 

Total Head 

2-4 in. (50-100 mm) 

2-4 in. (50-100 mm) 

Ponding 

TYPICAL INTERIOR DRAINS 

Water Buildup 

Total Head 

TYPICAL PERIMETER SCUPPERS 

Hydraulic Head 

Overflow Drain/Pipe 

Design Roof Line 

Ponding 

Actual Roof Slope 

(deflected) 

Fig. 8a. Typical primary and overflow systems for pitched roofs 




2.5.2.6 Maximum Design Head 

Design the primary roof drainage system such that the maximum total head (including hydraulic head) will 

not exceed 6 in. (150 mm) at the design flow rate. 

2.5.2.6.1 Adjusting Hydraulic Head for Different Drain Geometry 

A. For weir flow and transition flow regime designations in Tables 8a, 8b, 8c, and 8d: 

Where the primary drain bowl (sump) diameter, or the secondary (overflow) drain dam or standpipe diameter, 

differs from what is provided in Tables 8a, 8b, 8c, or 8d by: 

1. 15% or less, the effect on hydraulic head corresponding to a given flow rate is not significant; therefore, 

use the values in the tables as provided. 

2. More than 15%, use the following equation to approximate the effect on the hydraulic head with the 

flow rate held constant (but see Part a. below): 

H 2 = [(D 1/D 2) 0.67 ](H 1) 

Where: 

H 1 = known hydraulic head from the tables 

D 1 = drain bowl diameter for primary drains, or overflow dam or standpipe diameter for secondary 

(overflow) drains, corresponding to H 1 for a given flow rate 

H 2 = hydraulic head to be determined 

D 2 = drain bowl diameter for primary drains, or overflow dam or standpipe diameter for secondary 

(overflow) drains, corresponding to H 2 for a given flow rate 

a. Do not, under any circumstances, use less than 80% of the hydraulic head indicated in Tables 8a, 

8b, 8c, and 8d (for the given drain [outlet] size and flow rate) unless flow test results from a reputable 

testing laboratory, witnessed and signed by a licensed professional engineer, are provided to justify the 

hydraulic head values. 

b. Example: 

Roof Edge Overflow 

Roof Edge Cant Height 

Overflow Scupper 



Water Buildup 

TYPICAL INTERIOR DRAINS - ROOF EDGE OVERFLOW 

Hydraulic Head (Table 6) 

2-4 in. (50-100 mm) 

Total Head 

Total Head 

Ponding 



Actual Roof Line 

(deflected) 

Design Roof Line (flat) 


Ponding 


(deflected) 

Water Buildup 

TYPICAL INTEROIOR DRAINS - PERIMETER OVERFLOW SCUPPERS 

Fig. 8b. Typical primary and overflow drainage systems for flat roofs 


CL 

CL 

Interior Support 

Interior Support



Determine the total head for an 8 in. secondary drain (8 in. outlet diameter), with a 10 in. diameter x 

2 in. high overflow dam outlet, at a flow rate (Q) of 300 GPM. 

From Table 8c: 

D 1 = 12.75 in. (dam diameter) 

H 1 = 2.0 in. for 300 GPM, 8 in. outlet 

For the 10 in. diameter overflow dam on an 8 in. drain outlet: 

D 2 = 10 in. (dam diameter) 

Therefore: 

H 2 = [(D 1/D 2) 0.67 ](H 1) 

H 2 = [(12.75 in./10 in.) 0.67 ](2.0 in.)H 2 =(1.18)(2.0 in.) = 2.4 in. at Q = 300 GPM 

Total head = 2.4 in. hydraulic head + 2 in. dam height = 4.4 in. 

For secondary (overflow) drains with standpipes, the drain configuration is such that the drain bowl 

(sump) diameter typically has less impact than the standpipe diameter on the hydraulic head for a given 

flow rate; therefore make any adjustments to hydraulic head (H 2) based on standpipe diameter. 

B. For orifice flow regime designations for roof drains in Tables 8a, 8b, 8c and 8d, the total hydraulic head 

can include the depth of the drain bowl (sump). Therefore, where the depth of the drain bowl is less than that 

indicated in Tables 8a, 8b, 8c and 8d, add the difference in drain bowl depth to the hydraulic head from the 

tables to determine the design hydraulic head; however, where the depth of the drain bowl is greater than 

that indicated in the tables, use the hydraulic head in the tables as the design hydraulic head. 

2.5.3 Designing for Stability Against Ponding 

Roof instability due to ponding can be minimized or controlled in the initial roof design by any of the following 

methods: 

a) Provide sufficient overflow relief protection to remove the water before it reaches an excessive depth. 

b) Slope the roof sufficiently to ensure water will flow off the edges of the roof. 

c) Provide a sufficiently stiff and strong roof to limit the amount of deflection and to withstand ponding 

as well as the total load. 

d) Specify camber for roof supporting members (e.g., open web joists, structural shapes, and plate girders 

of steel). 

Design standards, such as the American Institute of Steel Construction (AISC) Specifications for Structural 

Steel Buildings, require that roof systems be investigated by structural analysis to ensure adequate strength 

under ponding conditions, unless the roof surface is provided with sufficient slope toward points of free 

drainage or other means to prevent the accumulation of water. The AISC specifications permit a reduction 

in safety factor to 1.25 (yield) with respect to bending stress due to ponding plus the total load supported by 

the roof (i.e., design rain and dead loads). See additional information in this section stability against ponding. 

Analyze roof framing systems according to the following recommendations (as applicable), to ensure 

instability from ponding does not occur based on the total load (dead plus snow and rain loads) supported 

by the roof framing before consideration of ponding, or by substantiating that a roof slope is sufficient. 

a) Dead-flat roofs: Ensure the total load supported is the design rain load plus the dead load of the roof. 

An acceptable analysis method for ponding of two-way framing systems is presented in the ASD and 

LRFD Specifications for Structured Steel Buildings, Commentary, Chapters K2, American Institute of Steel 

Construction (AISC). 

b) Sloped roofs to drains or scuppers: Ensure the total load supported is the design rain loads distributed 

locally to the low areas, plus the dead load of the roof. An acceptable analysis method, conservative for 

sloped roofs, is the AISC method given in Part a above using an appropriate equivalent uniform load 

based on the design rain load distribution plus dead load for the total load supported. Also, if the design 

roof slope is less than 1 ⁄ 4 in./ft (2%), ensure it is sufficient according to Section 2.5.4.1.13.2. 

c) Sloped roofs to free drainage over the roof edge: If the design roof slope is less than 1 ⁄ 4 in./ft (2%), 

ensure it is sufficient according to Section 2.5.4.1.13.2. 




2.5.4 Roof Drainage 

2.5.4.1 Conventional (Non-Siphonic) Roof Drainage 

2.5.4.1.1 Positive Drainage 

Design all roofs with positive drainage; however, dead-flat roofs consistent with this data sheet are acceptable. 

Sloping the roof surface 1 ⁄ 4 in./ft (2%) toward roof drains or scuppers or points of free drainage (roof edge) 

should be sufficient for positive drainage. If a slope of less than 1 ⁄ 4 in./ft (2%) is desired for positive drainage, 

use the analysis methods presented in Section 2.5.7. 

2.5.4.1.2 Secondary Drainage 

Provide secondary (overflow or emergency) roof drains or scuppers where blockage of the primary drains, 

if any, allows water to accumulate. This includes when roof gutters or other drains are located behind a 

parapet. 

2.5.4.1.3 Design Rainfall Intensities 

Design primary and secondary roof drainage to handle no less than the rainfall intensity (in./hr or mm/hour) 

based on a duration of 1-hr and frequency (MRI) of 100-years. For locations outside the United States, except 

as noted below, use the greater of the rainfall intensities determined using this data sheet or local codes 

and rainfall intensity maps. 

Rainfall intensity maps are in Appendix C. 

Note: The rainfall intensities will not necessarily correspond along the common boundary of the Western 

and Central United States because the Central and Eastern United States map is newer (1977 vs. 1961). 

The values expressed in inches are the most intense 60-min duration rainfalls having a 1% probability of being 

exceeded in one year. This is commonly designated as the ‘‘100-year, 1-hour rainfall.’’ 

In areas outside those covered by the maps and tabulation, or in local areas of intense rainfall history, obtain 

the rainfall intensities from local meteorological stations based on a 1-hr duration rainfall and a 100-yr MRI. 

Reasonable, but not exact, multiplication factors for converting a 1-hr duration rainfall of 30-yr and 50-yr 

MRI to a 100-yr MRI are 1.2 and 1.10, respectively. 

2.5.4.1.4 Design Drainage Area 

Use the roof area along with one-half ( 1 ⁄ 2) the area of any vertical walls that drain to the roof area in sizing 

drains and determining roof loads and stability from ponding. 

2.5.4.1.5 Roof Loads 

The roof primary and secondary drainage systems must be designed before the roof loads can be determined. 

2.5.4.1.6 Existing roofs 

Provide existing roofs (especially lightweight roof constructions) that have severely inadequate primary 

drainage and no overflow relief protection with additional drainage provisions. Determine the need for overflow 

drainage in conjunction with an evaluation of existing conditions. 

2.5.4.1.7 Roof Drains and Scuppers 

Roof drains may be used for conventional, or controlled-flow drainage systems. Roof drains and scuppers 

may be used separately or in combination for primary or overflow drainage systems. The following sections, 

when referring to drains, apply to conventional or controlled-flow drainage systems. 

2.5.4.1.7.1 Quantity 

Provide at least two roof drains or scuppers for total roof areas of 10,000 ft 2 (930 m 2 ) or less. For larger 

roof areas, provide a minimum of one drain or scupper for each 10,000 ft 2 (930 m 2 ) of roof area. The roof 

area may be increased to 15,000 ft 2 (1400 m 2 ) with a minimum drain diameter of 6 in. (150 mm) or scupper 

width of 8 in. (200 mm). 




2.5.4.1.7.2 Drain and Drain Leader Sizes 

Provide roof drains and vertical leaders in sizes of 4 in. to 10 in. (100 to 250 mm) diameter inclusive, except 

for areas less than 2500 ft 2 (230 m 2 ), such as canopies, where 3 in. (75 mm) diameter drains may be used. 

It is acceptable to use 10 in. (250 mm) diameter drains, but may be impractical due to drainage area 

limitations and drain flow restrictions imposed by drainage piping. 

Always use a drain leader (vertical and horizontal piping) with a diameter not less than the drain outlet 

diameter. 

2.5.4.1.7.3 Drain Strainers 

Provide strainers extending a minimum of 4 in. (102mm) above the roof surface over all roof drains. Use 

strainers with an available inlet area not less than one and one-half times the area of the conductor or leader 

connected to the drain. Flat-surface strainers with an inlet area not less than two times the area of the 

conductor can be used on flat decks, including parking decks and sun decks. 

2.5.4.1.7.4 Placement 

The placement of (primary) roof drains or scuppers are influenced by the roof structure’s support columns 

and walls, expansion joints, roof equipment, and other projections. When possible, locate roof drains at 

mid-bay low points, or within 20% of the corresponding bay spacing from the low points in each direction. 

If roof drains or scuppers are located at points of little deflection, such as columns and walls, slope the roof 

surface toward them at least 1 ⁄ 8 in./ft (1%) to compensate for minimum deflections at these locations. In 

general, do not locate interior (non-perimeter) drains more than 50 ft (15 m) from the roof perimeter, nor more 

than 100 ft (30 m) apart. Exception: Distances of 75 ft (23 m) from the perimeter and 150 ft (46 m) apart, 

may be used with a minimum drain diameter of 6 in. (150 mm). Place primary scuppers level with the roof 

surface in a wall or parapet as determined by the roof slope and the contributing area of the roof, but not 

located more than 50 ft (15 m) from a roof juncture, nor more than 100 ft (30 m) apart along the roof perimeter, 

except 60 ft (18 m) and 125 ft (38 m), respectively, may be used with a minimum scupper width of 8 in. 

(200 mm). Careful consideration of the above during the design phase is essential to provide adequate and 

uniform drainage of each roof section. 

2.5.4.1.7.5 Secondary Drainage 

Provide secondary drainage for both dead-flat and sloped roofs to prevent any possibility of water overload. 

The overflow relief provision establishes the maximum possible water level based on blockage of the primary 

drainage system. Ensure the provision is in the form of minimal height roof edges, slots in roof edges, 

overflow scuppers in parapets or overflow drains adjacent to primary drains. Ensure the overflow relief 

protection provides positive and uniform drainage relief for each roof section, with drainage areas preferably 

not exceeding those of the primary drainage or the drainage area limits in Section 2.5.4.1.7.1. When 

designing and sizing the secondary drainage system (overflow drains), assume the primary drains are 100% 

blocked and cannot flow water. 

Ensure the inlet elevation of overflow drains and the invert elevation (see sketches in Table 6a) of overflow 

scuppers are not less than 2 in. (50 mm) nor more than 4 in. (100 mm) above the low point of the (adjacent) 

roof surface unless a safer water depth loading, including the required hydraulic head to maintain flow, has 

been determined by the roof-framing designer. 

For secondary (overflow) roof drains, use a dam or standpipe diameter at least 30% larger the drain outlet 

diameter. 

For secondary (overflow) roof drains, use a drain bowl (sump) diameter not less than 75% of the dam 

diameter. 

2.5.4.1.7.5.1 Secondary drainage discharge: Discharge roof overflow drain or scupper drainage systems 

using vertical leaders, conductors, or piping separate from the primary drainage system and to an abovegrade 

location normally visible to building occupants. Discharge to points of free drainage, such as 

over-the-roof edges or through relief openings atop conductors, if this isn’t practical. 




2.5.4.1.8 Scuppers and Gutters 

Use three-sided channel-type roof scuppers whenever possible. For parapet walls, use the four-sided 

perimeter, closed-type scuppers (see sketch with Table 6a). Provide scuppers and leaders or conductors with 

minimum dimensions of 6 in. (150 mm) wide by 4 in. (100 mm) high and 5 in. (125 mm) diameter or equivalent, 

respectively. Ensure the height of the closed-type scupper is at least 1 in. (25 mm) higher than the estimated 

water buildup height (hydraulic head) developed behind the scupper (see Table 6a). 

Provide a watertight seal between gutters and the underside of the roof to ensure that rainwater will not enter 

the building, nor breach the building’s weather tight envelope, due to wind-driven rain or gutter overflow. 

2.5.4.1.9 Downspouts 

Provide downspouts that are protected or truncated above the highest expected level of snow banks and 

potential impacting objects (truck docks, etc.) or are of open-channel design. 

2.5.4.1.10 Inspection 

Inspect roofs and their drainage inlets after roof construction, prior to the start of the rainy or tropical cyclone 

seasons, and following storms, or at least every three months. Clear obstructions or accumulations of foreign 

matter as frequently as necessary. 

Inspects gutters to ensure that they are properly sealed at the underside of roofing to prevent rainwater from 

entering the building. 

2.5.4.1.11 Drainage System Sizing 

Determine the rainfall intensity for a given location using Section 2.5.4.1.3 and Appendix C, then calculate 

the number and sizes of roof drains and/or scuppers for the primary drainage system, as well as the sizes of 

vertical leaders or conductors and horizontal drainage piping as follows: 

Secondary drain sizing: Where provided, size secondary drains at least equivalent to the maximum capacity 

of the primary roof drains or scuppers as stated in Tables 5 and 6. For example, if the primary drainage system 

consists of six 6 in. (150 mm) drains each flowing 540 gpm (2044 L/min) and scuppers are used for the 

secondary drainage system, then the maximum drainage capacity of all scuppers should be equivalent to 

the 3,240 gpm (12,260 L/min) maximum drainage capacity of the primary roof drains. 

Total scupper capacity = 6 x the 540 gpm (2044 L/min) maximum capacity of each 6 in. (150 mm) drain per 

Table 5 = 3,240 gpm (12,260 L/min) 

1. Sizing Conventional Roof Drains/Vertical Leaders and Scuppers 

a. Determine the total number of roof drains or scuppers needed: 

Equation 1.1 English Units 

n = 

A ; or n = A 

10,000 15,000 

(for 6 in. dia. drains and 

8 in. wide scuppers per Section 2.5.4.1.7.1) 

Where n = number of drains needed (nearest higher whole no. ≥ 2) 

A = total roof drainage area (ft 2 ) 

Equation 1.2 Metric Units 

n = 

A 

930 ; or n = A 

1400 

(for 150 mm dia. drains 

and 200 mm wide scuppers per Section 2.5.4.1.7.1) 

Where n = Number of drains needed (nearest higher whole no. ≥ 2) 

A = Total roof drainage area (m 2 ) 

b. Determine the flow rate needed per roof drain, leader, or scupper: 

Equation 2.1 English Units 

Q = 0.0104 × i × A (See Note below) 

n 




Where Q = drain, leader or scupper flow needed (gpm) 

i = rainfall intensity (in./hr), Section 2.5.4.1.3 

A = total roof drainage area (ft 2 ) 

n = number of drains needed (Equation 1.1) 

Equation 2.2 Metric Units 

Q = 0.0167 × i × A (See Note below) 

n 

Where Q = drain, leader or scupper flow needed (dm 3 /min) 

i = rainfall intensity (mm/hr), Section 2.5.4.1.3 

A = total roof drainage area (m 2 ) 

n = number of drains needed (Equation 1.2) 

Note: The above coefficients (0.0104 or 0.0167) times ‘‘i’’ convert the rainfall intensity to an (average) 

flow rate per unit area (see Table 7); however, these coefficients may vary for controlled drainage 

systems (see Sizing Controlled Roof Drain/Vertical Leaders below). 

c. Determine the size needed for roof drains, leaders, or scuppers: 

Drains and Vertical Leaders 

Apply the flow, Q, needed per drain or vertical leader to Table 5 and select a drain or vertical leader diameter 

that provides adequate flow capacity. 

Scuppers 

Apply the flow, Q, needed per scupper to Table 6 and select a scupper type and size that provides adequate 

flow capacity. 

2. Sizing Controlled Roof Drains/Vertical Leaders 

a) Use the methodology in this section for controlled drainage systems by converting the rainfall intensity 

to the design peak flow rate rather than to the (average) flow rate. 

b) The design peak flow rate is usually approximated at twice the average flow rate for a controlled drain 

age system. 

c) The peak flow rate is the limited (controlled) flow rate required to maintain a predetermined depth of 

water on a roof and drain the roof within a 24-hour or 48-hour period. It varies according to the controlled 

drainage design criterion, rainfall intensity, and roof slope configuration. 

3. Sizing Horizontal Drainage Piping 

a) Determine the flow, Q p, needed per horizontal drainage pipe section: 

Q p = Q times the number of drains serviced by the pipe section. 

b) Determine the size of horizontal drainage piping needed: 

Apply the flow, Q p, needed per pipe section to Table 5 and select the pipe diameter and slope that provide 

adequate flow capacity. 




Table 5. Flow Capacity for Roof Drains 

⁄ ⁄ ⁄ 

Diameter of Drain or Pipe 

English Units 

Horizontal Drainage Piping, gpm Slopes (in. per ft) 

(in.) 

1 8 Slope 1 4 Slope 1 2 Slope 

3 34 48 69 

4 78 110 157 

5 139 197 278 

6 223 315 446 

8 479 679 958 

10 863 1217 1725 

12 1388 1958 2775 

15 2479 3500 4958 

Diameter of Drain or Pipe 

Metric Units 

Horizontal Drainage Piping, L/min Slopes (percentages) 

(mm) 

1 Slope 2 Slope 4 Slope 

75 130 180 260 

100 295 415 595 

125 525 745 1050 

150 845 1190 1690 

200 1815 2570 3625 

255 3265 4605 6530 

305 5255 7410 10,500 

380 9385 13,245 18,770 

2.5.4.1.12 Rain Loads with Drains and/or Scuppers 

2.5.4.1.12.1 Determine the hydraulic head and the total head (rainwater depth): 

a) Roof edges: Ignore the negligible hydraulic head needed to cause flow across a roof and over its edges. 

Base the total head on the height of the roof edge above the roof surface. 

b) Primary Roof Drains: Use Table 8a or 8b with the needed flow rate Q, and for the appropriate drain 

size, to determine the hydraulic head. Where the drain rim is flush with the roof surface (which is typical), 

the total head is equal to the hydraulic head. 

c) Secondary (overflow) Roof Drains: Use Table 8c or 8d with the needed flow rate Q, and for the 

appropriate drain size, to determine the hydraulic head above the rim of the drain dam or standpipe. Add 

the height of the dam or standpipe (height above the roof surface) to the hydraulic head listed in the table 

to determine the total hydraulic head. 

d) Overflow roof scuppers: Use Table 6a or 6b with the needed flow rate, Q (Section 2.5.4.1.11) under 

an appropriate scupper type and size, and determine the approximate depth of water above the scupper’s 

invert (by interpolation when necessary). 

Refer to Figures 8c and 8d for details of primary and secondary (overflow) roof drains, respectively, and 

hydraulic and total head for weir and transition flow regimes. 




Drain bowl (sump) 

diameter 

2.5.4.1.12.2 Determine the Rain Load 

The design rain load is based on the total head (rainwater depth) as follows: 

Rain Load = Total Head (in.) x 5.2 psf per in. 

Drain outlet 

size (diameter) 

Rain Load = Total Head (mm) x 0.01 kN/m 2 per mm 

Fig. 8c. Primary roof drain and hydraulic head 

Dam or standpipe 

diameter 

Drain outlet 

size (diameter) 

Water 

level 

Hydraulic 

head 

Top of overflow dam 

or standpipe 

Hydraulic head 

Total 

head 

Water 

level 

Fig. 8d. Overflow (secondary) roof drain with hydraulic head and total head 



surface 


surface



Table 6a. Hydraulic Head Versus Flow Capacity for Rectangular Roof Scuppers 

(Depth of Water Over Invert Versus Flow of Water Through Scupper) 

English Units 

Scupper Flows (gpm) 

Water 

Channel Type Closed Type 

Buildup h ≥ H Width b, in. Height h = 4 in. Height h = 6 in. 

H, in. 

Width b, in. 

6 8 12 24 6 8 12 24 6 8 12 24 

1 18 24 36 72 (see channel type) (see channel type) 

2 50 66 100 200 

3 90 120 180 360 

4 140 186 280 560 

5 194 258 388 776 177 236 354 708 

6 255 340 510 1020 206 274 412 824 

7 321 428 642 1284 231 308 462 924 303 404 606 1212 

8 393 522 786 1572 253 338 

Metric Units 

506 1012 343 456 686 1372 

Scupper Flows L/min 

Water 

Channel Type Closed Type 

Buildup h ≥ H Width b, mm Height h = 100 mm Height h = 150 mm 

H, mm 

Width b, mm 

150 200 300 500 150 200 300 500 150 200 300 500 

25 63 84 126 210 (see channel type) (see channel type) 

50 178 237 356 595 

75 327 437 656 1093 

100 505 673 1009 1682 

125 705 940 1411 2351 642 856 1284 2141 

150 927 1236 1854 3090 749 998 1497 2495 

175 1168 1558 2337 3894 841 1121 1681 2802 1105 1474 2211 3684 

200 1427 1903 2855 4758 923 1230 1846 3076 1249 1665 2498 4163 

Notes: Whenever h ≥ H for a closed-type scupper, the scupper flows under channel-type scuppers are appropriate. 

Interpolation is appropriate. 




Table 6b. Hydraulic Head and Flow Capacity for Circular Roof Scuppers 

Scupper Flow (gpm) 

H (in.) Scupper Diameter (in.) 

5 6 8 10 12 14 16 

1 6 7 8 8 10 10 10 

2 25 25 30 35 40 40 45 

3 50 55 65 75 75 90 95 

4 80 90 110 130 140 155 160 

5 115 135 165 190 220 240 260 

6 155 185 230 270 300 325 360 

7 190 230 300 350 410 440 480 

8 220 280 375 445 510 570 610 

Scupper Flow (L/min) 

H (mm) Scupper Diameter (mm) 

123 150 200 250 300 350 400 

25 22 26 30 32 37 38 38 

50 95 95 114 132 151 151 170 

75 189 208 246 284 284 341 360 

100 303 341 416 492 530 587 606 

125 435 511 625 719 833 908 984 

150 587 700 871 1022 1136 1230 1363 

175 719 871 1136 1325 1552 1666 1817 

200 833 1060 1420 1685 1931 2158 2309 

Notes: 

1) H = static head above scupper invert (design water level above base of scupper opening). 

2) Linear interpolation is acceptable. 

3) Extrapolation is not appropriate. 




Table 7. Conversion of Rainfall Intensity to Flow Rate and Rain Load per Unit Area 

English Units 

Rainfall Intensity (in./hr) Flow Rate (gpm/ft 2 ) Rain Load/hr (psf) 

1.0 .0104 5.2 

1.5 .0156 7.8 

2.0 .0208 10.4 

2.5 .0260 13.0 

3.0 .0312 15.6 

3.5 .0364 18.2 

4.0 .0416 20.8 

4.5 .0468 23.4 

5.0 .0520 26.0 

5.5 .0572 28.6 

6.0 .0624 31.2 

7.0 .0728 36.4 

8.0 .0832 41.6 

9.0 .0936 46.8 

10.0 .1040 52.0 

Metric Units 

Rainfall Intensity (mm/hr) Flow Rated (L/min per m 2 ) Rain Load/hr 

(kilonewtons [kN] per m 2 ) 

25 0.42 .25 

30 0.5 .29 

35 0.58 .34 

40 0.67 .39 

45 0.75 .44 

50 0.83 .49 

55 0.92 .54 

60 1.0 .59 

70 1.2 .69 

80 1.3 .79 

90 1.5 .88 

100 1.7 .98 

200 3.3 1.96 

300 5.0 2.94 

Note: Interpolation is appropriate. 




Table 8a. Hydraulic Head and Corresponding Drain Flow for Primary Roof Drains (US units) 

Hydraulic Head (in.) 

Drain Outlet Size (in.) 

3 4 5 6 8 10 

Diameter Bowl (Sump) Diameter (in.) 

10.5 10.5 10.5 10.5 11.75 15.25 

Flow Rate 

Drain Bowl (Sump) Depth (in.) 

Flow Rate 

(GPM) 2 2 2 2 3.25 4.25 (GPM) 

50 1.5 1.5 1.5 1.0 1.0 - 50 

75 1.5 1.5 1.5 1.5 - - 50 

100 2.5 2.0 2.0 2.0 2.0 2.0 100 

125 4 .0 2.5 - - - - 125 

150 5.0 3.0 2.5 2.5 - - 150 

175 5.5 4.0 - - - - 175 

200 - 4.5 3.0 3.0 3.0 2.5 200 

225 - 5.5 - - - - 225 

250 - 6.0 3.5 3.5 - - 250 

275 - - - - - - 275 

300 - - 4.5 4.0 4.0 3.0 300 

325 - - - - - - 325 

350 - - 5.5 4.5 - - 350 

375 - - - - - - 375 

400 - - 6.0 4.5 4.5 3.5 400 

450 - - - 4.5 - - 450 

500 - - - 5.0 4.5 3.5 500 

550 - - - 6.0 - - 550 

600 - - - - 4.5 4.0 600 

650 - - - - - - 650 

700 - - - - 5.0 4.0 700 

800 - - - - 5.5 4.5 800 

900 - - - - 6.0 4.5 900 

1000 - - - - - 5.0 1000 

1100 - - - - - 5.5 1100 

1200 - - - - - 6.0 1200 

Notes: 

1) Hydraulic head in this table is the height of water above the drain rim. Where the drain rim is at the same elevation as the surrounding 

roof surface, the total head is the same as the hydraulic head. 

2) Assume that the flow regime is either weir flow or transition flow, except where the hydraulic head values are in blue cells below the 

heavy line that designates orifice flow. 

3) Refer to Section 2.5.2.6 for recommendations for differing drain geometry. 






Table 8b. Hydraulic Head and Corresponding Drain Flow for Primary Roof Drains (SI units) 

Hydraulic Head (mm) 

Drain Outlet Size (mm) 

75 100 125 150 200 250 

Diameter Bowl (Sump) Diameter (mm) 

270 270 270 270 300 390 

Flow Rate 

Drain Bowl (Sump) Depth (mm) 

Flow Rate 

(L/min) 50 50 50 50 85 110 (L/min) 

190 38 38 38 25 25 - 190 

285 38 38 38 38 - - 285 

380 64 51 51 51 51 51 380 

475 102 64 - - - - 475 

570 127 76 64 64 - - 570 

660 140 102 - - - - 660 

755 - 114 76 76 76 64 755 

850 - 140 - - - - 850 

945 - 152 89 89 - - 945 

1040 - - - - - - 1040 

1135 - - 114 102 102 76 1135 

1230 - - - - - - 1230 

1325 - - 140 114 - - 1325 

1420 - - - - - - 1420 

1515 - - 152 114 114 89 1515 

1705 - - - 114 - - 1705 

1895 - - - 127 114 89 1895 

2080 - - - 152 - - 2080 

2270 - - - - 114 102 2270 

24600 - - - - - - 2460 

2650 - - - - 127 102 2650 

3030 - - - - 140 114 3030 

3405 - - - - 152 114 3405 

3785 - - - - - 127 3785 

4165 - - - - - 140 4165 

4540 - - - - - 152 4540 

Notes: 

1) Hydraulic head in this table is the height of water above the drain rim. Where the drain rim is at the same elevation as the surrounding 

roof surface, the total head is the same as the hydraulic head. 









Table 8c. Hydraulic Head and Corresponding Drain Flow for Secondary (Overflow) Roof Drains (US units) 

Flow Rate 

(GPM) 

Overflow Dam 

8 in. Diameter 

Hydraulic Head (in.) 

Overflow Dam 

12.75 in. Diameter 

Overflow 

Dam 17 in. 

Diameter 

Drain Outlet Size (in.) Drain Outlet Size (in.) Drain 

Outlet Size 

(in.) 

Overflow 

Standpipe 

6 in. 

Diameter 

Drain 

Outlet Size 

(in.) 

Flow Rate 

(GPM) 

3 4 6 6 8 10 4 

50 0.5 0.5 0.5 0.5 0.5 - 1.0 50 

75 1.0 - - - - - - 75 

100 1.5 1.0 1.0 1.0 0.5 1.0 1.5 100 

125 2.0 - - - - - - 125 

150 2.0 1.5 1.5 1.0 - - 2.5 150 

175 3.0 - - - - - - 175 

200 - 2.0 2.0 1.5 1.5 1.5 2.5 200 

225 - - - - - - - 225 

250 - 2.5 2.5 1.5 - - 2.5 250 

300 - 3.0 3.0 2.0 2.0 1.5 3.0 300 

350 - 3.5 3.5 2.5 - - 3.5 350 

400 - 5.5 3.5 3.0 2.5 2.0 - 400 

450 - - 4.0 3.0 - - - 450 

500 - - 5.0 3.5 3.0 2.5 - 500 

550 - - 5.5 4.0 - - - 550 

600 - - 6.0 5.0 3.5 2.5 - 600 

650 - - - - - - - 650 

700 - - - - 3.5 3.0 - 700 

800 - - - - 4.5 3.0 - 800 

900 - - - - 5.0 3.5 - 900 

1000 - - - - 5.5 3.5 - 1000 

1100 - - - - - 4.0 - 1100 

1200 - - - - - 4.5 - 1200 

Notes: 

1) To determine total head, add the height of the dam or standpipe (height above the roof surface) to the hydraulic head listed in this table. 

2) The drain bowl (sump) diameter is the same as for the primary drains of the same drain outlet size (see Table 8a). 

3) Assume that the flow regime is either weir flow or transition flow, except where the Hydraulic Head values are in blue cells below the 








Flow Rate 

(L/min) 

Table 8d. Total Head and Corresponding Drain Flow for Secondary (Overflow) Roof Drains (SI units) 

Overflow Dam 

200 mm Diameter 

Hydraulic Head (mm) above Dam or Standpipe 

Overflow Dam 

325 mm Diameter 

Overflow 

Dam 

430 mm 

Diameter 

Drain Outlet Size (mm) Drain Outlet Size (mm) Drain 

Outlet Size 

(mm) 

Overflow 

Standpipe 

150 mm 

Diameter 

Drain 

Outlet Size 

(mm) 

Flow Rate 

(L/min) 

75 100 150 150 200 250 100 

190 13 13 13 13 13 - 25 190 

285 25 - - - - - - 285 

380 38 25 25 25 13 25 38 380 

475 51 - - - - - - 475 

570 51 38 38 25 - - 64 570 

660 76 - - - - - - 660 

755 - 51 51 38 38 38 64 755 

850 - - - - - - - 850 

945 - 64 64 38 - - 64 945 

1135 - 76 76 51 51 38 76 1135 

1325 - 89 89 64 - - 89 1325 

1515 - 140 89 76 64 51 - 1515 

1705 - - 102 76 - - - 1705 

1895 - - 127 89 76 64 - 1895 

2080 - - 140 102 - - - 2080 

2270 - - 152 127 89 64 - 2270 

2460 - - - - - - - 2460 

2650 - - - - 89 76 - 2650 

3030 - - - - 114 76 - 3030 

3405 - - - - 127 89 - 3405 

3785 - - - - 140 89 - 3785 

4165 - - - - - 102 - 4165 

4540 - - - - - 114 - 4540 

Notes: 

1) To determine total head, add the height of the dam or standpipe (height above the roof surface) to the hydraulic head listed in this table. 

2) The drain bowl (sump) diameter is the same as for the primary drains of the same drain outlet size (see Table 8b). 






2.5.4.1.13 Roof Slope 

2.5.4.1.13.1 Roofs with interior drains: To ensure the points of maximum sag are no lower than the roof 

surface between these points and the drains of roofs with interior drainage provide a positive drainage slope 

of at least 1 ⁄ 4 in./ft (2%). In Figure 9 this is illustrated in the sloped roof detail where ponding occurs locally 

at the origin, whereas in the flat roof detail ponding occurs in every bay. 

If a slope less than 1 ⁄ 4 in./ft (2%) is desired, use deflection analysis to determine the needed slope. If water 

must flow across one bay into another, relatively complicated two-way deflection analysis is involved. The 

recommendations in Section 2.5.4.1.13.2 for roof slope with edge drainage are appropriate. Have the roof 

framing designer prepare calculations according to these recommendations, or other appropriate method, to 

substantiate that the design slope is sufficient to prevent roof instability due to ponding. 




CL 


Note: Overflow provision not shown 

CL 


(deflected) 

Actual Roof Line (deflected) 

Design Roof Line (sloped) 

No Ponding 

(with sufficient 

roof stiffness 

or slope) 

Note: Overflow provision not shown 

CL 

Ponding 

Flat Roof with Interior Drains 

CL 

Interior 

Roof Drain 

Sloped Roof to Interior Drains 

Water Buildup (locally) 

Fig. 9. Flat and sloped roofs with interior roof drains 

Interior 

Roof Drain 

Ponding (locally) 

2.5.4.1.13.2 Roofs with edge drainage: If interior drains are not provided and drainage is accomplished by 

causing the water to flow off the perimeter of the roof, sufficient roof slope is vital; at least 1 ⁄ 4 in./ft (2%). Under 

these circumstances, sufficient slope is needed to overcome the deflections caused by the dead load of the 

roof plus the weight of the 1-hour design storm less the effect of any specified camber. This is achieved when 

the actual downward pitch of the roof surface exceeds the upward slope for all deflected roof framing at or 

near their downward support column (or wall) (see Fig. 10). 

CL 

Exterior Support 

Ponding 

(insufficient roof 

stiffness or slope) 

Design Roof Line (sloped) 


(deflected) 

CL 

Interior Support 

No Ponding 

(sufficient roof 

stiffness or slope) 

Fig. 10. Sloped roof with roof edge drainage 


CL 

CL 

CL 

Typical 

Interior 

Support 

Typical 

Interior 

Support



If a design roof slope (S d) less than 1 ⁄ 4 in./ft (2%) is desired, have the roof framing designer prepare 

calculations according to the following recommendations or other appropriate method, to substantiate that 

the design slope is sufficient to prevent roof instability due to ponding: 

a) Ensure the actual slope (S a) under the dead load of the roof less the upward camber, when specified 

is at least 1 ⁄ 8 in./ft (1%). 

b) Ensure the actual slope (S a), from the perimeter of the roof, under the dead load of the roof plus the 

rain load, less the upward camber, when specified is greater than zero (i.e., upward positive slope, not 

flat). 

c) Ensure all primary and secondary members perpendicular to the roof edge, for the entire roof slope, 

have actual slopes (S a), calculated by the roof designer, meeting the slope criteria of (a) and (b) as follows: 


Sa (%) = Sd (%) + 240 × (Camber) − 

(D.L.) L 

≥ 1% 

L 1.44 × 24 × E × I 

Sa (%) = Sd (%) + 240 × (Camber) − (D.L. + 5.2 × i) L 3 

≥ 0% 

L 1.44 × 24 × E × I 

3 

Where: S a and S d = the actual and design roof slopes in percent, respectively. 

D.L. = the roof’s dead load in psf 

Camber = upward camber in inches when it is specified (not optional) by fabrication specifications 

(see Part e). 

I = rainfall intensity in in./hr 

L = span length of member in inches 

E = modulus of elasticity of members material, psi 

I = effective moment of inertia of member, (in.) 4 per inch of (tributary loaded) roof width 

To convert roof slope (percent) to in./ft multiply percent by 0.12 

Metric Units: 

Sa (%) = Sd (%) + 0.24 × (Camber) − 

L 

3 

(D.L.) L 

24 × E × I 

≥ 1% 

Sa (%) = Sd (%) + 0.24 × (Camber) − (D.L. + 0.01 × i) L 3 

> 0% 

L 24 × E × I 

Where: S a and S d = the actual and design roof slopes in percent, respectively. 

D.L. = Roof’s dead load in kN/m 2 

Camber = upward camber in mm when it is specified not optional by fabrication specifications 

(see Part e). 

I = rainfall intensity, in mm/hr 

L = span length of member in meters 

E = modulus of elasticity of members material, in kN/m 2 

I = effective moment of inertia of member, in (m) 4 per meter of (tributary loaded) roof width 

d) If secondary members are parallel to relatively stiff perimeter walls (e.g., masonry or metal panel walls), 

increase the actual roof slope to compensate for maximum deflection (adjusted for any specified camber) 

of the secondary member closest to the wall. Adjust the actual slope computed in the equations of Part 

c above by a decrease as follows: 

S a Decrease (%) = – 

(Max. Deflection of secondary member) 100 

(Distance secondary member from wall) 

Where: deflection and distance are in the same units (e.g., in. or mm) 




e) The following are cambers specified in the Standard Specifications of the Steel Joist Institute (SJI) 

for LH-Series (Longspan) and DLH-Series (Deep Longspan) Joists and Joist Girders: 

Top Chord Length ft (m) Approximate Camber in. (mm) 

20 (6) 1 ⁄ 4 (6) 

30 (9) 3 ⁄ 8 (10) 

40 (12) 5 ⁄ 8 (16) 

50 (15) 1 (25) 

60 (18) 1 1 ⁄ 2 (38) 

>60 (>18) See SJI Specifications 

Do not assume the above cambers for K-Series (Open Web) Joists because they are optional with the 

manufacturer. 

2.5.4.2 Siphonic Roof Drainage 

2.5.4.2.1 Restrictions 

2.5.4.2.1.1 For roofs with internal drains distributed throughout the roof , do not use siphonic roof drainage 

in hurricane-prone, tropical cyclone-prone, or typhoon-prone regions as defined in FM Global Data Sheet 

1-28. This recommendation does not apply to roofs with siphonic drains located only in eave (perimeter) 

gutters or valley gutters. 

2.5.4.2.1.2 Do not use siphonic roof drainage for roofs that will be prone to debris accumulation - such as 

roofs with nearby or overhanging vegetation where leaves, pine needles, or other vegetation is prone to 

substantially restrict roof drains flows or clog the siphonic piping system. Keep vegetation at least 50 ft (15 m) 

offset horizontally from the roof perimeter and no higher than the elevation of the lowest roof parapet. Ensure 

that a program is in place to control vegetation. 

2.5.4.2.1.3 Do not use siphonic roof drainage for gravel covered or stone ballasted roof, or for vegetated 

(green) roofs. 

2.5.4.2.2 Design Rainfall Intensity, Duration, and Frequency 

Rainfall intensity (i) is the rate that rainfall accumulates over time, is frequently expressed in inches or 

millimeters per hour (in/hr or mm/hr), and is a function of both duration (minutes or hours) and frequency 

(or return period, in years) for a given location and climate. 

For example, if the 100-year 2-minute rainfall is 10-inches (254 mm) per hour, then: 

Intensity (i) = 10 in./hr (254 mm/hr) 

Duration (D) = 2 minutes 

Frequency (F) = 100 years 

2.5.4.2.2.1 Determine the flow rate (Q [gpm or liter/min]) needed per roof drain, leader or scupper in the 

same manner as for gravity roof drainage, but with any adjustments as noted to the rainfall intensity (i). 

2.5.4.2.3 Acceptable Drainage Designs and Design Assumptions 

2.5.4.2.3.1 Acceptable Design Options 

Note that the recommendations in Section 2.5.4.2.3.2, General Design Assumptions and Requirements, apply 

to all acceptable design options. 

Option 1: 

Option 2: 

• Primary siphonic drainage designed for the 2-year 5-min rainfall intensity. 

• Secondary conventional (non-siphonic) drainage designed for the 100-year 15-min rainfall intensity, 

with primary drainage completely blocked. 

• Primary siphonic drainage designed for the 100-year 5-min rainfall intensity. 




• Secondary siphonic drainage designed for the 100-year 2-min rainfall intensity, with primary drainage 

completely blocked. 

• Roof structure designed to adequately support the 100-year 24-hour rainfall depth, unless depth 

reductions are appropriate where rainwater can freely overflow the roof area by gravity alone (e.g., 

at roof perimeter without parapets). This rain load may be considered an extreme design load, and 

therefore the use of a lower than normal rain load factor (partial safety factor), or net safety factor, 

is appropriate, provided that the resultant safety factor against structural material yielding or 

fracture/crushing (whichever occurs at the lower load) is not less than 1.25 when considering total 

load. Include ponding instability analysis under these conditions. 

2.5.4.2.3.2 General Design Assumptions and Requirements 

2.5.4.2.3.2.1 The design life of the drainage systems should not be less than the design life of the building, 

nor less than 50 years. 

2.5.4.2.3.2.2 Primary and secondary drainage must be completely independent systems for all acceptable 

roof drainage options. 

2.5.4.2.3.2.3 For secondary gravity drainage and scupper details (minimum sizes, inlet elevations) – follow 

the recommendations in Section 2.5.4.1 (the conventional drainage section) of DS 1-54. 

2.5.4.2.3.2.4 The siphonic drainage system must be designed to operate properly at all flow rates and rainfall 

intensities, up the maximum design flow rate and rainfall intensity. Ensure that the water depths on the roof 

or in the roof gutters will not exceed depths occurring at the maximum design flow rate and rainfall intensity. 

2.5.4.2.3.2.5 All secondary siphonic drainage systems should be designed to operate properly at the design 

rainfall intensity based on the following assumptions: 

a) All secondary siphonic roof drains are operating as designed (no clogging or blinding). 

b) At least one secondary siphonic drain per roof, but not less than 5% of the total secondary drains on 

a roof, are completely clogged or blinded — with the blocked or blinded secondary drains arranged to 

place the most demand on the roof drainage and roof structure. 

2.5.4.2.3.2.6 Do not credit any temporary storage of water on roofs or in gutters for the siphonic drainage 

design. 

2.5.4.2.3.3 Roof Load 

Arrange the secondary drain high enough above the primary drain so that water will reach a sufficient depth 

to ensure the primary drainage system operates properly but not so high that water reaches a depth that 

will overload the roof structure. 

2.5.4.2.4 Roof Slope, Positive Drainage, and Stability against Ponding 

Follow the recommendations for gravity drainage in Section 2.5.4.1 except as noted in Section 2.5.4.2.1 

(Restrictions in Hurricane-Prone Locations). 

2.5.4.2.5 Roof Drains 

2.5.4.2.5.1 Quantity (minimum number of drains per roof area): Follow the recommendations for gravity 

drainage in Section 2.5.4.1. 

2.5.4.2.5.2 Drain strainers (debris guards): Provide domed drain strainers extending at least 4-inches (100 

mm) above the roof surface for all siphonic roof drains, including those placed in roof gutters. Ensure that the 

open area of the strainer is at least three-times (3x) the cross-sectional area of the drain outlet or tailpipe, 

whichever is larger. Ensure that the hydraulic performance properties for the roof drain account for the 

presence of the drain strainers. 

2.5.4.2.5.3 Drain baffle (anti-vortex plate): All siphonic drains must have a baffle to prevent air entrainment 

into the siphonic system and allow for full-bore siphonic flow. Ensure that the baffle is clearly and permanently 

marked with a warning not to remove the baffle. 

2.5.4.2.5.4 Sump bowl or drainage basin: Roof drains on flat and low sloped roof (2% slope or less) should 

have a sump bowl or drainage basin to allow for siphonic flow while minimizing water depth on the roof. 




2.5.4.2.5.5 Provide roof drains with the manufacturer name and model number, drain outlet size (in 2 [mm 2 ]), 

and hydraulic resistance coefficient (e.g., “K” value), clearly and permanently marked on the drain body where 

it will be legible in its installed condition to an observer on the roof. 

2.5.4.2.6 Design Validation 

2.5.4.2.6.1 Siphonic design and analysis must be performed by a plumbing engineer licensed to practice in 

the project location. The design calculations, including computerized calculations and results, must be signed 

and stamped by the licensed plumbing engineer. 

2.5.4.2.6.2 The hydraulic properties and performance of manufactured roof drains used in the siphonic system 

must be based on physical test results from a testing program established in a nationally-recognized standard 

(such as ASME A112.6.9) and tested by a laboratory that has been verified to be qualified to perform the 

testing. Using roof drains with hydraulic performance based on calculation alone — or based on calculated 

hydraulic performance taken from test results of a different, albeit similar, roof drains — is not acceptable. 

2.5.4.2.7 Disposable (Available) Head 

2.5.4.2.7.1 Use the Design Disposable Head (H D), also known as the Design Available Head, as the vertical 

distance in ft (m) from the inlet (rim) of the roof drain to highest elevation (i.e., least vertical distance) of: 

a) Grade elevation at discharge inspection chamber(s) or manhole(s) 

b) Flood elevation 

c) Elevation of siphonic break (for discharge above grade) 

Refer to Figure 12, Elevation View of Siphonic System and Disposable (Available) Head. 

2.5.4.2.7.2 Use the Theoretical Disposable Head (H T) as the vertical distance in ft (m) from the water level 

directly upstream of the roof drain to the centerline of siphonic discharge pipe at or below grade. 

2.5.4.2.7.3 Ensure that either H T or H D, whichever provides for the more demanding condition, has been 

used when determining the properties or performance of the siphonic drainage system. For example, when 

determining if the siphonic drainage system has adequate capacity to drain the roof based on the design 

rainfall intensity (i), or to determine the maximum depth of water build-up on the roof based on the design 

rainfall intensity, use H D. However, when determining the minimum pressure or maximum velocity to compare 

to allowable values, use H T. 

2.5.4.2.7.4 Ensure that H D is at least 10% greater than the sum of the residual velocity head (at the last 

section of siphonic pipe just before the point of discharge) and the head losses. 

2.5.4.2.7.5 Ensure the calculated head losses account for pipe roughness values for both new and aged 

conditions, and that those roughness values which results in the most demanding condition are used. 

2.5.4.2.7.6 Ensure the designer’s calculated imbalance in head at the design flow rate between any two roof 

drains with a common downpipe (stack) and the point of discharge is not greater than 1.5 ft (0.46 m), or 

10% of H D, whichever is less. 

2.5.4.2.8 Minimum and Maximum Pressure 

2.5.4.2.8.1 Operating Pressure 

2.5.4.2.8.1.1 The operating pressure (gage) should not exceed 13 psig (90 kPa), or 30 ft (9.2 m) of water 

column head pressure. 

2.5.4.2.8.1.2 The operating pressure (gage) should be no less than: 

(3.5 psi [24.2 kPa]) – (local atmospheric pressure [P atm] accounting for site elevation) 

For example: 

a) At sea level P atm = 14.7 psi (101.6 kPa), therefore the minimum operating gage pressure is: 

3.5 psi (24.2 kPa) – 14.7 psi (101.6 kPa) = -11.2 psig (-77.4 kPa) 

b) At 3000 ft (915 m) above sea level, P atm = 13.2 psi (91.1 kPa), therefore the minimum operational gage 

pressure is: 

(3.5 psi [24.2 kPa]) – 13.2 psi (91.1 kPa) = -9.7 psig [-67.0 kPa] 

2.5.4.2.8.1.3 Minimum operating pressures are intended to prevent cavitation, air infiltration at pipe fittings 

and joints, and pipe overload (buckling or collapsing). 




2.5.4.2.8.1.4 See the recommendation for maximum velocity which relates to the minimum operational 

pressure limits above. 

2.5.4.2.8.2 Rated Pressure for Air Infiltration 

Pipe joints and fitting shall be rated to prevent air infiltration when subjected to negative (gage) pressure of 

-12.3 psig (-85.0 kPa) for 1-hour. 

2.5.4.2.9 Minimum and Maximum Velocity 

2.5.4.2.9.1 At the design flow rate, ensure that the velocity in tailpipes and horizontal collector pipes is at 

least 3.3 ft/sec (1 m/sec). 

2.5.4.2.9.2 The maximum velocity in the siphonic system should be based on maintaining allowable minimum 

pressures (maximum negative gage pressures) in the system. Generally, the velocity at design flow should 

not exceed 20 ft/sec (6 m/s) for the given minimum operational pressure. 

2.5.4.2.9.3 In order to ensure the siphonic action is broken where the siphonic drain system discharges (to 

either the underground storm drain system or the above-ground drainage system), provide an increaser and 

larger discharge pipe at least 10 pipe diameters in length. Base the larger discharge pipe diameter on 

adequate flow capacity when assuming open-channel flow. 

2.5.4.2.10 Priming 

The siphonic drainage system should be designed to prime (i.e., to begin full-bore siphonic flow) at not more 

than 1 ⁄ 2 the duration associated with the design rainfall intensity. A reasonable estimation can be made by 

determining the time required to fill the siphonic system based on the following equation: 

T f = 1.2(V p)(q t) ≤ 60 seconds 

Where: 

T f = time to fill the system (seconds) 

q t = the flow capacity (cfs or liter/sec) of all the contributing tailpipes when assumed to be acting siphonically, 

but also independently, and discharging to atmospheric pressure at the collector pipe. 

V p = the volume (cubic feet or liters) of the downpipe (to the point of theoretical siphonic discharge) and 

the contributing collector pipes. 

2.5.4.2.11 Siphonic Discharge 

2.5.4.2.11.1 Discharge the siphonic drainage system to the open atmosphere – either to a below-grade 

inspection chamber (manhole), or to an above grade trench or swale – to break the siphonic action. 

2.5.4.2.11.2 Below-Grade Inspection Chamber (Manhole): Provide a vented cover for the chamber (manhole) 

that is at least 50% open area, or where the total open area of the vented cover is not less than three times 

(3x) the cross-sectional area of the siphonic discharge pipe, whichever is greater. 

2.5.4.2.11.3 Keep the manhole cover clear or snow, ice and debris. Provide bollards or similar protective 

devices to keep materials, vehicles, etc from blocking the manhole cover. 

2.5.4.2.11.4 Avoid the use of vermin guards on discharge pipes since they could collect debris and block 

proper siphonic flow. The preferred alternative is to conduct frequent visual inspections to ensure that 

discharge pipes remain free of debris. See the Inspection and Maintenance section for additional details. 

2.5.4.2.12 Pipe Strength, Details and Materials 

2.5.4.2.12.1 Piping and Fittings – General 

2.5.4.2.12.1.1 Use metal pipe — such as cast iron, galvanized steel, stainless steel, or copper — rather than 

plastic pipe for better long-term durability and performance. Ensure pipe and associated joints, fittings, 

couplings, etc. meet or exceed nationally recognized plumbing materials standards such as those by ASTM, 

CSA, BS, or DIN. 

2.5.4.2.12.1.2 If plastic pipe cannot be avoided, then use Schedule 40 pipe or better (or an SI equivalent, 

based on the minimum ratio of pipe wall thickness to mean pipe diameter – See Table 10) for all piping 

components of the siphonic drain system. Ensure that plastic pipe (such as ABS, HDPE, or PVC) meets or 

exceeds applicable, nationally recognized plumbing materials standards such as those by ASTM, CSA, BS, 

or DIN. 




2.5.4.2.12.1.3 If plastic pipe is used, then take care to address the issues associated with thermal expansion, 

expansion joints, and pipe supports and restraints – and provide the necessary detailing to prevent damage. 

2.5.4.2.12.2 Expansion Joints 

Avoid the use of expansion joints in siphonic systems wherever possible since proper connection detailing 

and adequate long-term performance can be difficult to achieve. If the use of expansion joints cannot be 

avoided, then ensure that all of the following are met: 

a) Thermal expansion and contraction are based on temperature extremes associated with the local 

climate, building type, and location of building expansion joints; and 

b) Expansion joints are rated, with a minimum safety factor not less than 3.0, for both the maximum and 

minimum siphonic piping operating pressures, and with a critical buckling strength no less than that 

required of the adjacent siphonic piping; and 

c) The expansion joint and connections have smooth inner bores to prevent the accumulation of debris/ 

sediment and to avoid cavitation. 

2.5.4.2.12.3 Tailpipe 

To initiate adequately rapid priming, maintain full-bore siphonic flow, and reduce the likelihood of cavitation, 

ensure that: 

a) The diameter of the tailpipe is not greater than the diameter of the roof drain outlet. 

b) Pipe increasers are used only in the horizontal portion of the tailpipe, not in the vertical portion; and 

that only eccentric (not concentric) increasers are used with the crown (top) of the pipes set flush and the 

maximum offset at the pipe invert. 

c) 90-degree bends are used where transitioning from the vertical to the horizontal portion of the tailpipe 

(45-degree bends, or substantial slopes in the horizontal portion of the tailpipe, are not acceptable). 

2.5.4.2.12.4 Horizontal Collector Pipe 

Ensure all reducers or increasers are eccentric (not concentric) with the crown (top) of the pipe set flush 

and the offset at the pipe invert. 

2.5.4.2.12.5 Downpipe (Stack) 

2.5.4.2.12.5.1 Ensure the downpipe diameter is no greater than the diameter of the horizontal collector pipe. 

2.5.4.2.12.5.2 At the top of the downstack, where the collector pipe connection is made, use either two (2) 

45-degree bends, or a 90-degree bend with a minimum centerline bend radius equal to the pipe diameter. 

2.5.4.2.12.5.3 If a reducer is used just after an elbow, use an eccentric reducer with the pipes set flush at the 

outside radius of the elbow. 

2.5.4.2.12.6 Minimum Pipe Size 

Use pipe with an inside diameter of at least 1.6 in. (40 mm). 

2.5.4.2.12.7 Critical Buckling Strength of Pipe (P crit) 

All pipe sections used in siphonic systems must withstand a resultant (net) critical buckling pressure of at 

least three atmospheres based on all of the following conditions: 

a) Standard atmospheric pressure at sea level 

b) An assumed minimum out-of-roundness (maximum diameter – minimum diameter), or ovality, of one-half 

the pipe wall thickness 

c) Buckling strength based on the creep modulus of elasticity (E c) 

d) Assumed operating temperatures from 40°F (4°C) to 90°F (32°C)* 

That is, ensure P crit ≥ 3 atm (44.1 psi [304.8 kPa]) when considering the conditions listed above. 

*For piping that will be heat traced, ensure the pipe temperature will be within these assumed operating 

temperatures. 




2.5.4.2.12.8 Pipe Supports and Bracing 

2.5.4.2.12.8.1 Provide pipe supports and bracing as needed based on engineering analysis when accounting 

for all applicable conditions – including, but not limited to: Gravity loads, deflections, and material creep; 

siphonic pipe pressures; operational vibrations and fatigue; thermal expansion/contraction; and seismic 

bracing when located in an active earthquake zone based on FM Global Data Sheet 1-2, Earthquakes (50-, 

100-, 250-, and 500-year zones). 

2.5.4.2.12.8.2 For plastic pipe, provide pipe supports as described in the previous paragraph, but also ensure 

that the pipe supports and bracing conform to the following minimum requirements: 

a) Provide pipe supports every 4 ft (1.2 m) or less 

b) Provide pipe supports at every change in direction (e.g., at pipe elbows) 

c) Provide lateral bracing at every 30 ft (9.1 m) or less 

d) Provide lateral bracing at every change in pipe direction 

2.5.4.2.13 Icing, Freeze-up, and Impact and Environmental Damage 

2.5.4.2.13.1 Keep roof drains free of ice and snow. 

2.5.4.2.13.2 Use heat tracing on siphonic drain bodies and outlets exposed to freezing temperatures. 

2.5.4.2.13.3 Where siphonic drainage piping is used in an unheated building or is installed at the exterior 

of a building (e.g., downpipe attached to the building façade), and is exposed to freezing temperature, install 

heat tracing at all the exposed piping. 

2.5.4.2.13.4 Use only noncombustible metallic materials for drain components that are to be heat traced. 

2.5.4.2.13.5 Position above-grade secondary discharge pipes above the maximum expected snow level 

(including drift) and take special precaution to protect them from crushing or impact (such as protective 

bollards) when exposed to car parks or storage areas. 

2.5.4.2.13.6 Protect exposed siphonic drainage piping from ultra-violet radiation and other environmental 

sources of degradation. 

2.5.4.2.14 Testing and Handover 

2.5.4.2.14.1 The siphonic system must be verified as being clean and free of debris. Since it is very difficult 

to perform an in-situ operational test of the siphonic system, other means such as video verification can be 

used to ensure that the system is not clogged and will operate as designed. 

2.5.4.2.14.2 Verify that all roof drains have baffles (anti-vortex plates) and securely attached debris guards. 

2.5.4.2.14.3 Pressure test the siphonic system to 50% greater than the maximum pressure at design 

conditions but not less than 13 psig (89.9 kPa) or 30 ft (9.0 m) water column. Ensure the system holds the 

test pressure of at least 1-hour. 

2.5.4.2.15 Inspection and Maintenance 

Ensure facilities personnel visually inspect the roof drains and discharge pipes at least once every 3 months 

and keep a written log of the inspections. Inspect each roof drain to ensure that the debris guard and baffle 

plate is intact and that there is no debris clogging the opening around the baffle plate. Inspect each discharge 

pipe to ensure that the pipe is free of debris. Facilities personnel should remove any scattered debris on the 

roof that could clog or otherwise degrade the performance of the roof drains. 




Tail 

pipe 

Siphonic roof 

drain (typ.) 


parapet 

Siphonic roof 

drain 

Horizontal 

collector 

pipe 

Downpipe 

or Stack 

Horizontal 

collector 

pipe 

Tail 

pipe 

Fig. 11. Diagram of siphonic roof drain system 

Theoretical Disposable Head (H ) 

T 

Secondary overflow 

drainage (scupper) 

Design Disposable Head (H ) 

D 

Downpipe 

or Stack 

Water level directly 

upstream of roof drain 

Lip or rim of 

roof drain 

Vented 

cover 

Fig. 12. Elevation view of siphonic system and disposable (available) head 

Gravity storm 

drainage pipe 

Grade (or Flood) level 

Manhole or 

inspection chamber 

Gravity storm sewer 




Fig. 13. Siphonic roof drain (photo courtesy of Jay R. Smith Mfg. Co.) 

Fig. 14. Siphonic roof drain for gutters (without dome strainer or debris guard) 

(photo courtesy of Jay R. Smith Mfg. Co.) 




Table 9. Rainfall Intensity Conversion Rates 

Rainfall Intensity (i) 

in./hr mm/min L/sec-m 2 

1.4 0.6 0.01 

2.1 0.9 0.015 

2.8 1.2 0.02 

3.5 1.5 0.025 

4.3 1.8 0.03 

5.0 2.1 0.035 

5.7 2.4 0.04 

6.4 2.7 0.045 

7.1 3.0 0.05 

7.8 3.3 0.055 

8.5 3.6 0.06 

9.2 3.9 0.065 

9.9 4.2 0.07 

10.6 4.5 0.075 

11.3 4.8 0.08 

Note: (L/sec-m 2 ) x 141.7 = in./hr 

(mm/min) x 2.362 = in./hr 

where L = liter 

Table 10. Schedule 40 Pipe Dimensions and Geometric Properties 

Schedule 40 (Standard Weight) Pipe 

Nominal Size Wall Thickness (t) Inside Diameter Mean Diameter (DM) t/DM (t/DM) (DI) 3 

x1000 

(inch) (mm) (inch) (mm) (inch) (mm) (inch) (mm) 

1.5 38 0.145 3.7 1.610 40.9 1.755 44.6 0.0826 0.5640 

2 51 0.154 3.9 2.067 52.5 2.221 56.4 0.0693 0.3334 

2.5 64 0.203 5.2 2.469 62.7 2.672 67.9 0.0760 0.4385 

3 76 0.216 5.5 3.068 77.9 3.284 83.4 0.0658 0.2845 

3.5 89 0.226 5.7 3.548 90.1 3.774 95.9 0.0599 0.2147 

4 102 0.237 6.0 4.026 102.3 4.263 108.3 0.0556 0.1718 

5 127 0.258 6.6 5.047 128.2 5.305 134.7 0.0486 0.1150 

6 152 0.28 7.1 6.065 154.1 6.345 161.2 0.0441 0.0859 

8 203 0.322 8.2 7.981 202.7 8.303 210.9 0.0388 0.0583 

10 254 0.365 9.3 10.020 254.5 10.385 263.8 0.0351 0.0434 

12 305 0.375 9.5 12.000 304.8 12.375 314.3 0.0303 0.0278 

Nominal Size Cross Sectional (Open) Area 

(inch) (mm) (in 2 ) (ft 2 ) (mm 2 ) (m 2 ) 

1.5 38 2.03 0.0141 1313 0.0013 

2 51 3.35 0.0233 2164 0.0022 

2.5 64 4.79 0.0332 3087 0.0031 

3 76 7.39 0.0513 4767 0.0048 

3.5 89 9.88 0.0686 6375 0.0064 

4 102 12.72 0.0884 8209 0.0082 

5 127 20.00 0.1389 12900 0.0129 

6 152 28.88 0.2005 18629 0.0186 

8 203 50.00 0.3472 32259 0.0323 

10 254 78.81 0.5473 50848 0.0508 

12 305 113.04 .7850 72929 0.0729 




Elevation Above 

Sea Level 

Table 11. Standard Atmospheric Pressure at Various Elevations 

Pressure 

Head* Pressure 

(ft) (m) (ft) (m) (psi) (kPa) (atm) 

0 0 34.0 10.37 14.7 101.6 1.00 

1500 457 32.2 9.82 14.0 96.2 0.95 

3000 915 30.5 9.30 13.2 91.1 0.90 

4500 1372 28.8 8.78 12.5 86.0 0.85 

6000 1829 27.2 8.29 11.8 81.3 0.80 

7500 2287 25.7 7.84 11.1 76.8 0.76 

*Pressure head is feet (ft) or meter (m) of water column, with an assumed water density of 62.4 Lb/ft 3 (999.6 kg/m 3 ). 

Linear interpolation is appropriate. 

2.6 Other Roof Loads and Roof Overloading 

2.6.1 Reinforce existing roofs that are overloaded and subject to collapse from snow loading. Where 

reinforcing is impractical, use snow removal teams (as part of the emergency response team) to remove 

excessive snow. Determine the safe maximum snow depth for the roof areas. Have snow removal teams clear 

snow from the roofs when one-half of the safe maximum snow depth is reached. 

2.6.2 Evaluate and analyze existing roofs that have roof-mounted or roof-suspended equipment and 

structures added or modified. Include the supporting roof framing, columns and bearing walls in the analysis. 

Ensure the analysis and design of any needed reinforcing is performed by a qualified structural engineer. 

2.6.3 Design suspended or otherwise supported ceilings that allow access for maintenance workers for 

appropriate concentrated and uniform live loads based on the anticipated maintenance work. 

2.6.4 Indirect roof overloading: The overloading and collapse of the primary vertical support elements of the 

roof structure, such as columns and bearing walls, is another cause of roof collapse. 

Columns adjacent to traffic aisles for fork-lifts and other trucks are vulnerable to upset if not adequately 

protected from impact. Ensure the base plates of these columns are anchored to their foundations with a 

minimum of four (4) 1 in. (25 mm) diameter anchor bolts, and protected with concrete curbing, steel guard 

rails, or concrete-filled pipe bollards to resist and/or prevent impact loads from vehicles. 

Ensure walls, particularly masonry walls, are not be laterally loaded as a result of having bulk materials (e.g., 

sand, salt, grain) or rolled products (e.g., carpets or paper) placed against them, unless the wall and roof 

structure are designed to resist the resulting lateral loads. Likewise, ensure rack storage structures or vertical 

stays for confining rolled products in storage are not secured to the roof-framing system unless the framing 

and bracing systems are designed to resist the resulting laterally-induced loads. 

2.7 Use of Eurocode 

For use by European Committee for Standardization (CEN) member nations that have adopted, and comply 

with, the Eurocode as the national standard. 

2.7.1 Eurocode for Snow Loads 

Eurocode 1 (Eurocode 1, Actions on Structures, Part 1-3: General Actions — Snow Loads [EN 1991-1-3: 

2003]) may be used in CEN (European Committee for Standardization) member nations for snow load 

determination where it has been approved as the national standard, provided the following recommendations 

are adhered to. 

2.7.1.1 Snow Density 

a) For locations where the 50-year ground snow load is greater than 1.8 kN/m 2 (38 psf): 

1. In Equation 5.8, Section 5.3.6, Roof Abutting and Close to Taller Construction Works (leeward drifts 

on lower roofs) of Eurocode 1, use an upper limit bulk weight snow density of no less than 3 kN/m 3 (18.9 

lb/ft 3 ). 

2. In Equation 6.1, Section 6.2, Drifting at Projections and Obstructions (windward drift at projection or 

parapet) of Eurocode 1, use an upper limit bulk weight snow density of no less than 3 kN/m 3 (18.9 lb/ft 3 ). 




b) For locations where the 50-year ground snow load is less than or equal to 1.8 kN/m 2 (38 psf), use an 

upper limit bulk weight snow density of not less than 2 kN/m 3 (12.6 lb/ft 3 ) for windward and leeward snow 

drifts as recommended in Eurocode 1 (see Sections 5.3.6 and 6.2 of Eurocode 1). 

2.7.1.2 Hip and Gable (Pitched) Roofs 

For hip or gable sloped roofs (pitched roofs) of lightweight construction (metal roof, insulated steel deck, 

boards-on-joists, plywood diaphragm, and similar constructions) with slopes greater than or equal to 5 

degrees or slopes less than 60 degrees (5 ≤ Θ



a. For eave gutters, use a risk factor of at least 1.5. 

b. For all other cases, use a risk factor of at least 3.0. 

2.7.3.2 Effective Rainfall Intensity 

2.7.3.2.1 Ensure the primary drainage system is adequate for the 100-year 60-min rainfall intensity assuming 

that the secondary drainage system is completely blocked. 

2.7.3.2.2 Ensure the secondary drainage system is adequate for the 100-year 60-min rainfall intensity 

assuming that the primary drainage system is completely blocked. 

2.7.3.2.3 Ensure the secondary drainage system capacity is not less than the primary drainage system 

capacity. 

2.7.3.3 Do not use the following sections from EN 12056-3: 

6.2 Siphonic Systems 

6.3 Drains 

6.4 Connection to Sanitary Pipework 

7 Layout 

2.7.3.4 Follow the recommendations in Sections 2.5.2 through Section 2.5.4.1.13 of this data sheet, which 

include, but are not limited to, independence of primary and secondary drainage systems, minimum design 

rain depths, ponding instability requirements, roof slope requirements, minimum roof drain quantities 

(maximum roof drainage area per drain), drain placement, minimum drain sizes, drain strainers (debris 

guards), inlet elevations of secondary drains or scuppers relative to primary drain inlet elevations, and 

downspout recommendations (height above snow level, impact protection, freeze-up protection). 

2.7.3.5 Design Rainfall Intensities for Several Nations 

Germany: Use DIN EN 12056 and DIN 1986-100, including their rain intensity maps and tables, to determine 

the rainwater runoff for the primary and secondary drainage systems. These require independent secondary 

drainage systems for all flat roofs and roofs with internal drains with discharge to a free unobstructed 

location. 

United Kingdom: Use BS EN 12056-3:2000, including their rainfall intensity maps, to determine rainwater 

runoff for the primary and secondary drainage systems. 

France, Netherlands, and Switzerland: Use EN 12056-3:2000 with applicable rainfall intensity maps subject 

to the following minimum rainwater intensities noted in EN 12056-3:2000, Annex B, to determine the rainwater 

runoff for the primary and secondary drainage systems. Minimum intensities: France – 0.05 L/s/m 2 ; 

Netherlands and Switzerland – 0.03 L/s/m 2 . 

2.7.4 Different Partial Safety Factors for CEN Member Nations 

Use partial safety factors for loads (load factors), partial safety factors for materials (material factor), and 

design load combinations as prescribed in the Eurocode for all CEN member nations. Alternatively, different 

partial safety factors may be used if the Total Factored Demand for design can be verified to be no less than 

that which would result from using Eurocode partial safety factors for all failure modes. This comparison will 

only be practical when the all loads are uniform (e.g., for uniform snow load or roof live load, but not for drifting 

snow loads). 

Total Factored Demand = (Factored Loads) x (Partial Safety for Materials) 

Example: 

Let: 

Characteristic (Unfactored) Dead Load = 0.5 kN/m 2 

Characteristic (Unfactored) Uniform Snow Load = 2.0 kN/m 2 

From Eurocode: Dead load factor = 1.35 

Snow load factor = 1.5 

Material factor = 1.1 (steel beam) 




Total Factored Demand = (1.35(Dead Load) + 1.5(Snow Load))(1.1) 

= (1.35(0.5 kN/m 2 ) + 1.5(2.0 kN/m 2 ))(1.1) 

= 4.0 kN/m 2 

From a CEN National Annex, let: 

Dead load factor = 1.2 

Snow load factor = 1.6 

Material factor = 1.1 

Total Factored Demand = (1.2(Dead Load) + 1.6(Snow Load))(1.1) 

= (1.2(0.5 kN/m 2 ) + 1.6(2.0 kN/m 2 ))(1.1) 

= 4.2 kN/m 2 

Since 4.2 kN/m 2 ≥ 4.0 kN/m 2 , the use of the National Annex load factors and material factors is acceptable. 

2.8 Use of ASCE 7 for Snow Loads 

The provisions in Chapter 7 of ASCE 7-05 or ASCE 7-10 (ASCE/SEI 7-05, or 7-10, Minimum Design Loads 

for Buildings and Other Structures) may be used for the determination of snow loads provided the following 

recommendations are adhered to: 

2.8.1 Factors 

Importance factor (I) not less than 1.1 

Exposure Factor (C e) not less than 1.0 

Thermal Factor (C t) not less than 1.3 for structures intentionally kept below freezing; 1.2 for unheated 

structures; and not less than 1.1 for other buildings. 

2.8.2 Hip and Gable Roofs 

For unbalanced snow load on hip and gable roofs, use the provisions of this data sheet (see Section 2.3.9). 

2.9 Plan Review and Submissions 

2.9.1 General 

During initial design of buildings insured by FM Global, have the building designer submit the following 

information to the appropriate FM Global Operations office for confirmation that the design loads and drainage 

of each roof are in accordance with the recommendations in this data sheet (if the design does not follow 

the guidelines of this data sheet, proposed exceptions should be identified and compared): 

a) Roof framing and drainage system plans, sections and details 

b) The applicable building and plumbing codes and/or standards 

c) Identification, where a minimum roof live load of 20 psf (1.0 kN/m 2 ) governs, of reductions taken in 

the minimum roof live load for any primary or secondary members and their respective design dead and 

live loads 

d) The ground snow load, the mean recurrence interval (MRI), snow density, and the source, if different 

from the recommendations in this data sheet 

e) The balanced, unbalanced, drift, and sliding surcharge snow loads and drift length, and rain-on-snow 

surcharge loads as appropriate for the roof configurations, showing loading diagrams and denoting any 

differences from the recommendations in this data sheet. Focus on areas such as low roofs at roof steps, 

and roof projections, where substantial snow drifting can occur; verify that more substantial roof 

construction is provided at these areas. 

f) All roof drainage (both conventional [non-siphonic] and siphonic): 

• The rainfall intensity for the recommended duration (e.g., 60-minute, 15-minute, or 2-minute), 

frequency (mean recurrence interval [MRI], for example 100-year or 2-year), and the source, if 

different from the recommendations in this data sheet 

• Primary drains and/or scuppers: type, size, maximum drainage area and flow rate, roof surface slope 

to drainage point or dead-flat, and whether drains are located at mid-bay 




• Overflow drainage provisions: whether over the roof edge, or overflow scuppers or drains; type, size, 

maximum drainage area and flow rate for scuppers and drains; height to roof edge, invert (scuppers) 

or inlet (drains) from the (adjacent to) design roof line; and roof surface slope to overflow point or 

dead-flat 

• For conventional (non-siphonic) systems, maximum hydraulic head and total head for primary and 

overflow drains and scuppers; hydraulic head versus discharge rates for specific drains or scuppers 

to be used 

• Maximum design rain load for dead-flat roofs and for the low points of sloped roofs 

• Analysis method for dead-flat roofs and source used to substantiate that the roof is stable based 

on the design rain load and ponding recommendations in this data sheet. 

• Roof slope for roofs with drainage over the edge or sloped to drains or scuppers. If the slope is less 

than 1 ⁄ 4 in./ft (2%), substantiate with calculations that the design slope is sufficient based on Sections 

2.5.2.4, 2.5.3, and 2.5.4.1.13. 

g) Siphonic drainage: 

• Siphonic drainage calculations and construction documents stamped and signed by a licensed 

professional engineer. 

• Verification that the hydraulic properties and performance of the manufactured roof drains have been 

determined based on physical test results, from a qualified testing laboratory, in conformance with 

a nationally recognized standard (such as ASME A112.6.9). 

• Verification that the primary and secondary drainage systems are independent of each other. 

• Verification that all piping has adequate critical buckling strength for the full range of assumed 

operating temperatures. 

• Verification that expansion joints (if used) are properly detailed, are rated with an adequate safety 

factor, and have adequate critical buckling strength for the full range of assumed operating 

temperatures. 

• Verification that the design disposable head (H D) is at least 10% greater than the sum of the residual 

velocity head and the head losses. 

• Verification that the minimum and maximum operating pressures are in accordance with the 

recommended ranges. 

• Verification that adequate priming is provided, as indicated by the calculated time to fill (T f). 

• Ensure H D is appropriate based on the elevations differences between the roof height and discharge 

points. 

• Ensure the roof drain strainers (debris guards), and vented inspection chamber covers, have 

adequate open areas. 

• Ensure the minimum pipe size (inside diameter) is not less than of 1.6 in. (40 mm). 

• Ensure adequate pipe supports and bracing are provided. 

2.9.2 Other Codes and Standards 

When submitting a project that is in conformance with Section 2.1 of this data sheet, in addition to the 

recommendations in Sections 2.7 and 2.8, include the following: 

a) Edition/version of code and standard, including the date (for example, the 2005 edition of ASCE 7 [ASCE 

7-05]); the load classification (e.g., Variable Action for Persistent/Transient design for Eurocode); the loads 

factors; and applicable coefficients (e.g., C e, C t, and I). This information will typically be located on the 

general notes or structural notes sheet(s) of the construction drawings. 

b) For Eurocode-specific projects, verify the minimum roof dead load of 1.5 kN/m 2 (31 psf) if a reduced 

roof live (imposed) load is used per Section 2.7.2.1.a. 




3.0 SUPPORT FOR RECOMMENDATIONS 

3.1 General 

3.1.1 Use of Other Codes and Standards 

3.1.1.1 Eurocode 

The thirty CEN (European Committee for Standardization) member nations include the following (as of late 

2007): Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, 

Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, 

Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, United Kingdom. Consult the National 

Annex for additional guidelines and provisions. 

3.1.1.1.1 Snow Loads 

For the snow load provisions, refer to Eurocode 1 – Actions on Structures – Part 1-3: General Actions – Snow 

Loads (EN 1991-1-3). EN 1991-1-3 uses 50-year ground snow loads, and a recommended minimum snow 

density of 2 kN/m 3 (12.6 pcf) 

3.1.1.1.1.1 Return Period for Ground Snow Loads 

Eurocode 1 ground snow maps are based on a return period of 50 years. However, there may be a 

country-specific code or annex that uses a lesser return period; in these cases, an appropriate factor should 

be applied to obtain equivalent 50-year ground snow loads (see Section 2.3.3). 

3.1.1.1.1.2 Design Situations and Load Combinations 

Eurocode 1 (and Eurocode 0, Basis of Structural Design) allow for several types of Design Situation 

classifications for load combinations that include snow loads. The recommendation to consider 50-year snow 

and snow-drift loads to be characteristic values of Variable Actions for use in Persistent/Transient design 

situations, not Accidental or Exceptional design situations, will provide an acceptable design condition. 

3.1.1.1.1.3 Exposure and Thermal Coefficients 

The use of exposure coefficient (C e) and thermal coefficient (C t) not less than 1.0 (the recommended minimum 

values in Eurocode 1) should adequately address most reasonable unexpected design conditions; for 

example, where power and heating are lost in a building, or where a building becomes more sheltered due 

to future adjacent development. 

3.1.1.2 ASCE 7 (ASCE/SEI 7, Minimum Design Loads for Buildings and Other Structures) with exceptions 

ASCE 7-02 and 7-05 ground snow loads (P g) are based on a 2% annual probability of being exceeded 

(50-year MRI). The recommendations to use several minimum factors (Importance Factor of not less than 

1.1, Thermal Factor of not less than 1.1, and Exposure Factor of not less than 1.0), when allowing the use 

of ASCE 7-02 or ASCE 7-05 for the determination of snow loads will ensure the balanced snow loads will 

be adequate and sufficiently similar to the specific design snow loads recommended in this data sheet. 

3.1.2 Rainfall Intensity, Duration, and Frequency used for Roof Drainage 

Intensity (i) 

Intensity (i) is the rainfall rate, typically recoded as in./hr, mm/hr, or liter/sec-m 2 . See Table 9 for conversion 

rates. 

Duration 

Duration is the time over which the peak rainfall intensity is averaged. Duration for roof drainage is typically 

recorded in 2-min, 5-min, 15-min, or 60-min time intervals. Durations as much as 24 hours to 96 hours can 

be used for site/civil drainage analysis associated with flood events. The shorter the duration, the higher 

the rainfall intensity (2-min > 5-min > 15-min > 60-min intensity) for a given frequency. 

Frequency 




Frequency is the same as the return period or MRI (mean recurrence interval) of the event. For example, 

the “100-year event” has a probability of annual exceedance of approximately 1%, while the 5-year event has 

a probability of annual exceed of approximately 20%. 

3.1.3 Siphonic Drainage 

Conventional (atmospheric, or non-siphonic) roof drainage systems use the hydraulic head above the roof 

drain, which is typically no more that several inches, to create flow through the roof drain, and sloped 

horizontal piping to maintain flow to the vertical leaders or downpipes. A siphonic drainage system uses the 

head of the entire drainage system – in theory, from the elevation of the water directly upstream of the roof 

drain, to the discharge point at or below the grade elevation – which can be many ft (m), as the energy to 

drive drainage flow. A siphonic system’s horizontal runs of the piping generally are not sloped. 

A conventional drainage system operating at capacity could have roughly 20% to 30% of the cross-sectional 

area of the piping filled with water; however, a siphonic drainage system operating at capacity will be close 

to 100% full (full-bore flow). 

Since siphonic systems use the energy associated with the head of the entire drainage system, design 

velocities are achieved without pitching or sloping the horizontal pipe runs. Siphonic systems operate with 

full-bore flow velocities of roughly 10 ft/sec to 20 ft/sec (3 m/s to 6 m/s), while gravity systems operate with 

effective velocities of roughly 2 ft/sec to 5 ft/sec (0.6 m/s to 1.5 m/s). 

Conventional drainage systems operate at or near atmospheric pressure (gage pressure near zero). However, 

siphonic systems experience pressures less than atmospheric pressure – so that the operating negative 

gage pressure can be substantial. These negative operating pressures present a much more challenging 

task to the design engineer and installation contractor, as compared to a gravity drainage system, due to the 

concerns with air infiltration, pipe buckling and crushing, and overall performance and design sensitivity. 

The Design Disposable Head (H D) is based on the reasonable assumption the manhole or inspection chamber 

will experience surcharge from surface flow or site / storm drainage quite often over the life of the building. 

4.0 REFERENCES 

4.1 FM Global 

Data Sheet 1-35, Green Roof Systems 

Data Sheet 1-55, Weak Construction and Design 

4.2 Others 

American Institute of Steel Construction (AISC). Allowable Stress Design Specification for Structural Steel 

Buildings, Commentary K, Chapter K2. 

American Institute of Steel Construction (AISC). Load and Resistance Factor Design Specification for 

Structural Steel Buildings, Commentary K, Chapter K2. 

American Society of Civil Engineers (ASCE). Minimum Design Loads for Buildings and Other Structures. 

ASCE/SEI 7-02 and 7-05. 

European Committee for Standardization (CEN). Eurocode 0, Basis of Structural Design. EN 1990:2002 with 

2005 Amendment. 

European Committee for Standardization (CEN). Eurocode 1, Actions on Structures Part 1-1: General Actions: 

Densities, Self-weight, Imposed Loads for Buildings. EN 1991-1-1:2002. 

European Committee for Standardization (CEN). Eurocode 1, Actions on Structures Part 1-3: General Actions: 

Snow Loads. EN 1991-1-3:2003. EN 12056-1. 

European Committee for Standardization (CEN). Gravity Drainage Systems Inside Buildings, Part 3: Roof 

Drainage, Layout and Calculation. EN 12056-3:2000. 

International Code Council (ICC). International Plumbing Code. 2003 and 2006 editions. 

Steel Joist Institute (SJI). Standard Specifications for LH-Series (Longspan), DLH-Series (Deep Longspan) 

Joists and Joists Girders and K-series (Open Web) Joists. 



Page 54 FM Global Property Loss Prevention Data Sheets 

APPENDIX A GLOSSARY OF TERMS 

The following discussion of terms will facilitate use of this data sheet. When using building and plumbing 

codes, use the interpretations they provide. 

A.1 Roof Loads and Drainage 

A.1.1 Controlled Roof Drains 

The design of controlled roof drains is similar to conventional roof drains. The difference is that controlled 

drains are equipped with restrictive devices to accurately set the flow characteristics to the controlled drainage 

requirements. The purpose of controlling roof drains is to have the roof serve as a temporary storage reservoir 

of rainwater (e.g., to prevent flooding of storm sewers). 

A.1.2 Design Roof Line 

The design roof line is an imaginary line established during the design stage as either dead-flat or sloped 

by setting elevations at points of support (i.e., columns or walls) for roof framing members. The design roof 

line is not the actual roof line because framing members sag under the dead weight of the roof system, and 

sag additionally under super-imposed live loads such as snow and rain. (See Figs. 8a and 8b.) 

A.1.3 Ponding and Ponding Cycle 

Ponding refers to the retention of water due solely to the deflection of relatively flat roof framing (see Figs. 

8a and 8b). The deflection permits the formation of pools of water. As water accumulates, deflection increases, 

thereby increasing the capacity of the depression formed. This phenomenon is known as the ‘‘ponding cycle.’’ 

The amount of water accumulated is dependent upon the flexibility of the roof framing. If the roof framing 

members have insufficient stiffness, the water accumulated can collapse the roof. 

A.1.4 Dead Load 

The dead load of the roof is the weight of its permanent or fixed components, including supporting members, 

deck, insulation, roof covering, gravel, and suspended or supported ceilings or equipment, such as heaters, 

lighting fixtures, and piping, which were anticipated at the time of design. 

In some cases, dead loads that were not anticipated are added to existing buildings, or an allowance for 

future dead loads was included in the design dead load. For the purposes of this data sheet, any portion of 

the dead load exceeding the design dead load should be subtracted from the design snow, rain, or live load; 

any unused portion of the design dead load may be added to the design snow, rain, or live load. 

Dead load is normally expressed in pounds per square foot (lb/ft 2 or psf), kilonewtons per square meter 

(kN/m 2 ) or kilopascal (kPa). 

A.1.5 Live Load 

The live load of the roof is the weight allowance for temporary or movable loads, such as construction 

materials, equipment, and workers. In some cases, where roofs are accessible to building occupants or the 

general public, and where is it possible for people to congregate (such as a balcony, or rooftop deck or 

terrace) an occupancy live load (e.g., 100 psf [4.8 kPa] for balcony or assembly areas) is required to be 

considered as part of the total design load. In other cases, for example the top level of an exposed (uncovered) 

parking garage, an applicable vehicle live load must be considered. In cases where an occupied or accessible 

interior floor level or walkway (e.g., catwalk or maintenance platform) is be suspended from the roof framing, 

the live load of the occupied level (e.g., 60 psf [2.9 kPa] for an elevated walkway) must be considered. For 

occupancy or vehicle live loads, most codes and standards allow for reduction in the live load based on a 

function of the tributary area for each structural member, or for a reduction in live load as part of the total 

design load combination. However, the reduction of minimum roof live load (typically 20 psf [1.0 kN/m 2 ]) are 

only allowed when permitted by the local building code, and the reduced roof live load used for design 

purposes must not be less than that recommended in this data sheet. 

The live, snow, or rain load represents the superimposed weight that the roof system can support, within 

allowable design parameters, beyond its own dead load. In cases where re-roofing materials or equipment 

or structures that were not included in the design dead load are added to the roof system, their weight should 

be subtracted from the design rain or snow load. 




Most building codes and design standards permit reductions in minimum roof live loads, excluding snow or 

rain loads, based on the tributary loaded areas supported by roof members (joists, beams, etc.). This data 

sheet restricts live load reductions for lightweight roof constructions. Usually the minimum roof live load is 20 

psf (1.0 kN/m 2 ) with a reduction to 12 psf (0.6 kN/m 2 ) for members supporting a tributary area equal to or 

greater than 600 ft 2 (56 m 2 ) and with reduced roof live loads values based on a linear relationship for tributary 

areas from 200 ft 2 (19 m 2 ) to 600 ft 2 (56 m 2 ). For example, for a tributary area of 400 ft 2 (37 m 2 ), the design 

roof live load (reduced) is 16 psf (0.8 kN/m 2 ). This means that roofs assumed to have a 20 psf (1.0 kN/m 2 ) 

live load capacity, as commonly stated on the roof plan drawings, may actually only have an effective load 

capacity of 12 psf (0.6 kN/m 2 ). Usually, only the design calculations identify whether live load reductions 

have been taken. When code guidelines for live load reductions are followed, the practical result is the 

construction of very flexible roofs, highly susceptible to ponding and frequently unable to resist rain or 

unbalanced snow (drifts) loads. It is likely that live load reductions have been applied to minimum design live 

loads even in new construction, when rain loads due to drainage system blockage are not considered or 

appropriately understood. 

Live load is usually expressed in pounds per square foot (lb/ft 2 or psf), kilo-newtons per square meter (kN/m 2 ) 

or kilo-pascal (kPa). 

A.1.6 Total Load 

The total load of the roof is the combination of the dead load plus snow, rain, or live loads, excluding wind 

and earthquake loads. The design total load should be effectively resisted by each of the structural members 

of the roof system. Building codes and design standards establish allowable (design) working stresses and 

deflection limits, and these may only be exceeded when considering dead and snow, rain, or live loads in 

combination with wind or earthquake loads. 

A.1.7 Tributary Loaded Area (TA) 

The TA is that area of the roof supported by a roof (supporting) member. Tributary loaded areas for typical 

primary and secondary members are illustrated in Figure 15. For secondary members, such as joists, the TA 

is the joist length times the joist spacing. For primary members, such as beams, girders, or trusses usually 

supporting uniformly spaced joists, the TA is the beam or truss length times its spacing. As a rule of thumb, 

the TA for primary members is the area of a bay (a layout of four columns constitutes a bay) or precisely the 

product of the average column spacing in each direction. An exception to the rule of thumb is construction 

with members framed along exterior column lines or along double column lines at expansion joints; then the 

TA is the member length times one-half the member spacing plus the roof overhang beyond the column 

centerline. 





Overhang 

Exterior 

wall 

Primary (exterior) member 

A.1.8 Roof Strength 

Roof strength is the measure of a roof assembly and supporting system’s ability to support loads. Total roof 

strength is the measure of the roof system’s ability to support the design dead load plus snow, rain, or live 

loads without exceeding the allowable design parameters. Roof strengths are expressed in psf, kN/m 2 or kPa. 

Steel roof deck manufacturers often provide allowable uniform total load tables in their catalogs. This can 

be misleading since the strength of members supporting the deck is governed by the design total load and 

not the load capacity of the roof deck. The supporting members, because of this difference, will usually 

collapse well before a failure of the deck occurs. The primary determinant of roof strength, therefore, is the 

roof supporting members with appropriate adjustment for any live load reductions. 

A.1.9 Safety Factor 

½ x primary 

Member spacing 

Primary member spacing 

(average of adjacent spacings) 

Primary (interior) member 

Exterior bay Interior bay 

Secondary member 

Secondary member length 

Fig. 15. Typical tributary loaded areas for primary and secondary members 

The safety factor of a structural member can be defined as the ratio of its strength to its maximum anticipated 

design stress (working stress). In steel design using ‘‘elastic-design methods,’’ a design stress equal to 

two-thirds of the minimum yield stress of the material, is often used. This results in a safety factor for yield 

equal to 1/0.67 or 1.5. Although the initiation of yield may not entail fracture, once the yield stress in bending 

is reached, the joists and beam will start to deflect significantly (plastic deformation), thereby increasing the 

potential for substantial ponding and catastrophic failure. 

While it is helpful to recognize this safety margin, it is equally important to understand that safety factors 

are provided for the many uncertainties associated with materials, design, fabrication, installation, and 

unpredicted loads in excess of design values. The building designer should not compromise or use any portion 

of the safety margin for design purposes except when permitted for ponding analysis and wind or earthquake 

load combinations. 

Loads in ‘‘excess’’ of design values may occur when based on this data sheet, which establishes design 

values that reduce the risk of load-induced collapse to an acceptably low limit. The implications of such 

‘‘excess’’ loads, however, should be considered. For example, if a roof is deflected at the design snow load 

so that slope-to-drain is eliminated, ‘‘excess’’ snow load may cause ponding and perhaps progressive failure. 


Primary member length 

Secondary member 

spacing (average 

of adjacent spacings)



The rain-load to dead-load or snow-load to dead-load ratios of a roof structure are an important consideration 

when assessing the implications of ‘‘excess’’ loads. If the design rain or snow load is exceeded, the 

percentage increase in total load is greater for a lightweight structure (all metal, insulated steel deck, or 

boards-on-joists roof constructions) than for a heavy structure (concrete deck or plank-on-timber 

constructions). Thus, the lower the safety margin (expressed as a load), the higher the probability for roof 

collapse due to snow or rain ‘‘excess’’ loads. This fact is supported by loss history. 

APPENDIX B DOCUMENT REVISION HISTORY 

October 2012. The following changes were made: 

A. Revised and expanded the tables (flow rate versus corresponding hydraulic head) for primary and 

secondary roof drains. 

B. Added new table for flow rate and corresponding hydraulic head for circular roof scuppers. 

C. Added new recommendations for ground snow studies where ground loads are not mapped. 

D. Added new recommendations regarding partial safety factors (load factors) for Eurocode harmonization. 

E. Added new recommendations for ground snow loads in Russia. 

F. Revised rainfall intensity maps for the US and Puerto Rico. 

G. Revised the roof drainage illustrative problems. 

July 2011. Corrections were made to Table 12, Ground Snow Load for Alaskan Locations. 

January 2011. Minor editorial changes were made. A note was added to the China snow maps and tables 

to eliminate uncertainty regarding rounding/converting. 

September 2010. The following changes were made for this revision: 

• Added recommendations for Siphonic roof drainage, including new plan review guidance. 

• Added recommendations for using Eurocode provisions for roof drainage. 

• Added recommendations for ground snow loads in China. 

• Added background and guidance on rainfall intensity, duration, and frequency (i-D-F). 

January 2009. Minor editorial changes were made for this revision. 

July 2008. Completely revised. The following outlines the major changes: 

Added section that allows the use (with exceptions and changes) of Eurocode 1 for snow loads and roof 

live loads. 

Added section that allows the use (with exceptions and changes) of ASCE 7 for snow loads. 

Added updated ground snow load maps for the contiguous United States and ground snow load for Alaska. 

Added ground snow load tables for select cities in Korea and Japan (Tables 10 and 11, respectively). 

Added recommendations for rain-on-snow surcharge, intersecting snow drifts, drift distribution on dome roofs, 

and snow/ice load at overhanging eaves. 

Added flow chart for the use of live load reduction. 

Revised snow drift loads for hip and gable roofs, valley roofs, and roof projections. 

Revised sliding snow surcharge on low roofs. 

Revised the definition of live load to exclude variable loads such as snow and rain loads. 

Accepted using Eurocode EN 12056-3:2000 and rain intensity maps and data for determining rainwater runoff 

for France, Germany, Netherlands, Switzerland and the United Kingdom. 

September 2006. Minor editorial changes were done for this revision. 

May 2006. Minor editorial changes done for this revision. 

September 2004. Minor editorial changes were done for this revision. 




January 2001. This revision of the document has been reorganized to provide a consistent format. 

APPENDIX C SUPPLEMENTARY INFORMATION 


1-54 

Page 74 

45° 

40° 

35° 

30° 

25° 

(400) 

10 

(300) 

5 

(500) 

5 

(300) 

Zero 

(1800) 

10 

(1300) 

5 

(800) 

Zero 

(200) 

10 

(100) 

5 

CS 

(800) 

5 

(1500) 

Zero 

(2400) 

Zero 

(200) 

20 

CS 

CS 

(1000) 

20 

(800) 

15 

(600) 

10 

(2000) 

5 

(1500) 

Zero 

(4400) 

15 

CS 

(2800) 

5 

(1800) 

Zero 

(700) 

20 

(400) 

15 

(2800) 

20 

(1500) 

15 

(4300) 

20 

(4100) 

15 

(3200) 

10 

(5000) 

10 

(4000) 

5 

(3600) 

5 

(2000) 

Zero 

CS 

(1000) 

Zero 

(6000) 

15 

(4500) 

10 

(3600) 

5 

(2000) 

Zero 

(1900) 

20 

(1200) 

10 

(6400) 

15 

(5400) 

20 


10 

(4500) 

5 

(3000) 

Zero 

(3200) 

20 

Roof Loads for New Construction 

CS 

(5000) 

15 

(4800) 

10 

(2000) 

Zero 

(4000) 

Zero 

(3000) 

Zero 

(3300) 

20 (4000) 

(4600) 

20 

(4600) 

15 

(3800) 

10 

CS 

CS 

(5000) 

10 

(4600) 

5 

(3500) 

Zero 

20 

(3400) 

15 

(5200) 

20 

(4500) 

15 

(6300) 

15 


10 

(4500) 

5 

(3000) 

Zero 

(5000) 

5 

(3500) 

Zero 

Fig. 16a. Ground snow load (P g ) in psf for Western United States. 

©2008 Factory Mutual Insurance Company. All rights reserved 

CS 

(4100) 

25 

(6500) 

15 

(6600) 

20 

(6000) 

35 

(6000) 

30 

(6500) 

15 

(6400) 

10 

(5000) 

5 

CS 

(6000) 

15 

(3600) 

20 

CS 

(3000) 

25 

(4800) 

25 

(4500) 

20 

(6000) 

10 

(5000) 

5 

(5500) 

15 

(6000) 

25 

CS 

CS 

(2600) 

30 

(3600) 

10 

FM Global Operating Standards 

125° 120° 115° 110° 105° 100° 

CS 

(6000) 

15 

(5000) 

10 

(4500) 

20 

CS 

(3700) 

30 

CS 

(4500) 

20 

(6200) 

20 

(6500) 

15 

(5000) 

10 

(4800) 

15 

(4400) 

10 

(3200) 

5 

(4500) 

Zero 

(2600) 

30 

35 

15 

(5000) 

10 

20 

(3000) 

Zero 

25 

30 

20 

35 

50 

50 

25 35 

120° 115° 110° 105° 100° 

In CS CS areas, site-specific Case Studies Studies are required required to 

establish ground snow loads. Extreme Extreme local variations 

in in ground ground snow snow loads in these areas preclude mapping 

at at this scale. 

Numbers in perentheses represent the upper elevation elevation 

limits in feet for the ground snow load values values presented 

below. Site-specific case case studies studies are required required to establish 

ground snow loads at elevations not covered. covered. 

To convert convert lb/sq ft to kN/m², kN/m², multiply by by 0.0479. 

To convert feet to meters, multiply by 0.3048 

100 

100 

20 

CS 

15 

Zero 

15 

40 

40 

30 

20 

60 

20 

10 

Zero 

40 

5 

40 

0 100 200 300 400 Kilometers 

0 100 200 300 

50° 

45° 

40° 

35° 

30° 

Miles

Roof Loads for New Construction 

FM Global Operating Standards 

50° 

45° 

40° 

35° 

30° 

25° 

95° 90° 85° 80° 75° 70° 65° 

35 

70 

6070 

25 

50 

50 

15 

80 

100 

60 

50 

40 

35 

30 

25 

Zero 

90 

20 

10 

5 

CS 

CS 

60 

60 

15 

50 

40 

35 

30 

25 

20 

(2600) 

15 

(1800) 

10 

Fig. 16b. Ground snow load (P g ) in psf for Eastern United States. 


CS 

Zero 

20 

25 

(2500) 

20 

5 

(1000) 

40 

CS 

(800) 

35 

CS 

(1200) 

25 

(900) 

30 

(2500) 

25 

(800) 

60 

(1700) 

30 

CS 

10 

25 

15 

(700) 

50 

CS 

(1000) 

35 

(1700) 

30 

20 

30 

CS 

(900) 

50 

40 

40 

35 

CS 

20 

(1000) 

60 

CS 

30 

25 

(700) 

90 

(700) 

100 

25 

(500) 

60 

(600) 

80 

(500) 

70 

(500) 

50 (900) 

95° 90° 85° 80° 75° 

In CS areas, site-specific Case Studies are required to 

establish ground snow loads. Extreme local variations 

in ground snow loads in these areas preclude mapping 

at this scale. 

Numbers in perentheses represent the upper elevation 

limits in feet for the ground snow load values presented 

below. Site-specific case studies are required to establish 

ground snow loads at elevations not covered. 

To convert lb/sq ft to kN/m², multiply by 0.0479. 

To convert feet to meters, multiply by 0.3048 

100 

100 

50 

1-54 

Page 75 

45° 

40° 

35° 

30° 

25° 

0 100 200 300 400 Kilometers 

0 100 200 300 

Miles



Table 12. Ground Snow Load (P g) for Alaskan Locations, psf (kN/m 2 ) 

P g 

Location lb/ft Location 

Location 

2 

kN/m 2 

lb/ft 2 

kN/m 2 

lb/ft 2 

kN/m 2 

Adak 30 1.4 Galena 60 2.9 Petersburg 150 7.2 

Anchorage 50 2.4 Gulkana 70 3.4 St Paul 40 1.9 

Angoon 70 3.4 Homer 40 1.9 Seward 50 2.4 

Barrow 25 1.2 Juneau 60 2.9 Shemya 25 1.2 

Barter 35 1.7 Kenai 70 3.4 Sitka 50 2.4 

Bethel 40 1.9 Kodiak 30 1.4 Talkeetna 120 5.8 

Big Delta 50 2.4 Kotzebue 60 2.9 Unalakleet 50 2.4 

Cold Bay 25 1.2 McGrath 70 3.4 Valdez 160 7.7 

Cordova 100 4.8 Nenana 80 3.8 Whittier 300 14.4 

Fairbanks 60 2.9 Nome 70 3.4 Wrangell 60 2.9 

Fort Yukon 60 2.9 Palmer 50 2.4 Yakutat 150 7.2 

Table 13. Ground Snow Load (P g) for Locations in Korea, psf and kPa 

50-Year Ground Snow Load for Select Cities in Korea 

Cities Location 50-yr Ground Snow 

LONG (E) LAT (N) 

Load (kPa) 

Seoul 126°58’ 37°34’ 0.85 18 

Incheon 126°38’ 37°28’ 0.80 17 

Suwon 126°59’ 37°16’ 0.70 15 

Cheongju 127°27’ 36°38’ 1.10 23 

Daejeon 127°22’ 36°22’ 1.15 24 

Pohang 129°23’ 36°02’ 0.90 19 

Daegu 128°37’ 35°53’ 0.80 17 

Ulsan 129°19’ 35°33’ 0.60 13 

Masan 128°34’ 35°11’ 1.00 21 

Gwangju 126°54’ 35°10’ 1.05 22 

Busan 129°02’ 35°06’ 0.85 18 

Mokpo 126°23’ 34°49’ 0.95 20 

Icheon 127°29’ 37°16’ 1.05 22 

Cheonan 127°07’ 36°47’ 0.80 17 

Youngju 128°31’ 36°52’ 1.10 23 

Gumi 128°19’ 36°08’ 1.05 22 

Gunsan 126°45’ 36°00’ 0.95 20 

Jeonju 127°09’ 35°49’ 0.80 17 

Note: Snow Load is based on a snow weight density of 17.2 lb/ft 3 (2.73 kN/m 3 ) 


P g 

P g 

50-yr Ground Snow 

Load (psf)



Table 14. Ground Snow Load (P g) for Locations in Japan, psf and kPa 

50-Year Ground Snow Load for Select Cities in Japan 

City Location Altitude (m) Altitude (ft) 50-yr Ground 

LONG (E) LAT (N) 

Snow Load 

(kPa) 

50-yr Ground 

Snow Load 

(psf) 

Sapporo 141°19.9’ 43°03.4’ 17 56 4.70 98 

Yamagata 140°20.9’ 38°15.2’ 153 500 2.87 60 

Fukushima 140°28.5’ 37°45.4’ 67 221 1.29 27 

Nagano 138°11.7’ 36°39.6’ 418 1372 1.70 36 

Utsunomiya 139°52.3’ 36°32.8’ 119 392 0.72 15 

Fukui 136°13.6’ 36°03.2’ 9 29 5.85 122 

Maebashi 139°03.9’ 36°24.1’ 112 368 0.96 20 

Kumagaya 139°23.0’ 36°08.8’ 30 98 0.89 19 

Mito 140°28.0’ 36°23.0’ 29 96 0.66 14 

Gifu 136°45.9’ 35°23.8’ 13 42 1.21 25 

Nagoya 136°58.1’ 35°09.9’ 51 168 0.55 12 

Kofu 138°33.4’ 35°39.8’ 273 895 1.02 21 

Choshi 140°51.6’ 35°44.2’ 20 66 0.29 6 

Hamamatsu 137°43.4’ 34°42.4’ 32 104 0.12 3 

Tokyo 139°46.0’ 35°41.0’ 7 21 0.95 20 

Yokohama 139°39.4’ 35°26.2’ 39 128 1.11 23 

Hiroshima 132°28.0’ 34°24.0’ 4 12 0.45 9 

Kobe 135°10.8’ 34°41.3’ 58 189 0.25 5 

Osaka 135°31.3’ 34°40.7’ 23 76 0.40 8 

Fukuoka 130°22.6’ 33°34.8’ 3 8 0.41 8 

Miyazaki 131°25.4’ 31°55.2’ 6 21 0.07 2 

Note: Ground snow loads are based on the recommended unit snow weight densities provided in the guidelines of the Architectural Institute 

of Japan (AIJ). 



Table 15. Ground Snow Load (P g) for Locations in China* 

Cities by Numerical Order Cities by Alphabetical Order 

City No. City Name 50-Year Ground Snow Load City No. City Name 50-Year Ground Snow Load City No. City Name 50-Year Ground Snow Load City No. City Name 50-Year Ground Snow Load 

(kN/m 2 ) (psf) (kN/m 2 ) (psf) (kN/m 2 ) (psf) (kN/m 2 ) (psf) 

1 Urumqi 1.12 23 40 Luoyang 0.64 13 43 Anqing 0.80 17 40 Luoyang 0.64 13 

2 Lhasa 0.48 10 41 Hefei 0.96 20 14 Anshan 0.80 17 3 Mohe 1.12 23 

3 Mohe 1.12 23 42 Bangbu 0.80 17 42 Bangbu 0.80 17 69 Nanchang 0.80 17 

4 Qiqihar 0.64 13 43 Anqing 0.80 17 36 Baoji 0.48 10 50 Nanjing 0.80 17 

5 Harbin 0.80 17 44 Fuyang 0.96 20 17 Beijing 0.64 13 52 Nantong 0.48 10 

6 Jiamusi 1.12 23 45 Lianyungang 0.64 13 9 Changchun 0.64 13 59 Ningbo 0.64 13 

7 Ulanhot 0.48 10 46 Xuzhou 0.64 13 67 Changde 0.80 17 30 Qingdao 0.48 10 

8 Hohhot 0.48 10 47 Sheyang 0.48 10 65 Changsha 0.80 17 21 Qinhuangdao 0.64 13 

9 Changchun 0.64 13 48 Dongtai 0.64 13 51 Changzhou 0.64 13 4 Qiqihar 0.64 13 

10 Jilin 0.64 13 49 Zhenjiang 0.64 13 73 Chengdu 0.32 7 56 Shanghai 0.48 10 

11 Fushun 0.96 20 50 Nanjing 0.80 17 75 Chongqing 0.32 7 72 Shaowu 0.64 13 

12 Shenyang 0.96 20 51 Changzhou 0.64 13 16 Dalian 0.64 13 12 Shenyang 0.96 20 

13 Dandong 0.80 17 52 Nantong 0.48 10 13 Dandong 0.80 17 47 Sheyang 0.48 10 

14 Anshan 0.80 17 53 Wuxi 0.80 17 26 Datong 0.48 10 22 Shijiazhuang 0.64 13 

15 Jinzhou 0.80 17 54 Suzhou 0.64 13 48 Dongtai 0.64 13 54 Suzhou 0.64 13 

16 Dalian 0.64 13 55 Kunshan 0.48 10 74 Dujiangyan 0.32 7 25 Taiyuan 0.64 13 

17 Beijing 0.64 13 56 Shanghai 0.48 10 11 Fushun 0.96 20 19 Tanggu 0.64 13 

18 Tianjin 0.80 17 57 Jiaxing 0.64 13 44 Fuyang 0.96 20 18 Tianjin 0.80 17 

19 Tanggu 0.64 13 58 Hangzhou 0.80 17 71 Ganzhou 0.48 10 64 Tianmen 0.64 13 

20 Zhangjiakou 0.48 10 59 Ningbo 0.64 13 77 Guiyang 0.48 10 7 Ulanhot 0.48 10 

21 Qinhuangdao 0.64 13 60 Wenzhou 0.64 13 58 Hangzhou 0.80 17 1 Urumqi 1.12 23 

22 Shijiazhuang 0.64 13 61 Jinhua 0.96 20 5 Harbin 0.80 17 28 Weifang 0.64 13 

23 Xingtai 0.64 13 62 Wuhan 0.80 17 41 Hefei 0.96 20 32 Weihai 0.80 17 

24 Yinchuan 0.48 10 63 Yichang 0.64 13 8 Hohhot 0.48 10 60 Wenzhou 0.64 13 

25 Taiyuan 0.64 13 64 Tianmen 0.64 13 6 Jiamusi 1.12 23 62 Wuhan 0.80 17 

26 Datong 0.48 10 65 Changsha 0.80 17 57 Jiaxing 0.64 13 53 Wuxi 0.80 17 

27 Jinan 0.64 13 66 Yueyang 0.96 20 10 Jilin 0.64 13 37 Xi’an 0.48 10 

28 Weifang 0.64 13 67 Changde 0.80 17 27 Jinan 0.64 13 23 Xingtai 0.64 13 

29 Linyi 0.64 13 68 Jingdezhen 0.96 20 68 Jingdezhen 0.96 20 33 Xining 0.32 7 

30 Qingdao 0.48 10 69 Nanchang 0.80 17 61 Jinhua 0.96 20 46 Xuzhou 0.64 13 

31 Yantai 0.80 17 70 Jiujiang 0.80 17 15 Jinzhou 0.80 17 35 Yan’an 0.48 10 

32 Weihai 0.80 17 71 Ganzhou 0.48 10 70 Jiujiang 0.80 17 31 Yantai 0.80 17 

33 Xining 0.32 7 72 Shaowu 0.64 13 39 Kaifeng 0.80 17 63 Yichang 0.64 13 

34 Lanzhou 0.32 7 73 Chengdu 0.32 7 76 Kunming 0.64 13 24 Yinchuan 0.48 10 


Roof Loads for New Construction 1-54


Table 15. Continued 

Cities by Numerical Order Cities by Alphabetical Order 

City No. City Name 50-Year Ground Snow Load City No. City Name 50-Year Ground Snow Load City No. City Name 50-Year Ground Snow Load City No. City Name 50-Year Ground Snow Load 

(kN/m 2 ) (psf) (kN/m 2 ) (psf) (kN/m 2 ) (psf) (kN/m 2 ) (psf) 

35 Yan’an 0.48 10 74 Dujiangyan 0.32 7 55 Kunshan 0.48 10 66 Yueyang 0.96 20 

36 Baoji 0.48 10 75 Chongqing 0.32 7 34 Lanzhou 0.32 7 20 Zhangjiakou 0.48 10 

37 Xi’an 0.48 10 76 Kunming 0.64 13 2 Lhasa 0.48 10 38 Zhengzhou 0.80 17 

38 Zhengzhou 0.80 17 77 Guiyang 0.48 10 45 Lianyungang 0.64 13 49 Zhenjiang 0.64 13 

39 Kaifeng 0.80 17 78 Zunyi 0.32 7 29 Linyi 0.64 13 78 Zunyi 0.32 7 

* Note that the loads in this table include a snow load Importance Factor (I) of 1.2. Snow load values in psf have been converted and rounded-off from snow load values in kN/m 2 ; therefore, avoid converting 

from psf to kN/m 2 as this can result in round-off error. 


1-54 Roof Loads for New Construction


45° 

40° 

35° 

30° 

25° 

70° 

Kazakhstan 

Kyrgyzstan 

India 

Ground 

Snow Load 

2 kN/m (PSF) 

75° 

0.00 (0) 

0.27 (6) 

0.40 (8) 

0.53 (11) 

0.67 (14) 

0.80 (17) 

0.93 (19) 

1.06 (22) 

1.33 (28) 

1.60 (33) 

Above Snow Load Values 

are at All Elevations 

80° 

Nepal 

85° 

85° 

Russia 

Bhutan 

Bangladesh 

Bay of Bengal 

90° 

Mongolia 

Burma 

(Myanmar) 

2 

Fig. 17a, Ground Snow Load (P ) in kN/m for Western China 


g 

90° 

95° 100° 

95° 100° 

100 

100 

105° 

Laos 

0 100 200 300 400 Kilometers 

0 100 200 300 Miles


) 

Ground 

Snow Load 

2 kN/m (PSF) 

0.00 (0) 

0.27 (6) 

0.40 (8) 

0.53 (11) 

0.67 (14) 

0.80 (17) 

0.93 (19) 

1.06 (22) 

1.33 (28) 

1.60 (33) 

Above Snow Load Values 

are at All Elevations 

Vietnam 

Gulf 

Of 

Liaodong 

Korea 

Bay 

2 

Fig. 17b. Ground Snow Load (P ) in kN/m for Eastern China 


g 

100 

100 

North 

Korea 

Yellow Sea 

Taiwan Strait 

Pescadore 

Channel 

South 

Korea 

East 

China Sea 

Taiwan 

Bashi Channel 

0 100 200 300 400 Kilometers 

0 100 200 300 Miles



No 

Yes 

Refer to Section 2.2.1.2 

for the use of roof live 

load reduction. 

Start 

Roof slope < 4 in./ft (18.4°) 

or 

curved roof rise < 1/8 of span 

No 

Yes 

Ground snow 

load = 0 

or 

dead load + reduced live 

load > 28 psf 

2 

(1.33 kN/m )? 

No 

Yes 

Fig. 18. Roof live load reduction flow chart/decision tree 

Lightweight roof 

construction? 

Yes 

Roof slope > 1/4 in./ft (1.2°)? 


No 

Live load reduction not 

allowed. Use minimum roof 

2 

live load of 20 psf (1 kN/m ).


FM Global Operating Standards Page 97 

b 

Fig.19. Rainfall intensity(i)in inches per hour for the western United States(to convert to millimeters per hour 

multiplyby25.4.) 

©2008-2012 Factory Mutual Insurance Company. All rights reserved


Page 98 FM Global Operating Standards 

©2008-2012 Factory Mutual Insurance Company. All rights reserved 

Fig. 20. Rainfall intensity (i) in inches per hour for the central and eastern United States (to convert to millimeters per hour multiply by 25.4)



©2008=2012 Factory Mutual Insurance Company. All rights reserved 

Fig. 21a. Rainfall intensity (i) in inches per hour for Puerto Rico (to convert to millimeters per hour multiply by 25.4)


Page 100 FM Global Operating Standards 


Fig. 21b. Rainfall intensity (i) in inches per hour for Hawaiian Islands (to convert to millimeters per hour multiply by 25.4)




Fig. 22. Rainfall intensity (i) in inches per hour for Alaska (to convert to millimeters per hour multiply by 25.4)



APPENDIX D RESERVED FOR FUTURE PROVISIONS 

APPENDIX E ILLUSTRATIVE EXAMPLES AND JOB AIDS 

E.1 Snow Loading Illustrative Examples 

The following examples illustrate the methods used to establish design snow loads for most of the roof 

configurations discussed in this data sheet. 

Example 1: Determine the balanced and unbalanced design snow loads for a proposed building for 

Milwaukee, Wisconsin. It has galvanized steel, insulated panels on an unobstructed gable roof, sloped 8 on 

12 (see Fig. E1.1). 

a) Ground snow load (P g) from Figure 16b: 

P g = 30 psf (1.4 kN/m 2 ) 

b) Flat-roof snow load (Section 2.3.5) 

P f = 0.9 P g = 0.9 (30) = 27 psf (1.3 kN/m 2 ) 

c) Sloped-roof (balanced) snow load (Section 2.3.7): 

P s = C sP f = 0.66 (27) = 18 psf (0.9 kN/m 2 ) 

d) Sloped-roof (unbalanced) snow load — leeward (Section 2.3.9): 

1.5 P s = 1.5 (18) = 27 psf (1.3 kN/m 2 ) 

e) Sloped-roof (unbalanced) snow load – windward (Section 2.3.9) 

0.3 P s = 0.3 (18) = 5 psf (0.26 kN/m 2 ) 

Fig. E1.1. Design snow loads for Example 1 




Example 2: Determine the roof snow load for a proposed (bow-string truss) curved roof building for New 

Haven, Connecticut. The building has an 80 ft (24 m) clear span and 15 ft (4.6 m) rise, circular arc wood deck 

roof construction with insulation and built-up roofing (see Fig. E1.2). 

a. Ground snow load (P g) from Figure 16b: 

P g = 30 psf (1.4 kN/m 2 ) 

b. Flat roof snow load (Section 2.3.5): P f = 0.9 (30) = 27 psf (1.3 kN/m 2 ) 

c. Vertical angle measured from eave to crown (see Fig. E1.2): 

Tangent of vertical angle = rise = 15 = 0.375 

1 ⁄ 2 span 40 

Vertical angle = 21° 

d. Sloped-roof (balanced) snow load: 

P s = C s P f = 1.0 (27) = 27 psf (1.3 kN/m 2 ) 

where C s = 1.0 (Table 2 for cold, other surface roof) 

e. Unbalanced snow loads (Section 2.3.10): 

Eave slope = 41° (see Fig. E1.2) 

The 30° point is 30 ft (9.1 m) from the centerline (see Fig. E1.2). 

Unbalanced load at crown w/slope of 30° (Fig. 2a, Case I): 

0.5 P s = .5 (27) = 14 psf (0.6 kN/m 2 ) 

Unbalanced load at 30° point (Fig. 2a, Case II): 

2 P s = 2(27) = 54 psf (2.6 kN/m 2 ) 

Unbalanced load at eave (Fig. 2a, Case II): 

2 P s (1 − eave slope −30° ) 

40° 

2×27 (1 – 41° − 30° ) = 39 psf (1.9 kN/m 2 ) 

40° 

Fig. E1.2. Design snow loads for Example 2 

Example 3: Determine the design snow loads for the upper and lower flat roofs for a proposed building to 

be located in Lansing, Michigan. The elevation difference between the roofs is 10 ft (3 m). The upper roof is 

200 ft (61 m) wide and the lower roof is 40 ft (12.2 m) wide (see Fig. E1.3). 




a) Ground snow load (P g) from Figure 16b: 

P g = 35 psf (1.7 kN/m 2 ) 

b) Flat-roof (balanced) snow load for either roof (Section 2.3.5) 

P f = 0.9 (P g) = 0.9 (35) = 32 psf (1.5 kN/m 2 ) 

c) Maximum snow load at wall (lower roof) (Section 2.3.12.1): 

Max. load at wall = P d + P f = h r × D 

from Table 3, with P g = 35 psf and W b = 200 ft; D = 18.6 pcf and P d + P f = 125 psf = 10 × 18.6 = 

186 psf 

Max snow load (lower roof) = 125 psf (6 kN/m 2 ) 

d) Drift width (Section 2.3.12.1): 

W d = 4 h d when h d ≤ h c 

from Table 3, with P g = 35 psf and Wb = 200 ft; 

h d = 5.01 ft 

W d = 4 (5.01) = 20 ft (6.1 m) 

Fig. E1.3. Design snow loads for Example 3 (Leeward Drifting) 

e) See Figure E.1.3 for snow loads on both roofs. 

Example 4: Determine the design snow loads for the upper and lower flat roofs of the proposed building in 

Example 3, if the upper roof is 40 ft (12 m) wide and the lower roof is 200 ft (61 m) wide (see Fig. E1.4). 




(Note: This roof configuration forms the greatest snow drift by windblown snow across the lower roof because 

the lower roof is much wider than the upper roof, see Section 2.3.12.4) 

a) Items a and b from Example 3 are applicable. 

b) Maximum snow load at wall (lower roof) (Section 2.3.12.4): 

Max. load at wall = 3 ⁄ 4 (P d) + P f from Table 3, with P g = 35 psf and W b = 200 ft; 

P d = 93 psf, P f = 32 psf 

Max snow load (lower roof) = 3 ⁄ 4 (93) + 32 = 102 psf (4.9 kN/m 2 ) 

c) Drift width 

W d = 3 ⁄ 4 (4h d); 

from Table 3, with P g = 35 psf and W b = 200 ft; h d = 5.01 ft 

W d = 3 ⁄ 4 (4×5.01) = 15 ft (4.6 m) 

d) See Figure E1.4 for snow loads on both roofs. 

Fig. E1.4. Design snow loads for Example 4 (Windward Drifting) 

E.2 Roof Drainage and Rain Loading Illustrative Examples 

The following examples illustrate the methods used to establish design rain loads and roof drainage for some 

of the roof drainage systems discussed in the data sheet. 

These examples represent only the determination of roof rain loads. Other roof design loads, such as roof 

live load and snow load, also must be evaluated by the structural design engineer to establish the governing 

design roof load condition. 




Example 5: A proposed building has a roof 168 ft (57 m) by 336 ft (102 m), with bay dimensions of 28 ft 

(9 m) by 28 ft (9 m). The roof has eight 8-in. (200 mm) primary roof drains, with a contributory area of 84 ft 

x 84 ft (26 m x 26 m) for each drain. The roof edge has a continuous cant 3- 1 ⁄ 2 in. (88 mm) high, except a 

varying height parapet, 10- 1 ⁄ 2 in. (267 mm) max where scuppers are shown. The 100-year 1-hour rainfall 

intensity (i) is 2.75 in./hr (70 mm/hr). 

Check the size the primary roof drains and overflow provisions (using roof edges or scuppers as appropriate), 

denoting the required hydraulic head at the primary drainage device (drains), and the total head at the 

overflow provisions (roof edges or scuppers) and the design rain load to be used by the roof framing designer, 

when: 

1. The roof is dead-flat with interior roof drains (at mid-bay) and roof edge overflow relief as shown in 

Figure E1.5.124a, assuming there are no scuppers. 

2. The roof is sloped 1 ⁄ 4 in./ft (2%) to the low-point line where roof drains are placed. Overflow drainage 

is provided by four 24-in. (610 mm) wide scuppers set 2.5 in. (64 mm) above the low-point line at the 

perimeter of the roof as shown in Figure E1.5.2. 

Solution 1. Flat Roof — Figure E1.5.1 

a) Rainfall intensity (Appendix C): i = 2.75 in./hr (70 mm/hr) 

b) Minimum number of drains needed (Section 2.5.4.1.11): 

n = A /15,000 sf = (168 ft x 336 ft ) / 15,000 sf = 3.8, say a minimum of 4 drains 

c) Flow rate needed per drain (Section 2.5.4.1.11): 

Q = (0.0104 x i x A) / n = (0.0104 x 2.75 in./hr x 168 ft x 336 ft)/8 drains 

Q = 200 gpm (757 L/min) 

d) Hydraulic head at drain inlet (Table 8a): 

Hydraulic head = 3.0 in. (76 mm), when Q = 200 gpm (757 L/min) 

e) Total head at roof edge overflow provision (See Fig. 8b): 

Total head = Roof edge height* 

Fig. E1.5.1 Flat roof plan for Example 5 




Total head = 3.5 in. (88 mm) ≥ 3 in. minimum head for dead-flat roofs (Section 2.5.2.3) 

f) Design rain load (Section 2.5.2.2): 

Design rain load (psf) = Total head (in.) x (5.2 psf/in.) = 3.5 in. x 5.2 = 18.2 psf (0.86 kN/m 2 ) 

g) Check that the flat roof will adequately support the maximum depth of water of 3.5 in. (88 mm) or 

18.2 psf (0.86 kN/m 2 ) over its entirety. Verify that the roof framing design has been checked for instability 

due to ponding based on this rain load.* 

Note that, due to the very long overflow length (the entire roof perimeter), the hydraulic head for overflow 

drainage (i.e., the height of water above the roof edge) is negligible, and therefore the total head (including 

the hydraulic head) is assumed to be equal to the height of the roof perimeter cant of 3.5 in. (88 mm). 

Solution 2. Sloped Roof with Scuppers — see Fig. E1.5.2. 

a. Items (a) through (d) from Solution (1) are applicable. 

b. Minimum number of scuppers needed (Section 2.5.4.1.11): 

n = A/15,000 sf = 3.8, say 4 scuppers (four 8 in. [200 mm] min. width scuppers) 

c. Flow rate needed per overflow scupper (Section 2.5.4.1.11): 

Q = (0.0104 i × A)/n = (0.0104 × 2.75 in./hr x 168 ft x 336 ft)/4 

Q = 400 gpm (1514 L/min) 

Fig. E1.5.2 Sloped roof plan for Example 5 

d. Overflow scupper size needed (Sections 2.5.4.1.11): 

The scupper flow rate needs to meet or exceed the design flow rate of the eight roof drains. 

Q >= (8 x 200 gpm)/4 = 800 gpm (3025 L/min) per scupper 

From Table 6a, for a channel-type scupper (with width [b] of 24-in. [610 mm]), determine the hydraulic 

head (H) needed for a flow rate (Q) of 800 gpm (3025 L/min): 

Q (gpm) = 2.9 x b [in.] x H^1.5 = 2.9 x 24 in. x H^1.5 = 800 gpm 




Therefore, 

H = 5.1 in. (130 mm) 

According to Section 2.5.4.1.8, the scupper height (h) should be at least 1 in. (25 mm) higher than the 

(estimated) water depth H. Therefore, the minimum height of the scupper opening (h) is: 

h = 5.1 in. (130 mm) + 1.0 (25 mm) = 6.1 in. (155 mm). 

e. Total head at scupper overflow provision (see Fig. 8b) w/scupper invert set 2.5 in (64 mm) above roof 

surface: 

Total head = hydraulic head (H) + height to scupper invert 

Total head = 5.1 in. + 2.5 in. = 7.6 in. (190 mm) ≥ 6 in. (150 mm) minimum head at low points for sloped 

roofs. (Section 2.5.2.3). 

f. Design rain load at low-point line (overflow scuppers) (Section 2.5.2.2): 

Design rain load (max) = total head (max) × 5.2 ≥ 30 psf (1.5 kN/m 2 ) 

Design rain load (max) = 7.6 in. x 5.2 psf/in. = 39.5 psf (1.9 kN/m 2 ) 

g. Design the sloped roof to support a rain load of 40 psf (1.9 kN/m 2 ) at the low-point lines of the roof 

decreasing linearly to near zero at 30 ft (9.3 m) away from the drains at the low-point lines. Verify that the 

roof framing design has been checked for instability due to ponding based on this rain load. 

Example 6 

Fig. E1.5.3 Sloped roof section for Example 5 

A proposed building has a roof area 200 ft (61 m) by 400 ft (122 m) with six 6-inch (150 mm) primary roofs 

drains and six 8-in. (200 mm) secondary (overflow) roof drains located at mid-bay. The overflow drains are 

placed adjacent to the primary drains with dam set 3 in. (75 mm) above the roof surface. The primary drains 




have drain bowl diameter of roughly 10.5 in. (270 mm) and the overflow drain dam diameter is roughly 12.75 

in. (325 mm). The roof slopes a minimum of 1/4 in./ft (2%) as shown in Figure E1.6. The 100-year 1-hour 

rainfall intensity (i) is 4.0 in./hr (100 mm/hr). 

For the given primary and overflow roof drains, determine the hydraulic head and total head, and the resulting 

design rain load to be used by the roof framing designer. 

a. Rainfall intensity (Appendix C): i = 4.0 in./hr (100 mm/hr) 

b. Check minimum number of drains (n): 

n = A / 15,000 ft 2 = 80,000 ft 2 /15,000 ft 2 = 5.3 drains 

Therefore using 6 primary drains (and 6 secondary drains) is acceptable when the drains outlet diameter 

is at least 6 in. (150 mm). 

c. Flow rate needed per drain (primary and overflow) (Section 2.5.4.1.11): 

Q = (0.0104 i × A)/n = (0.0104 × 4.0 in./hr x 80,000 ft 2 )/6 

Q = 555 gpm (2100 L/min) 

d. Check primary drains: 

From Table 8a, for the 6-inch primary drain with Q = 555 gpm (say 550 gpm), 

Hydraulic Head = 6 in. (150 mm) 

e. Check secondary (overflow) drains: 

From Table 8c, for the 8-in. secondary drain with Q = 550 gpm, 

Hydraulic Head = 3.25 in. (interpolated) 

Fig. E1.6. Roof plan for Example 6 

Total Head = 3.25 in. + 3 in. (dam height) = 6.25 in. (160 mm) 

f. Determine the design rain load to be used for the roof framing: 

The depth of rainwater is greater when for the secondary drainage system is flowing (and the primary 

drains are assumed to be completely blocked). Therefore, use 6.25 in. (160 mm) as the minimum design 

heads at the low points of the sloped roof. 

Design rain load at roof low points = 6.25 in. x 5.2 psf/in. = 32.5 psf (1.5 kN/m 2 ). For the roof slope of 

¼ in. per ft (2%), at 25 ft (7.6 m) from the drains the rain load will be near zero. Verify that the roof framing 

design has been checked for instability due to ponding based on this rain load.

DS 1-54 Roof Loads for New Construction (Data Sheet) - FM Global

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