Factory Mutual Data Sheet 1-54 Roof Loading

September 2006Page 1 of 67

ROOF LOADS FOR NEW CONSTRUCTION

Table of ContentsPage

1.0 SCOPE ................................................................................................................................................... 31.1 Changes ............................................................................................................................................ 3

2.0 LOSS PREVENTION RECOMMENDATIONS ....................................................................................... 32.1 Roof Loads and Drainage ............................................................................................................... 3

2.1.1 Construction and Location .................................................................................................... 33.0 SUPPORT FOR RECOMMENDATIONS ............................................................................................. 39

3.1 Loss History ................................................................................................................................... 393.1.1 Roof Collapse-Snow, Rain and Other Roof Loads ............................................................. 39

4.0 REFERENCES ..................................................................................................................................... 394.1 FM Global ...................................................................................................................................... 394.2 Others ............................................................................................................................................ 39

APPENDIX A GLOSSARY OF TERMS ..................................................................................................... 40A.1 Roof Loads and Drainage ............................................................................................................. 40

A.1.1 Controlled Roof Drains ........................................................................................................ 40A.1.2 Design Roof Line ................................................................................................................ 40A.1.3 Ponding and Ponding Cycle ............................................................................................... 40A.1.4 Dead Load .......................................................................................................................... 40A.1.5 Live Load ............................................................................................................................ 41A.1.6 Total Load ........................................................................................................................... 41A.1.7 Tributary Loaded Area (TA) ................................................................................................ 41A.1.8 Roof Strength ...................................................................................................................... 41A.1.9 Safety Factor ....................................................................................................................... 42

APPENDIX B DOCUMENT REVISION HISTORY ..................................................................................... 42APPENDIX E ILLUSTRATIVE EXAMPLES AND JOB AIDS ................................................................... 43

E.1 Snow Loading Illustrative Examples ............................................................................................. 43E.2 Roof Drainage and Rain Loading Illustrative Examples: .............................................................. 46E.3 Job Aids—Snow and Rain Loads and Drainage .......................................................................... 52

List of FiguresFig. 1. Snow loads for hip and gable roofs. ................................................................................................... 9Fig. 2. Snow loads for curved roofs. ............................................................................................................ 10Fig. 3. Snow loads for valley roofs. .............................................................................................................. 11Fig. 4. (To be used with Table 4.) Snow loads for lower roofs. ................................................................... 11Fig. 5. Snow loads for lower roof of adjacent structures. ............................................................................ 14Fig. 6. Sliding snow load for lower roofs. .................................................................................................... 14Fig. 7. Snow load at roof projections. .......................................................................................................... 15Fig. 8a. Typical primary and overflow drainage systems for pitched roofs. ................................................ 16Fig. 8b. Typical primary and overflow drainage systems for flat roofs. ....................................................... 17Fig. 9. Flat and sloped roofs with interior roof drains. ................................................................................. 24Fig. 10. Sloped roof with roof edge drainage. ............................................................................................. 25Fig. 11a. Ground snow load (Pg ) in psf for western United States.To obtain kN/m2, multiply by 0.048.) . 28Fig. 11b. Ground snow load (Pg ) in psf for central United States.To obtain kN/m2, multiply by 0.048.) ... 29Fig. 11c. Ground snow load (Pg ) in psf for eastern United States.To obtain kN/m2, multiply by 0.048.) .. 30Fig. 12. Ground snow load (Pg) in kg/m2, for 50-yr MRI, for Western Europe. (To convert kg/m2

to kN/m2, multiple by 0.0098. To convert kg/m2 to psf, divide by 4.88.) (June 1984.) For furtherinformation on Germany, Switzerland and Austria, see notes for Fig. 12. .................................. 31

FM GlobalProperty Loss Prevention Data Sheets 1-54

©2006 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.

Fig. 13. Rainfall intensity (i) in inches per hour for the western United States.(To convert tomillimeters per hour multiply by 25.4.) ........................................................................................... 33

Fig. 14. Rainfall intensity (i) in inches per hour for the central and eastern United States.(To convert to millimeters per hour multiply by 25.4.) .................................................................... 34

Fig. 15. Rainfall intensity (i) in inches per hour for Puerto Rico. (To convert to millimeters per hourmultiply by 25.4.) ............................................................................................................................. 35

Fig. 16. Rainfall intensity (i) in inches per hour for Hawaiian Islands.(To convert to millimeters per hourmultiply by 25.4.) ............................................................................................................................. 36

Fig. 17. Rainfall intensity (i) in inches per hour for Alaska.(To convert to millimeters per hour multiplyby 25.4.) .......................................................................................................................................... 37

Fig. 18. Rainfall intensity (i) in millimeters per hour for Western Europe, 50-yr 60-minute rainfall.Multiply values on map by 1.07 to convert to 100-yr 60-minute rainfall intensities.(To convert to in./hr, divide by 25.) (November 1982.) ................................................................... 38

Fig. 19. Typical tributary loaded areas for primary and secondary members. ............................................ 42Fig. 20. Design snow loads for Example 1. ................................................................................................. 43Fig. 21. Design snow loads for Example 2. ................................................................................................. 44Fig. 22. Design snow loads for Example 3. ................................................................................................. 45Fig. 23. Design snow loads for Example 4. ................................................................................................. 46Fig. 24a. Flat roof plan for Example 5. ........................................................................................................ 47Fig. 24b. Sloped roof plan for Example 5. ................................................................................................... 48Fig. 25. Roof plan for Example 6. ................................................................................................................ 49Fig. 26. Roof plan for Example 7. ................................................................................................................ 50

List of TablesTable 1. Ground Snow Load (Pg) for Alaskan Locations in psf (kN/sq m) ................................................... 7Table 2. Ground Snow Load (Pg) versus Balanced Flat-Roof Snow Load (Pf), Denisty (D),and

Height of Balanced Snow Load (hb) for Flat and Low-sloped Roofs ............................................. 7Table 3. Roof Slope Factor Cs ...................................................................................................................... 8Table 4. (To be used with Figure 4) Ground Snow Load (Pg) versus Balanced Snow Load (Pf),

Density (D), Balance Snow Load Height (hb), Drift Height (hd), Max Drift Load (Pd) andMax Load (Pd+Pf) ......................................................................................................................... 12

Table 4, Continued. (To be used with Figure 4) Ground Snow Load (Pg) versus Balanced SnowLoad (Pf), Density (D), Balance Snow Load Height (hb), Drift Height (hd), Max Drift Load (Pd)and Max Load (Pd+Pf) .................................................................................................................. 13

Table 5. Flow Capacity for Roof Drains and Piping1 .................................................................................. 20Table 6. Hydraulic Head Versus Flow Capacity for Roof Scuppers ............................................................ 21Table 7. Conversion of Rainfall Intensity to Flow Rate and Rain Load per Unit Area ............................... 22Table 8. Hydraulic Head versus Roof Drain Flow ........................................................................................ 22

1-54 Roof Loads for New ConstructionPage 2 FM Global Property Loss Prevention Data Sheets

©2006 Factory Mutual Insurance Company. All rights reserved.

1.0 SCOPE

This loss prevention data sheet presents guidelines principally for snow and rain loadings and drainage forthe design of new roofs of buildings and other structures. The emphasis is placed upon the design of newconstruction because structural and mechanical changes in existing buildings can be very expensive.

In general, it is the function of this data sheet to present background details and guidelines for buildingdesigners to use in carrying out the requirements or intent of typical building and plumbing codes regardingdesign roof loads and roof drainage.

It should be noted that the various guidelines presented are not based upon the worst conditions possible,or even the worst conditions recorded. A probabilistic approach is used to establish design values that reducethe risk of a snow-load-induced or rain-load-induced roof collapse to an acceptable low level.

1.1 Changes

September 2006. Minor editorial changes were done for this revision.

2.0 LOSS PREVENTION RECOMMENDATIONS

2.1 Roof Loads and Drainage

2.1.1 Construction and Location

2.1.1.1 General

2.1.1.1.1 Roofs should be designed to withstand their dead load plus the more restrictive of the followinglive loads:

a) The balanced (uniform) or unbalanced snow loads in accordance with Section 2.1.1.2.

b) The rain loads in accordance with Section 2.1.1.3 and precluding (i.e., ruling out in advance) instabilityfrom ponding.

c) Other superimposed live loads, as specified, to account for the use and maintenance of the roof andthe occupancy of the building/structure.

d) A minimum design live load of 20 psf (1.0 kN/m2) for flat roofs, sloped roofs less than 4 in./ft (18.4%)and curved roofs with rise less than 1⁄8 of span, except when a reduction in the minimum design live loadis appropriate, see Recommendation 2.1.1.1.2.

2.1.1.1.2 Reductions in the minimum roof design live load of 20 psf (1.0 kN/m2) when permitted by applicablebuilding code should be restricted for lightweight roof constructions. These constructions include all metal,insulated steel deck, boards-on-joists, plywood diaphragm, etc. Reductions in the minimum roof design liveload should only be taken whenever the roof slopes at least 1⁄4 in./ft (2%) and both of the following are met:

a) The resultant minimum roof design live load is greater than the snow or rain loads described in thedata sheet and instability due to ponding is precluded (see Section 2.1.1.1.8).

b) The roof snow load is zero or the supported combined design dead load plus live load is at least 28 psf(1.4 kN/m2).

2.1.1.1.3 During initial design, the building (structure) designer should submit the following information toFactory Mutual Engineering Association for confirmation that the design live loads and drainage of each roofare in accordance with 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.

c) Identification, where a minimum design live load of 20 psf (1.0 kN/m2) governs, of reductions takenin the minimum design live load for any primary or secondary members and their respective design deadand live loads.

d) The ground snow load and the source if different from the guidelines of this data sheet.

Roof Loads for New Construction 1-54FM Global Property Loss Prevention Data Sheets Page 3


e) The balanced, unbalanced, drift and sliding surcharge snow loads and drift length, as appropriate forthe roof configurations, showing loading diagrams and denoting any differences from the guidelines ofthis data sheet.

f) The rainfall intensity for a 1-hour duration, the Mean Recurrence Interval (MRI) and the source if differentfrom the guidelines of this data sheet.

g) Primary drains and/or scuppers: type, size, maximum drainage area and flow rate, roof surface slopeto drainage point or dead-flat, and whether drains are located at mid-bay.

h) 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.

i) Maximum hydraulic head and total head for primary and overflow drains and scuppers.

j) Maximum design rain load for dead-flat roofs and at the low points of sloped roofs.

k) Analysis method for dead-flat roofs and source used to substantiate that the roof is stable based onthe design rain load of this guideline and ponding.

l) Roof slope for roofs with drainage over the edge or sloped to drains or scuppers. If the slope is lessthan 1⁄4 in./ft (2%), substantiate with calculations that the design slope is sufficient based on Section2.1.1.3.7.2

2.1.1.1.4 Roof drainage should be designed in accordance with one of the methods described in Section2.1.1.3, or to applicable building and plumbing codes, whichever results in better drainage.

2.1.1.1.5 Overflow relief protection should be provided in accordance with Section 2.1.1.3.5.3.

2.1.1.1.6 Roofs should be designed for a rain load or total head consisting of the maximum possible depthof rainwater that could accumulate, as determined by the relative levels of the roof surface and overflowprovisions, except for water retained (ponding) due solely to the deflected roof, as described in Section2.1.1.3.2 and shown in Figures 8a and 8b. The roof framing designer, however, should preclude instabilitydue to ponding based on the estimated water buildup (total head), consistent with this guideline.

2.1.1.1.7 Roofs should be designed with positive drainage: however, dead-flat roofs consistent with thisguideline are acceptable. Sloping the roof surface 1⁄4 in./ft (2%) toward roof drains or scuppers or points offree drainage (roof edge) should be sufficient for positive drainage. If a slope of less than 1⁄4 in./ft (2%) isdesired for positive drainage, analysis methods presented in Section 2.1.1.3.7 should be used.

2.1.1.1.8 Roof framing systems should be analyzed by the designer according to the following guidelines(as applicable), to preclude instability from ponding based on the total (dead plus live) load supported by theroof framing before consideration of ponding or by substantiating that a roof slope is sufficient.

a) Dead-flat roofs: the total load supported should be the design rain load, according to Section 2.1.1.3.6,plus the dead load of the roof. An acceptable analysis method for ponding of two-way framing systemsis presented in the ASD and LRFD Specifications for Structured Steel Buildings, Commentary, ChaptersK2, American Institute of Steel Construction (AISC).

b) Sloped roofs to drains or scuppers: the total load supported should be the design rain loads, accordingto Section 2.1.1.3.6, distributed locally to the low areas, plus the dead load of the roof. An acceptableanalysis method, conservative for sloped roofs, is the AISC method given in Part a, using an appropriateequivalent uniform load based on the design rain load distribution plus dead load for the total loadsupported. Also, if the design roof slope is less than 1⁄4 in./ft (2%), it should be substantiated that it issufficient according to Section 2.1.1.3.7.2.

c) Sloped roofs to free drainage over the roof edge: if the design roof slope is less than 1⁄4 in./ft (2%), itshould be substantiated that it is sufficient according to Section 2.1.1.3.7.2.

2.1.1.1.9 Unless roofs are sufficiently sloped for drainage over the roof edges (see Section 2.1.1.3.7.2),drains or scuppers should be provided in quantity, placement and size as described in Section 2.1.1.3.5.

2.1.1.1.10 Roofs with controlled flow drains should have an overflow drainage system at a higher elevationas described in Section 2.1.1.3.2.4. Such roofs should otherwise be designed in accordance with this datasheet.



2.1.1.1.11 Roof drains and vertical leaders in sizes of 4 to 6 in. (100 to 150 mm) diameter inclusive shouldbe used, except for areas less than 2500 ft2 (230 m2), such as canopies, where 3 in. (75 mm) diameter drainsmay be used. It is usually impractical to use 8 in. (200 mm) diameter drains because of drainage arealimitations and drain flow restrictions imposed by drainage piping and/or water buildup loads.

2.1.1.1.12 Three-sided channel type roof scuppers should be used whenever possible. For walls andparapets, the four-sided perimeter, closed type scuppers should be used (see sketch with Table 6). Scuppersand leaders or conductors should have minimum dimensions of 6 in. (150 mm) wide by 4 in. (100 mm) highand 5 in. (125 mm) diameter or equivalent, respectively. The height of the closed type scupper should beat least 1 in. (25 mm) higher than the estimated water buildup height (hydraulic head) developed behind thescupper (see Table 6).

2.1.1.1.13 To assure adequate drainage, two roof drains or scuppers, as applicable, should be installed forroof areas of approximately 10,000 ft2 (930 m2) or less. For larger areas, the number of drains or scuppersshould be in accordance with Section 2.1.1.3.5.1.

2.1.1.1.14 The sizing of vertical leaders or conductors, and piping for horizontal drainage systems, shouldbe in accordance with Section 2.1.1.3.5.4 and tabulated in Table 5.

2.1.1.1.15 Where overflow roof drains or scuppers are provided, they should be at least equivalent to theprimary roof drains or scuppers, and placed and sized in accordance with Section 2.1.1.3.5 as applicable.

2.1.1.1.16 Roof overflow drain or scupper drainage systems should have vertical leaders, conductors orpiping to points of discharge independent of the primary roof drainage system. If these points of dischargecan experience backup, then points of free drainage, such as over-the-roof edges or through relief openingsatop conductors, should be used.

2.1.1.1.17 Roofs and their drainage inlets should be inspected at least every three months and followingstorms. They should be cleared of obstructions or accumulations of the foreign matter (described in Section2.1.1.4.1) as frequently as individual judgment deems necessary.

2.1.1.1.18 Existing roofs (especially lightweight roof constructions) which have severely inadequate primarydrainage and no overflow relief protection should be provided with additional drainage provisions. The needfor overflow drainage should be an individual judgment in conjunction with an evaluation of existingconditions.

2.1.1.1.19 Existing roofs which are severely exposed to collapse from snow loading should be reinforcedin the exposed area. But, where reinforcing is impractical, the Emergency Organization should include snowremoval teams. Snow removal must be safe, practical and reliable in order to be effective. A safe maximumsnow depth should be determined and snow cleared from the roofs when one-half of this depth is reached.

2.1.1.1.20 Existing roofs having roof-mounted or roof- suspended equipment and structures added ormodified, as described in Section 2.1.1.4.1, should have the supporting roof framing and columns structurallyanalyzed for the resulting dead load plus live load and reinforced as determined by a qualified engineer.

2.1.1.1.21 Rack storage structures or vertical stays should not be secured to roof framing, and bulk materialsshould not be placed against roof supporting walls (as described in Section 2.1.1.4.2) unless the roof andwall systems, respectively, are so designed.

2.1.1.1.22 Columns in the traffic areas of manually operated trucks or other mobile equipment should beprotected and anchored as described in Section 2.1.1.4.2.

2.1.1.1.23 Suspended or otherwise supported ceilings which allow access for maintenance workers shouldbe designed for appropriate concentrated and uniform live loads based on the anticipated maintenance work.

2.1.1.2 Snow Loads

2.1.1.2.1 General

Design snow loads should be determined in accordance with the guidelines of this section. However, thedesign superimposed roof load should not be less than the minimum live or snow loads designated by theapplicable building code or less than the rain loads covered in Section 2.1.1.3. For roofs of unusual shape orconfiguration, wind-tunnel or analytical modeling techniques should be used to help establish design snowloads.



2.1.1.2.2 Snow Load Notation

(For reference purposes, most of the nomenclature appears in Figure 4 of Section 2.1.1.2.9.1.)

Cs = slope factor

D = snow density in pcf (kN/m3) of drifted snow (i.e., density of wind blown snow)

hb = height of balanced (uniform) snow load in ft (m) (i.e., balanced snow load Pf or Ps divided by D)

hc = clear height from top of balanced snow load in ft (m) to the closest point(s) on adjacent upper roof;to the top of parapet; or to the top of a roof projection

hd = maximum height of snow drift surcharge above balanced snow load in ft (m)

hr = difference in height between the upper roof (including parapets) and lower roof or height of roofprojection in ft (m)

Pd = maximum intensity of drift surcharge load in psf (kN/m2 or kPa)

Pf = flat-roof snow load in psf (kN/m2 or kPa)

Pg = ground snow load in psf (kN/m2 or kPa)

Ps = sloped-roof snow load in psf (kN/m2 or kPa)

S = separation distance between buildings in ft (m)

Wb = horizontal distance of roof upwind of drift in ft (m), but not less than 25 ft (7.6 m). Wb shall equal theentire upwind distance of roofs with multiple elevation differences provided the predicted drift height at eachelevation difference exceeds hc

Wd = width of snow drift surcharge in ft (m)

2.1.1.2.3 Ground Snow Loads

Ground snow loads (Pg) used in determining design snow loads for roofs are given in the three-part mapfor the Contiguous United States in Figures 11a, 11b and 11c at the end of Section 2.0. The maps wereprepared at the Cold Regions Research and Engineering Laboratory (CRREL) for the ANSI/ASCE 7-88Standard (formerly ANSI A58.1). The maps present snow-load zones with estimated ground snow loads basedupon a 50-year Mean Recurrence Interval (MRI). In some areas the amount of local variation in snow loadsis so extreme, it precludes meaningful mapping. Such areas are not zoned but instead are shown in black.In other areas the snow load zones are meaningful, but the mapped values within these zones should not beused for certain geographic settings such as high elevations or areas close to warmer large bodies of water(e.g., oceans and the Great Lakes). Such areas are shaded as a warning that the zoned value for thoseareas applies to normal settings therein. The local office of the National Weather Service, local codes or thebuilding officials having jurisdiction should be contacted for areas not zoned or for areas of high elevationor where a more precise estimate is needed. When possible, use data based upon a 50-year MRI.Reasonable, but not exact, multiplication factors for converting from 25-year and 30-year MRI to 50-year MRIare 1.2 and 1.15, respectively.

Snow loads are zero for Hawaii and Puerto Rico.

Ground snow loads (Pg) for Alaska are presented in Table 1 for specific locations only and generally do notrepresent appropriate design values for other nearby locations. In Alaska, extreme local variations precludestatewide mapping of ground snow loads.

Ground snow loads (Pg) for Canada should be taken as 1.15 times the snow component of the ground snowload in kPa units from the tabulation of 690 locations in the 1990 Supplement to the National Building Codeof Canada as recalculated and mapped by the Atmospheric Environment Service (Ontario, Canada). Forsnow load values not included in the Supplement or where a more precise estimate is needed and includingaltitude effects, maps are available from the Canadian Climate Centre in Ontario. The rain component ofthe ground snow load, from the above sources, should be added as a uniform load to the roof snow loadsof Sections 2.1.1.2.5 through 2.1.1.2.9. To convert kPa to psf, multiply kPa × 20.9. Units of kPa and kN/m2 areequivalent. The 1.15 multiplier is a reasonable conversion from the 30-yr MRI basis of the Canadian Codeto the 50-yr MRI for this guideline.

Ground snow loads (Pg) for Western Europe are given in the map, Figure 12.



2.1.1.2.4 Snow Density

As used in this data sheet (to evaluate the heights of roof snow loads), the snow (weight) density (D) shouldbe determined as a function of the ground snow load (Pg) according to Table 2 or the following formulas:

English Units:

D (pcf) = 0.13 Pg + 14 ≤ 35 where Pg in psf

Metric Units:

D (kN/m3) = 0.43 Pg + 2.2 ≤ 5.5 where Pg in kN/m2

2.1.1.2.5 Flat-Roof Snow Loads

The balanced (uniform) snow load (Pf) on an unobstructed flat roof of well insulated roofs or unheatedbuildings, including any roof with a slope less than 5° (1 in./ft or 8%), should be determined using Table 2or the following formulas:

Pf = Pg where Pg ≤ 20 psf (1.0 kN/m2)

Pf = 0.9 Pg ≥ 20 psf (1.0 kN/m2) where 20 < Pg ≤ 40 psf (1.0 < Pg ≤ 1.9 kN/m2)

Pf = 0.8 Pg ≥ 36 psf (1.7 kN/m2) where Pg > 40 psf (1.9 kN/m2)

2.1.1.2.6 Minimum Snow Loads for Low-Sloped Roofs

The minimum allowable snow loads should be the balanced snow loads (Pf) of Section 2.1.1.2.5 or Table 2and should apply to shed, hip and gable roofs with slopes less than 15°, and curved roofs where the verticalangle (see Fig. 2) from the eave to the crown is less than 10°. The formulas in Section 2.1.1.2.5 satisfy thefollowing minimum snow load guidelines: for locations where the ground snow load (Pg) is 20 psf (1.0 kN/m2)or less, the flat roof snow load (Pf) for such roofs should not be less than the ground snow load (Pg); inlocations where the ground snow load (Pg) exceeds 20 psf (1.0 kN/m2), the flat-roof snow load (Pf) for suchroofs should not be less than 20 psf (1.0 kN/m2).

In building codes, minimum roof live loads and live load reductions do not apply to snow loads. Snow orrain loads greater than such live loads govern the determination of design live loads.

Table 1. Ground Snow Load (Pg) for Alaskan Locations in psf (kN/sq m)

Adak 20 (0.96) Galena 65 (3.1) Petersburg 130 (6.2)Anchorage 45 (2.2) Gulkana 60 (2.9) St. Paul Islands 45 (2.2)Angoon 75 (3.6) Homer 45 (2.2) Seward 55 (2.6)Barrow 30 (1.4) Juneau 70 (3.3) Shemya 20 (0.96)Barter Island 60 (2.9) Kenai 55 (2.6) Sitka 45 (2.2)Bethel 35 (1.7) Kodiak 30 (1.4) Talkeetna 175 (8.4)Big Delta 60 (2.9) Kotzebue 70 (3.3) Unalakleet 55 (2.6)Cold Bay 20 (0.96) McGrath 70 (3.3) Valdez 170 (8.2)Cordova 100 (4.8) Nenana 55 (2.6) Whittier 400 (19.0)Fairbanks 55 (2.6) Nome 80 (3.8) Wrangell 70 (3.3)Fort Yukon 70 (3.3) Palmer 50 (2.4) Yakutat 175 (8.4)

Table 2. Ground Snow Load (Pg) versus Balanced Flat-Roof Snow Load (Pf), Denisty (D),and Height of BalancedSnow Load (hb) for Flat and Low-sloped Roofs

English Units:Ground Snow Load, Pg (psf) Balanced Flat-Roof Snow Load, Pf (psf)

5 10 15 20 25 30 35 40 50 60 70 80 90 1005 10 15 20 23 27 32 36 40 48 56 64 72 80

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

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 27.00.3 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



Metric Units:Ground Snow Load, Pg (kN/m2) Balanced Flat-Roof Snow Load, Pf (kN/m2)

0.25 0.5 0.6 0.9 1.0 1.4 1.9 2.0 3.0 4.0 5.00.25 0.5 0.6 0.9 1.0 1.3 1.7 1.7 2.4 3.2 4.0

Density D, (kN/m3) Balanced Flat-Roof 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 4.40.1 0.2 0.2 0.3 0.4 0.4 0.6 0.6 0.7 0.8 0.9

Note: Interpolation is appropriate.

2.1.1.2.7 Sloped-Roof Snow Loads

The balanced (uniform) snow load (Ps) on sloped roofs, such as shed, hip, gable and curved roofs, shouldbedetermined by multiplying the flat-roof load (Pf) by the roof slope factor (Cs):

Ps = Cs x Pf

Values of Cs are given in Table 3. Cold roof values should be used. The exception is warm roof values whichapply for uninsulated glass or metal panel, plastic (e.g., acrylic or reinforced plastic panels) and fabric roofswith R-value less than 1.0 ft2-hr.-°F/Btu (0.2 m2-°K/W) of buildings continuously heated above 60°F (16°C).‘‘Slippery surface’’ values should be used where the sliding surface is of aluminum, copper, galvanized orenameled steel panels (such as on all-metal buildings) and unobstructed with sufficient space below theeaves to accept all sliding snow. Balanced load diagrams for hip and gable roofs are shown in Figure 1 andfor curved roofs in Figure 2. Roof slope for curved roofs is the vertical angle (see Fig. 2) measured fromthe eave to the crown.

Table 3. Roof Slope Factor Cs

Roof Slope

Cs Values1,2

Unobstructed Slippery Surfaces All Other SurfacesCold Roof Warm Roof Cold Roof Warm Roof

≤5° 1.0 1.0 1.0 1.014° (3 on 12) 1.0 0.8 1.0 1.018.4° (4 on 12) 0.94 0.74 1.0 1.026.6° (6 on 12) 0.79 0.62 1.0 1.030° 0.73 0.57 1.0 1.033.7° (8 on 12) 0.66 0.52 1.0 0.9145° (12 on 12) 0.46 0.36 1.0 0.6360° 0.19 0.14 0.4 0.2570° 0 0 0 0

1. Use cold roof values unless conditions discussed in Section 2.1.1.2.7 apply.2. Interpolation is appropriate within any column.

2.1.1.2.8 Unbalanced Roof Snow Loads

Balanced and unbalanced snow loads should be considered separately. Winds from all directions shouldbe considered when establishing unbalanced snow loads. Wind directions at 90° to each other should beconsidered simultaneously; however, winds from opposite directions, 180°, should be considered separately.The figures of this section show the snow loads for roofs with balanced and unbalanced load diagrams.

2.1.1.2.8.1 Hip and Gable Roofs. Unbalanced snow loads should be considered, on the (downwind) leewardroof side, for roof slopes between 5° and 70° (1 on 12 and 33 on 12) inclusive. Unbalanced snow loadingdiagrams, cases I and II, for hip and gable roofs with roof slopes from 5° to 15° and 15° to 70°, respectively,appear in Figure 1. For Case I, no reduction in snow load should be applied for roof slopes from 5° to 15°(i.e., Cs = 1.0 and Ps = Pf) and the snow surface above the eave should not be at a higher elevation than thesnow above the ridge. Snow depths should be determined by dividing the snow loads by the appropriatesnow density (D) from Table 2.



2.1.1.2.8.2 Curved Roofs (e.g., bow-string truss roofs). Unbalanced snow loads should be considered forslopes where the ‘‘vertical angle’’ from the eave to the crown is between 10° and 60°. Portions of curved roofshaving a roof slope exceeding 70° should be considered free of snow. The point at which the roof slopeexceeds 70° should be considered the ‘‘eave’’ for such roofs. Unbalanced loading diagrams, Cases I, II andIII, for curved roofs with roof slopes at the eave of 30°, 30°–70°, and greater than 70°, appear in Figure 2.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, thesnow load should not be decreased between the 30° roof slope point and the eave, but should remainconstant at 2.0 Ps as shown by the dashed line.

2.1.1.2.8.3 Valley-Roofs. Valleys are formed by multiples of folded plate, gable, sawtooth and barrel vaultroofs. No reduction in snow load should be applied for roof slope (i.e., Cs = 1.0 and Ps = Pf; see Section2.1.1.2.5). For valleys formed by the minimum slopes stated in Section 2.1.1.2.8.1 and 2.1.1.2.8.2, theunbalanced snow load should increase from one-half the balanced load (0.5 Pf) at the ridge (or crown) tothree times the balanced load at the valley (3.0 Pf) (see Fig. 3). The snow surface above the valley, however,should not be at a higher elevation than the snow above the ridge (or crown). Snow depths should bedetermined by dividing the snow loads by the appropriate snow density (D) in Table 2. The above snow loadmethodology is also applicable to multiple gable and barrel vault roofs.

2.1.1.2.9 Drifts on Lower Roofs—Snow Loads

In areas where the ground snow load (Pg) is less than 5 psf (0.25 kN/m2) or the ratio hc/hb is less than 0.2,drift loads need not be considered. Otherwise, lower levels of multilevel roofs should be designed to sustain

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



localized loads from snow drifts caused by wind over higher portions of the same structure, adjacent structuresor terrain feature within 20 ft (6 m). For snow drifts formed on lower roofs by windblown snow across thelower roof, see Section 2.1.1.2.9.4.

2.1.1.2.9.1 Drift Load. The drift load on lower roofs should be taken as a triangular surcharge loadingsuperimposed on the balanced roof snow load (Pf) as shown in Figure 4.

Maximum drift height (hd) in ft (m) should be determined from Table 4 or the following formulas:

English Units:

hd (ft) = 0.43 √3

Wb √4

Pg+10 − 1.5 ≤ hc

where Pg in psf; Wb and hc in ft

Fig. 2. Snow loads for curved roofs.



Metric Units:

hd (m) = 0.42 √3

Wb √4

Pg+0.48 − .457 ≤ hc

where Pg in kN/sq m; Wb and hc in meters

Drift surcharge load (maximum intensity), Pd = hd × D ≤ hc × D

Maximum snow load (at wall) = Pd + Pf ≤ hr × D

The drift surcharge load (Pd) and the maximum snow load at the wall (see Fig. 4) may also be determinedby Table 4 provided the product of the density (D) times hc or hr does not govern.

Drift width (Wd) should equal 4 hd except for rare cases when the calculated hd exceeds hc by more than1 ft (0.3 m). For these cases, the minimum Wd should be established so that the cross-sectional area of thedrift (0.5 Wd × hc) is equal to the cross-sectional area of the hypothetical drift (0.5 hd × 4 hd = 2 hd

2 ) thatwould be computed if hd were less than hc; however, Wd should not be less than 6 hc. Thus,

Wd = 4 hd, except when hd > hc + 1 ft (0.3 m), then Wd = 4 hd

2

hc

≥ 6 hc

If Wd exceeds the width of the lower roof (this occurs frequently with canopy roofs), the drift should betruncated at the far edge of the roof and not reduced to zero.

Fig. 3. Snow loads for valley roofs.

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



Table 4. (To be used with Figure 4) Ground Snow Load (Pg) versus Balanced Snow Load (Pf), Density (D), BalanceSnow Load Height (hb), Drift Height (hd), Max Drift Load (Pd) and Max Load (Pd+Pf)

English Units:Ground Snow Load, Pg (psf)

Balanced Snow Load, Pf (psf)5 10 15 20 25 30 35 40 50 60 70 80 90 1005 10 15 20 23 27 32 36 40 48 56 64 72 80

Density, D (pcf)Balanced Snow Load Height, hb (ft)

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 27.00.3 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

UpwindDistanceWb (ft)

Drift Height, hd (ft)1

Max. Drift Load, Pd (psf)1

Max. Load at Wall, Pd + Pf (psf)1

25 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 2.5714 18 21 24 27 30 33 35 41 47 52 58 63 6419 28 36 44 50 57 65 71 81 95 108 122 135 144

50 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 3.6224 28 33 37 41 44 48 52 60 67 75 82 90 9829 38 48 57 64 71 80 88 100 115 131 146 162 178

100 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 4.9636 42 47 53 58 63 68 73 83 93 103 113 124 13441 52 62 73 81 90 100 109 123 141 159 177 196 214

200 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 6.6451 58 66 73 80 86 93 100 113 126 139 153 166 17956 68 81 93 103 113 125 136 153 174 195 217 238 259

300 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 7.8261 70 79 87 95 103 111 118 133 149 164 180 195 21166 80 94 107 118 130 143 154 173 197 220 244 267 291

400 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 8.7669 80 89 98 107 116 125 133 150 167 184 202 219 23774 90 104 118 131 143 157 169 190 215 240 266 291 317

500 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 9.5576 88 98 108 118 127 137 146 164 182 201 220 239 25881 98 113 128 141 154 169 182 204 230 257 284 311 338

600 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 10.383 94 106 116 127 136 147 156 176 196 216 236 256 27888 104 121 136 150 163 179 192 216 244 272 300 328 358

800 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 11.493 106 119 130 142 153 164 175 197 219 241 264 286 30898 116 134 150 165 180 196 211 237 267 297 328 358 388

1000 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 12.4102 116 130 142 155 167 179 191 215 238 262 286 311 335107 126 145 162 178 194 211 227 255 286 318 350 383 415

Note: Interpolation is appropriate.1The drift height (hd), maximum drift load (Pd) and maximum load at wall (Pd + Pf) are limited to hc, (hc × D) and (hr × D), respectively.



Table 4, Continued. (To be used with Figure 4) Ground Snow Load (Pg) versus Balanced Snow Load (Pf), Density (D),Balance Snow Load Height (hb), Drift Height (hd), Max Drift Load (Pd) and Max Load (Pd+Pf)

Metric Units:Ground Snow Load, Pg (kN/sq m)

Balanced Snow Load, Pf (kN/sq m)0.25 0.5 0.6 0.9 1.0 1.4 1.9 2.0 3.0 4.0 5.00.25 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 4.40.1 0.2 0.2 0.3 0.4 0.4 0.6 0.6 0.7 0.8 0.9

UpwindDistanceWb (m)

Drift Height, hd (m)1

Max. Drift Load, Pd (kN/sq m)1

Max. Load at Wall, Pd + Pf (kN/sq m)1

10 .37 .43 .46 .51 .53 .59 .66 .67 .77 .85 .91.85 1.04 1.14 1.34 1.38 1.66 1.97 2.07 2.68 3.30 4.02

1.10 1.54 1.74 2.24 2.38 2.92 3.67 3.77 5.08 6.50 8.0215 .49 .56 .59 .65 .67 .74 .82 .83 .91 1.03 1.11

1.13 1.35 1.47 1.70 1.75 2.08 2.45 2.53 3.18 4.03 4.891.38 1.85 2.07 2.60 2.75 3.34 4.15 4.23 5.58 7.23 8.89

30 .74 .83 .86 .94 .97 1.06 1.15 1.16 1.31 1.42 1.521.69 1.99 2.15 2.45 2.52 2.96 3.44 3.61 4.58 5.55 6.691.94 2.49 2.75 3.35 3.52 4.22 5.14 5.31 6.98 8.75 10.69

50 .96 1.07 1.10 1.2 1.23 1.34 1.44 1.46 1.63 1.77 1.892.20 2.56 2.76 3.13 3.20 3.74 4.33 4.54 5.72 6.91 8.302.45 3.06 3.36 4.03 4.20 5.00 6.03 6.24 8.12 10.11 12.30

100 1.32 1.46 1.51 1.63 1.67 1.8 1.94 1.96 2.18 2.35 2.493.05 3.51 3.77 4.25 4.34 5.04 5.81 6.08 7.62 9.16 10.973.30 4.01 4.37 5.15 5.34 6.30 7.51 7.78 10.02 12.36 14.97

120 1.44 1.58 1.63 1.76 1.80 1.94 2.09 2.11 2.34 2.52 2.683.30 3.80 4.08 4.59 4.69 5.44 6.26 6.55 8.17 9.84 11.783.55 4.30 4.68 5.49 5.69 6.70 7.96 8.25 10.57 13.04 15.78

150 1.58 1.74 1.79 1.94 1.98 2.13 2.29 2.31 2.56 2.76 2.923.64 4.18 4.48 5.03 5.14 5.96 6.86 7.17 8.96 10.75 12.853.89 4.68 5.08 5.93 6.14 7.22 8.56 8.87 11.36 13.95 16.85

180 1.71 1.88 1.93 2.09 2.13 2.29 2.46 2.49 2.75 2.96 3.133.93 4.51 4.83 5.42 5.54 6.41 7.37 7.71 9.62 11.53 13.784.18 5.01 5.43 6.32 6.54 7.67 9.07 9.41 12.02 14.73 17.78

200 1.79 1.96 2.02 2.18 2.22 2.39 2.56 2.59 2.86 3.08 3.264.11 4.70 5.05 5.66 5.78 6.68 7.58 8.03 10.01 12.00 14.344.36 5.20 5.65 6.56 6.78 7.94 9.38 9.73 12.41 15.20 18.34

300 2.11 2.31 2.38 2.56 2.61 2.80 3.00 3.03 3.34 3.59 3.84.86 5.54 5.94 6.65 6.79 7.84 8.99 9.40 11.70 14.00 16.715.11 6.04 6.54 7.55 7.79 9.10 10.69 11.10 14.10 17.20 20.71

Note: Interpolation is appropriate.1The drift height (hd), maximum drift load (Pd) and maximum load at wall (Pd + Pf) are limited to hc, (hc × D) and (hr × D), respectively.



2.1.1.2.9.2 Adjacent Structures and Terrain Features. A drift load should be applied to lower roofs orstructures sited within 20 ft (6 m) of a higher structure or terrain feature (i.e., tanks, hills) as shown in Figure5. The drift load should be determined using the methodology of Section 2.1.1.2.9.1; the factor 1-S/20 withS in ft (1-S/6 with S in meters) should be applied to the maximum intensity of the drift Pd to account for thehorizontal separation between structure S, expressed in ft (m). Drift loads need not be considered forseparations greater than 20 ft (6 m).

2.1.1.2.9.3 Sliding Snow. Lower roofs which are located below roofs having a slope greater than 20° shouldbe designed for an increase in drift height of 0.4 hd, except that the total drift surcharge (hd + 0.4 hd) shouldnot exceed the height of the roof above the uniform snow depth, hc, as shown in Figure 6. Sliding snow neednot be considered if the lower roof is separated a distance S greater than hr, or 20 ft (6 m), whichever is less.

2.1.1.2.9.4 Roof Projections and Parapets. Projections above lower roofs, such as high bays or higher roofsof the same building, penthouses and mechanical equipment, can produce drifting on the lower roof asdepicted in Figure 7. Such drift loads should be calculated on all sides of projections having horizontaldimensions (perpendicular to wind direction) exceeding 15 ft (4.6 m) using the methodology of this Section,even though the surcharge loading shape may be quadrilateral rather than triangular. To compensate for a

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

Fig. 6. Sliding snow load for lower roofs.



probable lower drift height, three-quarters of the drift height (hd) should be used, based upon a value of Wb

taken as the maximum distance upwind from the projection to the edge of the roof.

Drift loads created at the perimeter of the roof by a parapet wall should be computed using one-half of thedrift height (hd) with Wb equal to the length of the roof upwind of the parapet.

2.1.1.3 Rain Loads

2.1.1.3.1 General

Design rain loads should be determined in accordance with the guidelines of this section. However, the designsuperimposed roof load should not be less than the minimum live or rain loads designated by the applicablebuilding codes or the snow loads given in Section 2.1.1.2.

2.1.1.3.2 Bases for Design Rain Loads

Minimum roof live loads and live load reductions in building codes do not apply to rain loads. Rain or snowloads greater than such live loads govern the determination of ‘‘design’’ live loads. Rain loads should bedesigned in accordance with the following guidelines, or for applicable building and plumbing codes,whichever is the more restrictive:

2.1.1.3.2.1 Roof Drainage Systems. Roof drainage systems should be designed in accordance with one ofthe methods discussed in this section, or the applicable codes, whichever results in better drainage.

2.1.1.3.2.2 Roof Stability. Roofs should be designed to preclude (i.e., ruling out in advance) instability fromponding.

2.1.1.3.2.3 Drainage System Blockage. Roofs should be designed to sustain the load of the maximumpossible depth of water that could accumulate if the primary drainage system of the roof is blocked. The designrain load or load due to the depth of water (total head) should be determined by the relative levels of theroof surface (design roof line) and overflow relief provisions, such as flow over roof edges or through overflowdrains or scuppers. Roof instability due to ponding (the retention of water due solely to the deflected roofframing) should be considered in this situation. If the overflow drainage provisions contain drain lines, suchlines should be independent of any primary drain lines. (See Figs. 8a and 8b.)

2.1.1.3.2.4 Controlled Drainage Provisions. Roofs with controlled flow drains should be equipped with anoverflow drainage system at a higher elevation which prevents rainwater buildup on the roof above thatelevation, except for the resulting hydraulic head (e.g., see ‘‘typical roof drains’’ in Fig. 8a). Such roofs shouldbe designed to sustain the load of the maximum possible depth of water to the elevation of the overflowdrainage system plus any load due to the depth of water (hydraulic head) needed to cause flow from theoverflow drainage system. Roof instability, due to ponding, should be considered in this situation. Likewise,the overflow drainage system should be independent of any primary drain lines.

Fig. 7. Snow load at roof projections.



2.1.1.3.3 Rainfall Intensities

Roof drainage provisions should be designed to handle a rainfall intensity of at least a 1-hr event with a 100-yrMRI. Rainfall intensity (i) is expressed in inches (mm) per hour.

Rainfall intensity maps are shown at the end of Section 2.0 (see Figs. 13–17) for the Western U.S., Centraland Eastern U.S., Puerto Rico, Hawaiian Islands and Alaska. Interpolation is appropriate between rainfallintensity lines. These maps were obtained from various Technical Papers of the U.S. Weather Bureau.

Note: The rainfall intensities will not necessarily correspond along the common boundary of the Westernand Central U.S. because the Central and Eastern U.S. map is newer [1977 vs. 1961].) The values expressedin inches are the most intense 60-min duration rainfalls having a 1% probability of being exceeded in oneyear. This is commonly designated as the ‘‘100-yr 1-hr rainfall.’’

Rainfall intensity (i) map for Western Europe is shown in Figure 18. The values on the map are expressedin millimeters per hour and a 50-yr return period or 50-yr 1-hr rainfall. To convert the values to 100-yr 1-hrrainfall for data sheet use, multiply the values by 1.07. (To convert millimeters to inches divide by 25.)

Rainfall intensity (i) for locations in Canada should be taken as 4.0 times the values tabulated by theAtmospheric Environment Service (Ontario, Canada) in the 1990 Supplement to the National Building Codeof Canada. The tabulation presents the probable rainfall intensity in millimeters for a 15-min duration anda 10-yr return period or ‘‘10-yr 15-min rainfall.’’ To convert millimeters to inches divide by 25. The 4.0 multiplieris a slightly conservative conversion from the 10-yr 15-min rainfall basis of the Canadian Code to the 100-yr1-hr rainfall of this guideline.

In areas outside of those covered by the maps and tabulation, or in local areas of intense rainfall history,the rainfall intensities should be obtained from local meteorological stations based upon a 1-hr duration rainfalland a 100-yr MRI. Reasonable, but not exact, multiplication factors for converting a 1-hr duration rainfallof 30-yr and 50-yr MRI to a 100-yr MRI are 1.2 and 1.07, respectively.

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



2.1.1.3.4 Stability Against Ponding

Roof instability due to ponding can be minimized or controlled in the initial roof design by one or a combinationof 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 assure that 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 pondingas well as the total load.

d) Specify camber for roof supporting members (e.g., open web joists, structural shapes and plate girdersof steel). (Note: The Standard Specifications of the Steel Joist Institute specifies camber for all joist seriesexcept K-Series [Open Web] Joists, which are optional with the manufacturer [see Section 2.1.1.3.7.2,Part e.)

Design standards, such as the AISC Specifications for Structural Steel Buildings, require that roof systemsbe investigated by structural analysis to assure adequate strength under ponding conditions, unless the roofsurface is provided with sufficient slope toward points of free drainage or other means to prevent theaccumulation of water. The AISC Specifications permit a reduction in safety factor to 1.25 with respect tostress due to ponding plus the total load supported by the roof (i.e., design rain and dead loads).

2.1.1.3.5 Roof Drains and Scuppers

Roof drains may be used for a conventional or controlled drainage system. Roof drains and scuppers maybe utilized separately or in combination for primary or overflow drainage systems. The sections to follow, whenreferring to drains, apply to both conventional and controlled flow drains, unless otherwise noted.

2.1.1.3.5.1 Quantity. At least two roof drains or scuppers should be provided for total roof areas of 10,000ft2 (930 m2) or less. For larger roof areas, a minimum of one drain or scupper should be provided for each

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



10,000 ft2 (930 m2) of roof area, except the roof area may be increased to 15,000 ft2 (1400 m2) with a minimumdrain diameter of 6 in. (150 mm) or scupper width of 8 in. (200 mm).

2.1.1.3.5.2 Placement. The placement of (primary) roof drains or scuppers are influenced by the roofstructure’s support columns and walls, expansion joints, roof equipment and other projections. Preferably,roof drains should be located at mid-bay low points, or within 20% of the corresponding bay spacing from thelow points in each direction. If roof drains or scuppers are located at points of little deflection, such as columnsand walls, the roof surface should be sloped toward them at least 1⁄8 in./ft (1%) to compensate for minimumdeflections at these locations. In general, interior (non-perimeter) drains should not be located 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) fromthe perimeter, and 150 ft (46 m) apart, may be used with a minimum drain diameter of 6 in. (150 mm).Scuppers (primary) should be placed level with the roof surface in a wall or parapet as determined by theroof slope and the contributing area of the roof, but should not be located more than 50 ft (15 m) from a roofjuncture 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 aboveduring the design phase is essential to provide adequate and uniform drainage of each roof section.

2.1.1.3.5.3 Overflow Relief Protection. Overflow drainage should be provided for both dead-flat and slopedroofs to prevent any possibility of water overload. The overflow relief provision establishes the maximumpossible water level based on blockage of the primary drainage system. The provision should be in the formof miminal height roof edges, slots in roof edges, overflow scuppers in parapets or overflow drains adjacentto primary drains, see Figures 8a and 8b. The overflow relief protection should provide positive and uniformdrainage relief for each roof section with drainage areas preferably not exceeding those of the primarydrainage or the drainage area limits in Section 2.1.1.3.5.1. Flow through the primary drainage system shouldnot be considered when sizing overflow relief drains and scuppers.

The inlet elevation of overflow drains and the invert elevation (see sketches in Table 6) of overflow scuppersshould be not less than 2 in. (50 mm) nor more than 4 in. (100 mm) above the low point of the (adjacentto) roof surface unless a safer water depth loading, including the required hydraulic head to maintain flow,has been determined by the roof framing designer.

2.1.1.3.5.4 Drainage System Sizing. After the rainfall intensity for a given location has been determined(Section 2.1.1.3.3), one can determine the number and sizes of roof drains and/or scuppers, as well as thesizes of vertical leaders or conductors and horizontal drainage piping, for either primary or overflow drainagesystems, as follows:

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

(for 6 in. dia. drains and8 in. wide scuppers per Section 2.1.1.3.5.1)

n = A10,000

; or n = A15,000

Where n = number of drains needed (nearest higher whole no. ≥2)A = total roof drainage area (ft2)

Equation 1.2 Metric Units

(for 150 mm dia. drainsand 200 mm wide scuppers per Section 2.1.1.3.5.1)

n = A930

; or n = A1400

Where n = Number of drains needed (nearest higher whole no. ≥2)A = Total roof drainage area (m2)



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 2.1.1.3.3A = total roof drainage area (ft2)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 (dm3/min)i = rainfall intensity (mm/hr), Section 2.1.1.3.3A = total roof drainage area (m2)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 drainagesystems (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 leaderdiameter which provides adequate flow capacity.

Scuppers

Apply the flow, Q, needed per scupper to Table 6 and select a scupper type and size which providesadequate flow capacity.

2. Sizing Controlled Roof Drains/Vertical Leaders:

a) The methodology of this section should also be used for controlled drainage systems by simplyconverting 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 controlleddrain-age system.



Table 5. Flow Capacity for Roof Drains and Piping1

English Units:Diameter of Drain or

Pipein.

Roof Drains andVertical Leaders

gpm

Horizontal Drainage Piping, gpm Slopes— in. per ft

1⁄8 Slope 1⁄4 Slope 1⁄2 Slope

3 90 34 48 694 180 78 110 1575 360 139 197 2786 540 223 315 4468 21170 479 679 95810 — 863 1217 172512 — 1388 1958 277515 — 2479 3500 4958

Metric Units:Diameter of Drain or

Pipemm

Roof Drains andVertical Leaders

dm3/min

Horizontal Drainage Piping, dm3/min Slopes— percentages

1 Slope 2 Slope 4 Slope

75 340 130 180 260100 680 295 415 595125 1360 525 745 1050150 2040 845 1190 1690200 24420 1815 2570 3625255 — 3265 4605 6530305 — 5255 7410 10,500380 — 9385 13,245 18,770

1 To ensure achieving these flow capacities, roof drains must be placed at mid-bay, or the roof surfaces must be sloped toward the roofdrains (see Sections 2.1.1.3.5 and 2.1.1.3.7).

2 Design flow of this capacity is impractical; water must build up approximately 4.5 in. (113 mm) to achieve this flow.



Table 6. Hydraulic Head Versus Flow Capacity for Roof Scuppers(Depth of water over invert versus flow of water through scupper)

CHANNEL TYPE SCUPPER

Flow (gpm) = 2.9 bH1.5

when h ≥ H; units—in.

Flow (dm3/min) = 0.0034 bH1.5

when h ≥ H; units—mm

CLOSED TYPE SCUPPER

Flow (gpm) = 2.9 b(H1.5—h11.5)

when h < H; units—in.

Flow (dm3/min) = 0.0034 b(H1.5—h11.5)

when h < H; units—mm

English Units

Scupper Flows, gpmWater

BuildupH, in.

Channel Type Closed Typeh ≥ H

Width b, in.Height h = 4 in. Height h = 6 in.

Width b, in.6 8 12 24 6 8 12 24 6 8 12 24

1 18 24 36 722 50 66 100 2003 90 120 180 360 (see channel type)4 140 186 280 560 (see channel type)5 194 258 388 776 177 236 354 7086 255 340 510 1020 206 274 412 8247 321 428 642 1284 231 308 462 924 303 404 606 12128 393 522 786 1572 253 338 506 1012 343 456 686 1372

Metric Units

Scupper Flows, dm3/minWater

BuildupH, mm

Channel Type Closed Typeh ≥ H

Width b, mmHeight h = 100 mm Height h = 150 mm

Width b, mm150 200 300 500 150 200 300 500 150 200 300 500

25 63 84 126 21050 178 237 356 59575 327 437 656 1093 (see channel type)

100 505 673 1009 1682 (see channel type)125 705 940 1411 2351 642 856 1284 2141150 927 1236 1854 3090 749 998 1497 2495175 1168 1558 2337 3894 841 1121 1681 2802 1105 1474 2211 3684200 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 7. Conversion of Rainfall Intensity to Flow Rate and Rain Load per Unit Area

English Units:Rainfall Intensity, i in./hr Flow Rate gpm/ft2 Rain Load/hr, psf

1.0 .0104 5.21.5 .0156 7.82.0 .0208 10.42.5 .0260 13.03.0 .0312 15.63.5 .0364 18.24.0 .0416 20.84.5 .0468 23.45.0 .0520 26.05.5 .0572 28.66.0 .0624 31.27.0 .0728 36.48.0 .0832 41.69.0 .0936 46.8

10.0 .1040 52.0Metric Units:

RainfallIntensity, i

mm/hr

Flow Ratedm3/minper 1 m2

Rain Load/hr,kilonewtons (kN)

per 1 m2

25 0.42 .2530 0.5 .2935 0.58 .3440 0.67 .3945 0.75 .4450 0.83 .4955 0.92 .5460 1.0 .5970 1.2 .6980 1.3 .7990 1.5 .88

100 1.7 .98200 3.3 1.96300 5.0 2.94


Table 8. Hydraulic Head versus Roof Drain Flow

Hydraulic Head(Approx. Water

Depth Over Inlet)

Drain Diameter4 in.

(100 mm)6 in.

(150 mm)8 in.

(200 mm)Approximate Flow in gpm (dm3/min)

1.0 in. (25 mm) 80 (300) 100 (380) 125 (470)1.5 in. (38 mm) 120 (450) 140 (530) 170 (640)2.0 in. (50 mm) 170 (640) 190 (720) 230 (870)2.5 in. (63 mm) 180 (680) 270 (1020) 340 (1290)3.0 in. (75 mm) 380 (1440) 560 (2120)3.5 in. (88 mm) 540 (2040) 850 (3220)4.0 in. (100 mm) 1100 (4160)4.5 in. (113 mm) 1170 (4430)




c) The peak flow rate is the limited (controlled) flow rate required to maintain a predetermined depth ofwater on a roof and drain the roof within a given 24-hour or 48-hour period. It varies according to thecontrolled drainage design criterion, rainfall intensity and roof slope configuration.

3. Sizing Horizontal Drainage Piping:

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

Qp = Q times the number of drains serviced by the pipe section

b) Determine the size of horizontal drainage piping needed:

Apply the flow, Qp, needed per pipe section to Table 5 and select the pipe diameter and slope which providesadequate flow capacity.

2.1.1.3.6 Rain Loads with Drains and/or Scuppers

2.1.1.3.6.1 Adequate Roof Strength. It is imperative that all roof structures be strong enough to support themaximum buildup of water that could accumulate if the primary drainage system is blocked. Roof structuresmust also preclude instability due to ponding. In determining the corresponding load (total head), one shouldinclude the loads due to the depth of water from the roof surface (design roof line) to the overflow provision(i.e., roof edges, overflow drains or overflow scuppers) plus the depth of water (hydraulic head) needed tocause the water to flow out of the overflow drainage system. The load due to ponding (the retention of waterdue solely to the deflected roof framing) should not be included. However, it is accounted for in the roofstability analysis. The typical primary and overflow drainage systems shown in Figures 8a and 8b illustratetotal head, hydraulic head and ponding.

2.1.1.3.6.2 Hydraulic Head. The water depth needed to cause flow out of overflow drainage systems shouldbe determined as follows:

a) Roof Edges—neglect the negligible hydraulic head needed to cause flow across a roof and over itsedges.

b) Overflow Roof Drains—use Table 8 with the needed flow rate Q (Section 2.1.1.3.5.4), under anappropriate drain diameter and determine the approximate depth of water over the drain’s inlet, byinterpolation when necessary.

c) Overflow Roof Scuppers—use Table 6 with the needed flow rate, Q (Section 2.1.1.3.5.4), under anappropriate scupper type and size, and determine the approximate depth of water over the scupper’s invert,by interpolation when necessary.

2.1.1.3.6.3 Design Rain Load. The general expression given below for the design rain load for roof supportingmembers is the total head times the weight of the water. Total head is measured from the design roof lineto the maximum water level (overflow discharge), as illustrated in Figures 8a and 8b of Section 2.1.1.3.2.3.The total head includes the depths of water from the design roof line to the overflow provision plus thehydraulic head corresponding to either an overflow drain or scupper. In addition, the roof framing designershould prepare calculations substantiating that the roof design precludes roof instability due to ponding.

Structural roof support members should be designed to support at least a 3 in. (75 mm) depth of water ondead-flat roofs, or at least a 6 in. (150 mm) depth of water at the low point locations of drains and scupperson sloped roofs, but not less than the total head. The actual rain load distribution to the structural memberswill depend upon any roof slope and the overflow relief provisions.

Total Head = Maximum water depth from design roof line to overflow discharge level including any hydraulichead (see Section 2.1.1.3.6.2).

English Units:

Design Rain Load (psf) = Total Head (in.) × 5.2 ≥ 15 psf for dead-flat roofs and 30 psf at low-point of slopedroofs.

Metric Units:

Design Rain Load (kN/m2) = Total Head (mm) × 0.01 ≥ 0.7 kN/m2 for dead-flat roofs and 1.4 kN/m2 atlow-point of sloped roofs.



2.1.1.3.7 Roof Slope

2.1.1.3.7.1 Roofs with Drains. To ensure that the points of maximum sag are no lower than the roof surfacebetween these points and the drains of roofs with interior drainage, a positive drainage slope of at least1⁄4 in./ft (2%) should be provided. In Figure 9 this is illustrated in the sloped roof detail where ponding occurslocally 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, the needed slope should be individually determined by deflectionanalysis. If water must flow across one bay into another, relatively complicated two-way deflection analysisis involved. The guidelines of Section 2.1.1.3.7.2 for roof slope with edge drainage are appropriate. The roofframing designer should prepare calculations according to these guidelines, or other appropriate method, tosubstantiate that the design slope is sufficient to preclude roof instability from ponding.

2.1.1.3.7.2 Roofs with Edge Drainage. If drains are not provided and drainage is accomplished by causingthe water to flow off the perimeter of the roof or if drains or scuppers are located only at the perimeter,sufficient roof slope is vital, preferably at least 1⁄4 in./ft (2%). Under these circumstances, sufficient slope isneeded to overcome the deflections caused by the dead load of the roof plus the weight of the 1-hour designstorm less the effect of any specified camber. This is achieved when the actual downward pitch of the roofsurface exceeds the upward slope for all deflected roof framing at or near their downward support column(or wall). (See Fig. 10)

If a design roof slope (Sd) less than 1⁄4 in./ft (2%) is desired, the roof framing designer should preparecalculations, according to the following guidelines or other appropriate method, to substantiate that the designslope is sufficient to preclude roof instability from ponding:

a) The actual slope (Sa) under the dead load of the roof less the upward camber, when specified, shouldbe at least 1⁄8 in./ft (1%).

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



b) The actual slope (Sa), upward from the perimeter of the roof, under the dead load of the roof plus1-hour of rain load (see Table 7), less the upward camber, when specified, should be greater than zero(i.e., upward positive slope, not flat).

c) All primary and secondary members perpendicular to the roof edge, for the entire roof slope, shouldhave actual slopes (Sa), calculated by the roof designer, meeting the slope criteria of a) and b) as follows:

English Units:

Sa (%) = Sd (%) + 240 × (Camber) − (D.L.) L ≥ 1%L 1.44 × 24 × E × I

3

Sa (%) = Sd (%) + 240 × (Camber) − (D.L. + 5.2 × i) L ≥ 0%L 1.44 × 24 × E × I

3

Where: Sa and Sd are the actual and design roof slopes in percent, respectively.

D.L. is 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

Fig. 10. Sloped roof with roof edge drainage.



Metric Units:

Sa (%) = Sd (%) + 0.24 × (Camber) − (D.L.) L ≥ 1%L 24 × E × I

3

Sa (%) = Sd (%) + 0.24 × (Camber) − (D.L. + 0.01 × i) L > 0%L 24 × E × I

3

Where: Sa and Sd are the actual and design roof slopes in percent, respectively.

DD.L., Roof’s dead load in kN/m2

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/m2

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),the actual roof slope should be increased to compensate for maximum deflection (adjusted for anyspecified camber) of the secondary member closest to the wall. Therefore, the actual slope computed inthe equations of Part c above should be adjusted by a decrease as follows:

Sa 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 Approximate Camberft (m) in. (mm)

20 ( 6) 1⁄4 ( 6)30 ( 9) 3⁄8 (10)40 (12) 5⁄8 (16)50 (15) 1 (25)60 (18) 11⁄2 (38)>60 (>18) See SJI Specifications

The above cambers should not be assumed for K-Series (Open Web) Joists because it is optional withthe manufacturer.

2.1.1.4 Other Roof Loads

2.1.1.4.1 Roof Overloading

Subjecting the roofs of buildings and other structures (e.g., silos) to temporary or fixed loads which werenot anticipated at the time of initial roof design creates the possibility of roof overloading and subsequentcollapse. Roofs are seldom designed to withstand accumulation of coal or cement dust, sand, sawdust, woodchips, spillages from baghouses and conveyors and other debris. Any such accumulations on roofs shouldbe removed since they normally prevent adequate roof drainage, and may greatly increase the total roof liveload and significantly compromise the design live load.

Another source of possible overloading is the addition or modification of roof-mounted or roof-suspendedequipment and structures. These fixed loads become critical if coupled with other loadings such as rain andsnow. Thus, suitable structural reinforcement is warranted.

2.1.1.4.2 Indirect Roof Overloading

The overloading and collapse of the primary vertical support elements of the roof structure, such as columnsand bearing walls, is another cause of roof collapse. Columns adjacent to traffic aisles for manually operatedfork-lift and other trucks are vulnerable to upset if not adequately protected.



The baseplates of these columns should be anchored to their foundations with a minimum of four 1 in. (25 mm)diameter anchor bolts, and protected with concrete curbing or concrete-filled pipe bumpers to resist and/orprevent impact loads from these vehicles. Walls, particularly masonry, should not be laterally loaded by placingbulk materials (e.g., sand) or rolled products (e.g., carpets or paper) against them, unless the wall and roofstructure is designed to resist significant lateral loads. Likewise, rack storage structures or vertical stays,for confining rolled products in storage, should not be secured to the roof framing system unless the framingand bracing systems are designed to resist significant laterally-induced loads.





50 0

Miles

10050

50 0

Kilometers

100 200

25

30

20

25

30

25

30

35

35

3035

40

1525

4025

40

35

2015

15

10

20

20

20 20

40

25

35

35

K

W

G

PM

R

S

N

EH

B

R

M

B

F

WC

N

W

F

W

CF F

W

W

M

T

K

S

C

30

20 15

15

10

15

10

Zero

10

10

10 15

15

1515

15

10

20

1015

W

Y

G

I

S

P

R

P

V

I

C

W

B

C

S

B

R

D

B

L

WY

20

10

10

V

F

20

G

15

B

O15

A

B

O

15

20

A

B

L

20

15

20

S

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O

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L

C

K

20

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Zero

15

20

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10

10

5

15

20

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10

10

15

15

10

10

10

15

10

20

15

15

10

15

1015

10 15

20

5

15

5

2020

Y

R

C

P

H

T

B

L

S

A

CB

W

R

D LS

R

L

L

L

A

L

F

R

10

C

L

V

FC

F

K

B

20

J

W

F

LQ

T

W

20K

L

M

C

T

5

15

10

15

5

Zero

5

5

Zero

5

15

5

5

15

Zero

Zero

5

Zero

10

5

Zero

Zero

10

Y

15

W

C

10A

Y

W

GP

P

H

R

M

G

R

E

P

T

C

L

W

B

L

L

O

W

H

R

M

O

H

S

R

E

P

A

E

I

G

T

15

I

20

115° 110° 105° 100°

30°

35°

40°

45°

50°

130° 125° 120° 115° 110° 105°

25°

30°

35°

40°

45°

Dots are included to assist in defining theposition of boundaries. The letter adjacent tothe dot is the first letter of the place namethere.

The zoned value is not appropriate for certaingeographic settings, such as high country inthese areas

In these areas extreme local variations insnow loads preclude mapping at this scale

Fig. 11a. Ground snow load (P ) in psf for western United States.To obtain kN/sq m, multiply by 0.048.)

g



60

50

40

6070

60

50

70

70

60

50

60

60B

C

M

I W

L

M

S

B

WK

GT

60

B

W

H

L

E

A

30

20

25

30

25

30

35

3535

30

3035

40

1550

30

25

4025

40

35

60

2015

15 20

20

20

40

40

25

35

35

40

50K

W

G

P

W

N

M

A

ML

P

R

S

N

H

EA

H

J

W

C

H

G

P

H

B

R

F

M

B

AF

WC

N

F

C

W

F

W

CF F

W

W

G

T

K

S

35

30

25

20

5

10

W

R

P

V

D

C

F

C

W

5

20

40

25

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Zero

35

Zero

15

15

10

15

40

5035

20

5

10

20

20

3025

5

E

P

L

M

S

A

M

N

E

HS

S

J

P

C

M

O

L

E

S

W

A

V

N

O

M

L

W

G

L

L

C

D

S

K

C

E

T

O

L

O

F

F

O

L

A

20

15

Zero

10

10

5

15

20

20

15

15

10

10

10

20

15

15

10

15

10

5

15

2020H

B

S

A

C

S

R

L

L

L

A

L

F

R

10

C

L

V

FC

F

B

20

F

Q

20K

L

C

T

50 0

Miles

10050

50 0

Kilometers

100 200

100° 95° 90°

30°

35°

40°

45°

50°105° 100° 95° 90° 85°

30°

35°

40°

45°


Dots are included to assist in defining theposition of boundaries. The letter adjacent tothe dot is the first letter of the place namethere.In these areas extreme local variations insnow loads preclude mapping at this scale

Fig. 11b. Ground snow load (P ) in psf for central United States.To obtain kN/sq m, multiply by 0.048.)

g



50 0

Miles

10050

50 0

Kilometers

100 200

5060

40

403530

30

90

100

50 6050

40 35 20

70

60

50

70

60

8070

C

W

C

P

C

A

60

W

BP

WC

A

G

E

L

A

P N

C

NP

BN

N

SG

US

SGR

M

W

W

W40

S

C50

60

N

B

40

50

G

S

40

60

30N

10

15

25

20

15

10

20

25

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2025

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15

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30

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252540

35

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20

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T

C

F

F

W

C

S

A

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P

D

V

GC H

M

D

FLG

H

G

T

T

F

L

L

S

DJK

D

W

35

P

C

C

N

EIET

W

G

P

YR

C

R

S

R

P

R Y

W

LM

W

C S

I

D

C

C

A

F

S

35

30O

W

C T

S

10

ro

40

T

C

E

M

N

S

P

J


In these areas extreme local variations insnow loads preclude mapping at this scale

Dots are included to assist in defining theposition of boundaries. The letter adjacent tothe dot is the first letter of the place namethere.

Fig. 11c. Ground snow load (P ) in psf for eastern United States.To obtain kN/sq m, multiply by 0.048.)

g



Kilometers

0 100 200 300

Miles

0 200 4005° 0° 5° 10° 15°

35°

20°

40°

45°

50°

55°

60°

65°

30° 25° 20° 15° 10° 5° 0° 5° 10° 15° 20° 25° 30° 35°

35°

40°

45°

50°

55°

60°

65°

Reykjavik

Belfast

Vienna

Bern

Warsaw

Palermo

Sevilla

Taranto

The Hague

Amsterdam

Trondheim

Budapest

Frankfurt

Munich

Berlin

Hamburg

Essen

Goteborg

Dublin

Liverpool

Copenhagen

Porto

Lisbon

London

Glasgow

Brussels

Paris

Lyon

Birmingham

Riga

Vilnius

Bordeaux

MadridBarcelona

Marseille

Starvnger Stockholm

Helsinki

Tallinn

Sofia

Tirane

Milan

Florence

Rome

Belgrade

Prague

Inverness

95

180110

95

135

135

135220

135

220

135

110

145

220

200

150150

100

40

70120

110

50 145170

40

40

40

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40

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50

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70

40

40

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30

3040

40

50

50

55

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70

85

65

65

40

11090

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6060

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80

100

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20060

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6080

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40 40

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200 50100 150180

60

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6060

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200150250

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500350

300300

500

200

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400400

150

150

60

BALTICSEA

NORTHSEA

ATLA

NTI

CO

CE

AN

222

ADRIATIC SEA

2

NORWEGIA

N SE

A

MEDITERRANEAN SEA

Fig. 12. Ground snow load (Pg) in kg/m , for 50-yr MRI, for Western Europe. (To convert kg/mto kN/m , multiple by 0.0098. To convert kg/m to psf, divide by 4.88.) (June 1984) For further

information on Germany, Switzerland and Austria, see notes for Fig. 12.

Notes for Figure 12:

The ground snow load map for Europe is to be used when the property is located in the countriesindicated. Interpolation for snow loadings between isopleth lines is generally acceptable in regions nothaving high hills or mountains (see footnote).

The snow loadings for Switzerland, Austria and Germany should be determined by the discussion below.

Switzerland

The following is from the Swiss Association of Engineers and Architects.

For H < 2000 m:

Ps = 40 + ( H55 )

2

, but at least 90 kg/m2

where H is the altitude above sea level in meters and Ps is the snow load for the altitude in question.

For H > 2000 m: Use measured snow loads.

Austria

Snow load is not always in direct proportion to altitude above sea level. The following guidelines areapproximate and should be confirmed with local data.

Altitude (m) Snow Load (kg/m2)Up to 300 75300-500 75-100500-750 100-160750-1000 160-200

1000-1500 200-3001500-2000 300-4002000-3000 400-600

Germany

For Germany multiply the loads shown on the map by the following:

Altitude (m) Multiplier700-800 1.3800-900 1.6900-1000 1.9

Footnote. This guideline should be used with caution when the building is to be located on a high hill or mountain. In that case,consult local records for maximum weight of snowfall. To convert kg/m2 to kN/m2, multiply by 0.0098. To convert kg/m2 to psf, divideby 4.88.





Kilometers

0 100 200 300

Miles

0 200 400

115° 110° 105° 100°

25°

30°

35°

40°

45°

50°

130° 125° 120° 115° 110° 105° 100°

20°

25°

30°

35°

40°

45°

PACI

FIC

OCE

AN

1.51.5

1.5 2.0

2.52.5

1.5

2.0 1.51.0

2.0

3.0

1.5

1.0

1.01.0

3.02.01.5

1.03.0

2.0 2.5

1.5

1.0

1.0

3.0

2.01.5

2.0 2.5

1.5

1.5

1.5

1.0

2.5 2.0

1.5

2.53.0

2.0

2.5

Source: U.S.Weather Bureau, Technical Paper No. 40, 1961

100 Year 1 hour duration

Fig. 13. Rainfall intensity (i) in inches per hour for the western United States



Kilo

met

ers

010

020

030

0

Mile

s

020

040

0

105°

100°

95°

90°

85°

80°

75°

25°

30°

35°

40°

45°

110°

105°

100°

95°

90°

85°

80°

75°

70°

65°

25°

30°

35°

40°

45°

50°

LAKE

E

RIE

GU

LF O

F M

EXIC

O

LAK

E O

NTA

RIO

ATLANTIC OCEAN

LAK

E M

ICH

IGA

N

LAK

E S

UP

ERIO

R

LAKE

HU

RO

N

2.0

2.0

2.75

2.75

2.52.5

3.5

3.0

3.0

3.25

3.75

4.25

4.5

3.75

4.75

2.25

4.5

4.5

3.25

4.28

4.5

3.25

2.6

*

3.5

4.25

4.0

3.25

4.0

3.25

2.25

4.25

4.0

3.2 5

3 .0

3.2 5

3.75

3.25

3.75

2.75

3.0

2.75

2.75

2 .5

2.5

2.25

2.25

5.0

2.2 5

2 .52.

5

2 .52 .

25

2.25

2 .5

2.75

2.75

3.0

3.0

2.7 5

2.75

2.75

3.5

3.0

2.75

3.25

4.25

2.5

2.25

3.5

3.75

2.5

4.75

2.5

2.25

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Berlin

Frankfurt

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Brussels

Paris

ViennaMunich

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Bern

Yon

Bordeaux

Barcelona

Fig. 18. Rainfall intensity (i) in millimeters per hour for Western Europe, 100-yr 60-minute rainfall. (To convert to in./hr, divide by 25.)

Kilometers

0 100 200 300

Miles

0 200 400

15°10°5°0°

65°

20°5°

45°

50°

55°

60°

40°

3.0 SUPPORT FOR RECOMMENDATIONS

3.1 Loss History

3.1.1 Roof Collapse-Snow, Rain and Other Roof Loads

Large accumulations of snow, rain or other means of overloading the roofs of industrial or commercialbuildings can play havoc with business operations. According to the FM Global Research loss statistics, over1700 losses occurred, some involving multiple buildings and/or structures which suffered roof collapses from1977 through 1996 as a result of such roof overloadings.

The primary cause of rain overloading is ponding water accumulating in depressions and the excessivebuildup of rainwater on roofs. Most rain loading collapses occur on essentially flat roofs, slopes of 1⁄8 in./ft(1%) or less, of relatively lightweight construction and designed for minimum live loads.

A roof designed as dead-flat or sloped only 1⁄8 in./ft (1%) to either interior drains or free drainage (roof’s edges)probably contains low spots by the time it is constructed. Water ponding on a roof surface will remain untilit evaporates. Ponding can be troublesome because it may involve freeze-thaw cycles or prolonged moisture,both of which are detrimental to the roofing materials. The National Roofing Contractors Association (NRCA)recommends a minimum 1⁄4 in./ft (2%) slope on roofs. The most serious difficulty with ponding is that it canlead to overload problems on the roof structure.

As rainwater or snow meltwater flows to low areas, these areas tend to sag increasingly, allowing a deeperpond to form. If the roof structure does not possess enough stiffness to resist this progression and stabilize,collapse by localized overloading results. This mechanism is commonly known as the ‘‘ponding cycle’’ and ithas been responsible for most roof collapses due to rain loads or the combination of rain and snow loads.

Lightweight, flexible roof structures are particularly susceptible because they do not have enough stiffness toresist or stabilize the ponding cycle. Theoretical calculations for ponding are very complicated. The AISCSpecifications describes a method for determining minimum stiffness necessary for two-way framing systemsto preclude roof instability or the prevention of ponding collapse.

Excessive buildup of rainwater is also caused by inadequate interior drains or perimeter scuppers, rainfallintensities greater than predicted, clogged drainage devices, and the backup of storm drainage systems. Allof the above caused overloading and subsequent roof collapse.

When code guidelines for live load reductions are followed, the practical result is that very flexible roofs areconstructed which are highly susceptible to ponding and frequently unable to resist rain or unbalanced snowloads.

4.0 REFERENCES

4.1 FM Global

Data Sheet 1-14, Construction Systems.

Data Sheet 1-55, Weak Construction and Design.

4.2 Others

American Institute of Steel Construction (AISC), Allowable Stress Design Specification for Structural SteelBuildings, Commentary K, Chapter K2.

American Institute of Steel Construction (AISC), Load and Resistance Factor Design Specification forStructural Steel Buildings, Commentary K, Chapter K2.

American Iron and Steel Institution (AISI), Base Test Method for Purlins Supporting a Standing Seam Roof.

American Iron and Steel Institution (AISI), Specification for the Design of Cold-Formed Steel StructuralMembers.

ANSI A58.1, Minimum Design Loads for Buildings and Other Structures.

ANSI/ASCE 7-88, Minimum Design Loads for Buildings and Other Structures.

Arctic Environmental Information and Data Center (AEIDC).



Cold Regions Research and Engineering Laboratory (CRREL).

European Committee for Standardization (CEN).

Eurocode 1: Part 2.3 Action on Structures — Snow Loads, DD ENV 1991-2-3:1966.

National Roofing Contractors Association (NRCA).

National Weather Service, Technical Notes.

1990 Supplement to the National Building Code of Canada.

Standard Specifications of the Steel Joist Institute (SJI) for LH-Series (Longspan), DLH-Series (DeepLongspan) Joists and Joists Girders and K-series (Open Web) Joists.

Swiss Association of Engineers and Architects, Snow Loads.

The Division of Building Research, National Research Council, and the Meteorological Branch, Departmentof Transport, Canada.

U.S. Weather Bureau, Technical Papers.

APPENDIX A GLOSSARY OF TERMS

The following discussion of terms will facilitate use of this data sheet. When using building and plumbingcodes, use the interpretations provided therein.

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 controlleddrains are equipped with restrictive devices to accurately set the flow characteristics to the controlled drainagerequirements. The purpose of controlling roof drains is to have the roof serve as a temporary storage reservoirof 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 roofline is not the actual roof line because framing members sag under the dead weight of the roof system, andsag 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 framingmembers 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 which were not anticipated are added to existing buildings or an allowance forfuture dead loads was included in the design dead load. For the purposes of this data sheet, any portion ofthe dead load exceeding the design dead load should be subtracted from the design live load; any unusedportion of the design dead load may be added to the design live load.

Dead load is normally expressed in pounds per square foot (psf), kilonewtons per square meter (kN/m2) orkilopascal (kPa).



A.1.5 Live Load

The live load of the roof is the weight allowance for temporary or movable loads, such as rain, snow orconstruction materials, equipment and workers, which will occasionally be present.

The design live load represents the superimposed weight which the roof system can support, within allowabledesign parameters, beyond its own dead load. In cases where re-roofing materials or equipment or structureswhich were not included in the design dead load are added to the roof system, their weight should besubtracted from the design live load.

Most building codes and design standards permit reductions in minimum (roof) design live loads, excludingsnow 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, see Section 2.1.1.1.2. Usuallythe minimum design live load is 20 psf (1.0 kN/m2) with a reduction to 16 psf (0.8 kN/m2) for memberssupporting a tributary area of 201 to 600 ft2 (19 to 56 m2), and a reduction to 12 psf (0.6 kN/m2) for memberssupporting a tributary area greater than 600 ft2 (56 m2). This means that roofs assumed to have a 20 psf(1.0 kN/m2) live load capacity, as commonly stated on the roof plan drawings, may actually only have aneffective load capacity of 12 psf (0.6 kN/m2). Usually, only the design calculations identify whether live loadreductions have been taken. When code guidelines for live load reductions are followed, the practical resultis that very flexible roofs, highly susceptible to ponding and frequently unable to resist rain or unbalanced snow(drifts) loads, are constructed. It is likely that live load reductions have been applied to minimum design liveloads, even in new construction, when rain loads due to drainage system blockage is not considered orappropriately understood (Section 2.1.1.3.2).

Live load is usually expressed in pounds per square foot (psf), kilonewtons per square meter (kN/m2) orkilopascal (kPa).

A.1.6 Total Load

The total load of the roof is the combination of the dead plus 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, andthese may only be exceeded when considering dead and live loads in combination with wind or earthquakeloads.

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 typicalprimary and secondary members are illustrated in Figure 19. For secondary members, such as joists, the TAis the joist length times the joist spacing. For primary members, such as beams or trusses usually supportinguniformly spaced joists, the TA is the beam or truss length times its spacing. As a rule of thumb, the TA forprimary members is the area of a bay (a layout of four columns constitutes a bay) or precisely the product ofthe average column spacing in each direction. An exception to the rule of thumb is construction with membersframed along exterior column lines or along double column lines at expansion joints; then the TA is themember length times one-half the member spacing plus the roof overhang beyond the column centerline.All of the above is applicable to single span or cantilevered roof framing (Data Sheet 1-14, ConstructionSystems).

A.1.8 Roof Strength

The term ‘‘roof strength’’ and ‘‘total roof strength,’’ for the purposes of this data sheet, are used interchangeablywith design live load and total load, respectively. Roof strength is the measure of a roof assembly andsupporting system’s ability to support loads above the design dead load without exceeding the allowabledesign parameters. Total roof strength is the measure of the roof system’s ability to support the design deadplus live loads without exceeding the allowable design parameters. Roof strengths are also expressed inpsf, kN/m2 or kPa.

Steel roof deck manufacturers often provide allowable uniform total load tables in their catalogs. This canbe misleading since the strength of members supporting the deck is governed by the design total load andnot the load capacity of the roof deck. The supporting members, because of this difference, will usuallycollapse well before a failure of the deck occurs. The primary determinant of roof strength, therefore, is theroof supporting members with appropriate adjustment for any live load reductions.



A.1.9 Safety Factor

The safety factor of a structural member is defined as the ratio of its strength (yield stress) to its maximumanticipated design stress (working stress). In steel design using ‘‘elastic-design methods,’’ a design stressequal to two-thirds of the minimum yield stress of the material is often used. This results in a safety factor equalto 1/0.67 or 1.5.

While it is helpful to recognize this safety margin, it is equally important to understand that safety factorsare provided for the many uncertainties associated with materials, design, fabrication, installation andunpredicted loads in excess of design values. The building designer should not compromise or use any portionof the safety margin for design purposes except when permitted for ponding analysis and wind or earthquakeload combinations.

Loads in ‘‘excess’’ of design values may occur when based on this guideline, which establishes design valuesthat reduce the risk of load-induced collapse to an acceptable 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 thatslope-to-drain is eliminated, ‘‘excess’’ snow load may cause ponding and perhaps progressive failure. Therain-load to dead-load or snow-load to dead-load ratios of a roof structure are an important consideration whenassessing the implications of ‘‘excess’’ loads. If the design rain or snow load is exceeded, the percentageincrease in total load is greater for a lightweight structure (all metal, insulated steel deck or boards-on-joistsroof constructions) than for a heavy structure (concrete deck or plank-on-timber constructions). Thus, thelower 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 collapse loss history.

APPENDIX B DOCUMENT REVISION HISTORY


May 2006. Minor editorial changes done for this revision.


September 2001. Redrawn maps for ground snow load and rainfall intensity were updated.

Fig. 19. Typical tributary loaded areas for primary and secondary members.



January 2001. This revision of the document has been reorganized to provide a consistent format.

GIRTS Issued: Subject: ZEE SECTION PROPERTIES 2/1/93 Table C-22.

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 roofconfigurations discussed in the data sheet.

Example 1: Determine the balanced and unbalanced design snow loads for a proposed building forMilwaukee, WI. It has galvanized steel, insulated panels on a gable roof, sloped 8 on 12 (see Fig. 20).

a) Ground snow load (Pg) from Figure 11b:

Pg = 30 psf (1.4 kN/m2)

b) Flat-roof snow load (Section 2.1.1.2.5 or Table 2)

Pf = 0.9 Pg = 0.9 (30) = 27 psf (1.3 kN/m2)

c) Sloped-roof (balanced) snow load (Section 2.1.1.2.7):

Ps = CsPf = 0.66 (27) = 18 psf (0.9 kN/m2)

where Cs = 0.66 (Table 3 for slippery, cold, 8 on 12 sloped roof)

d) Sloped-roof (unbalanced) snow load (Section 2.1.1.2.8.1): 1.5 Ps = 1.5 (18) = 27 psf (1.3 kN/m2)

e) See Figure 20 for both snow loading conditions.

Example 2: Determine the roof snow load for a proposed (bow-string truss) curved roof building for Boston,MA. The building has an 80 ft clear span and 15 ft rise, circular arc wood deck roof construction with insulationand built-up roofing (see Fig. 21).

a. Ground snow load (Pg) from Figure 11c:

Pg = 30 psf (1.4 kN/m2)

b. Flat roof snow load (Section 2.1.1.2.5 or Table 2):

Pf = 0.9 (30) = 27 psf (1.3 kN/m2)

c. Vertical angle measured from eave to crown (see Fig. 21):

Tangent of vertical angle = rise = 15 = 0.3751⁄2 span 40

Vertical angle = 21°

Fig. 20. Design snow loads for Example 1.



d. Sloped-roof (balanced) snow load:

Ps = Cs Pf = 1.0 (27) = 27 psf (1.3 kN/m2)

where Cs = 1.0 (Table 3 for cold, other surface roof)

e. Unbalanced snow loads (Section 2.1.1.2.8.2):

Eave slope = 41° (see Fig. 21)

The 30° point is 30 ft (9.1 m) from the centerline (see Fig. 21).

Unbalanced load at crown w/slope of 30° (Fig. 2, Case I):

0.5 Ps = .5 (27) = 14 psf (0.6 kN/m2)

Unbalanced load at 30° point (Fig. 2, Case II):

2 Ps = 2(27) = 54 psf (2.6 kN/m2)

Unbalanced load at eave (Fig. 2, Case II):

2 Ps (1 − eave slope −30° )40°

2×27 (1 – 41° −30°) = 39 psf (1.9 kN/m2)40°

f. See Figure 21 for both loading conditions.

Example 3: Determine the design snow loads for the upper and lower flat roofs for a proposed building tobe located in Lansing, MI. 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. 22).

a) Ground snow load (Pg) from Figure 11c:

Pg = 35 psf (1.7 kN/m2)

b) Flat-roof (balanced) snow load for either roof (Section 2.1.1.2.5)

Pf = 0.9 (Pg) = 0.9 (35) = 32 psf (1.5 kN/m2)

c) Maximum snow load at wall (lower roof) (Section 2.1.1.2.9.1):

Max. load at wall = Pd + Pf ≤ hr × D

from Table 4, with Pg = 35 psf and Wb = 200 ft; D = 18.6 pcf and

Pd + Pf = 125 psf ≤ 10 × 18.6 = 186 psf

Max snow load (lower roof) = 125 psf (6 kN/m2)




d) Drift width (Section 2.1.1.2.9.1):

Wd = 4 hd when hd ≤ hc + 1 ft

from Table 4, with Pg = 35 psf and Wb = 200 ft;hd = 5.01 ft

Wd = 4 (5.01) = 20 ft (6.1 m)

e) See Figure 22 for snow loads on both roofs.

Example 4: Determine the design snow loads for the upper and lower flat roofs of the proposed building inExample 3, if the upper roof is 40 ft (12 m) wide and the lower roof is 200 ft (61 m) wide (see Fig. 23). (Note:This roof configuration forms the greatest snow drift by windblown snow across the lower roof because thelower roof is much wider than the upper roof, see Section 2.1.1.2.9.4.)

a) Items a and b from Example 3 are applicable.

b) Maximum snow load at wall (lower roof) (Section 2.1.1.2.9.4):

Max. load at wall = 3⁄4 (Pd) + Pf from Table 4, with Pg = 35 psf and Wb = 200 ft;

Pd = 93 psf, Pf = 32 psf

Max snow load (lower roof) = 3⁄4 (93) + 32 = 102 psf (4.9 kN/m2)

c) Drift width (Section 2.1.1.2.9.1) Wd = 3⁄4 (4hd);

from Table 4, with Pg = 35 psf and Wb = 200 ft; hd = 5.01 ft

Wd = 3⁄4 (4×5.01) = 15 ft (4.6 m)

d) See Figure 23 for snow loads on both roofs.




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 someof the roof drainage systems discussed in the data sheet.

Example 5: A proposed building for Dallas, TX, has a roof, 168 ft (57 m) by 336 ft (102 m), with baydimensions, 28 ft (9 m) by 28 ft (9 m). Joists are spaced 5.6 ft (1.8 m) o.c., normal to beams which spanfrom column to column. The roof edge has a continuous cant 3-1⁄2 in. (88 mm) high except a varying heightparapet, 10-1⁄2 in. (267 mm) max where scuppers are shown. Size the (primary) roof drains and overflowprovisions (using roof edges or scuppers as appropriate), denoting the required hydraulic head at the primarydrainage device (drains), and the total head at the overflow provisions (roof edges or scuppers) and thedesign rain load to be used by the roof framing designer, when:

a) The roof is dead-flat with interior roof drains (at mid-bay) and roof edge overflow relief as shown inFigure 24a.

b) The roof is sloped, 1⁄4 in./ft (2%), to the low-point line where roof drains are placed. Overflow relief isby scuppers set 3-1⁄2 in. (89 mm) above the low-point line at the perimeter of the roof as shown in Figure24b.

Solution (a.) Flat Roof — Figure 24a

1. Rainfall intensity (Fig. 14): i = 4 in./hr (100 mm/hr)

2. Number of drains needed (Section 2.1.1.3.5.4, Part 1.a):

n = A = 168 × 336 = 3.8 ≤ 8 (using eight 6 in. [150 mm] dia. drains)15,000 15,000

3. Flow rate needed per drain (Section 2.1.1.3.5.4, Part 1.b):

Q = 0.0104 × i × A = 0.0104 × 4 × 168 × 336n 8

Q = 294 gpm (1110 dm3/min)




4. Roof drain size needed (Section 2.1.1.3.5.4, Part 1.c):

According to Table 5, with 6 in. dia. drain, Q = 540 ≥ 294 gpm (with 150 mm dia. drain,Q = 2040 ≥ 1110 dm3/min)

5. Hydraulic head at drain inlet (Section 2.1.1.3.6.2 and Table 8):

Hydraulic head (by linear interpolation) = 2.5 + (294 − 270) (1/2)(380 − 270)

Hydraulic head = 2.6 in. (65 mm) < 3.5 in. (88 mm) roof edge height

Therefore, overflow relief allows the needed drain flow.

6. Total head at roof edge overflow provision (See Fig. 8b):

Total head = Roof edge height

Total head = 3.5 in. (88 mm)

7. Design rain load (Sect. 2.1.1.3.6.3):

Design rain load (psf) = Total head×5.2≥15 psf.

Design rain load = 3.5×5.2 = 18.2 psf (0.86 kN/m2)

8. The flat roof should support the maximum depth of water of 3.5 in. (88 mm) or 18.2 psf (0.86 kN/m2)over its entirety. The roof framing designer should check the roof for instability due to ponding based on thisload. Installing wide, low hydraulic head scuppers along the roof edges could reduce the design rain loadand also the minimum design live load for the roof framing, see Section 2.1.1.1.2. (Note: Snow load shouldnot govern at this location.)

Solution (b.) Sloped Roof — Fig. 24b

1. Items 1 through 5 from Solution (a) are applicable.

Fig. 24a. Flat roof plan for Example 5.



2. Number of scuppers needed (Section 2.1.1.3.5.4, Part 1.a):

n = A = 3.8 ≤ 4 (using four 8 in. [200 mm] min. width scuppers)15,000

3. Flow rate needed per overflow scupper (Section 2.1.1.3.5.4, Part 1.b):

Q = 0.0104 i × A = 0.0104 × 4 × 168 × 336n 4

Q = 587 gpm (2220 dm3/min)

4. Overflow scupper size needed (Section 2.1.1.3.5.4, Part 1.c):

Size of four overflow scuppers to be equivalent to the eight roof drains (Sect. 2.1.1.1,Recommendation 2.1.1.1.15.):

Needed flow capacity Q = 8 (540) = 1080 gpm (4090 dm3/min)4

Select equivalent scupper from Table 6 for needed flow capacity Q (Part 4). Assume a scupper, 7 in.(180 mm) by 24 in. (610 mm) wide with H = h. Flow capacity (under channel type) Q = 1284 ≥ 1080 gpm(4860 ≥ 4080 dm3/min).

According to Sect. 2.1.1.1, Recommendation 2.1.1.1.12, the scupper height h should be 1 in. (25 mm) higherthan the (estimated) water depth H. Check Table 6 for H = 7−1 = 6 in. (150 mm) under channel type (H < h)for the needed flow rate (Part 3):

Q = 1020 ≥ 587 gpm (Q = 3860 ≥ 2220 dm3/min)

5. Hydraulic head (H) at scupper (Section 2.1.1.3.6.2 and Table 6):

Hydraulic head (H) = 4 + 587 − 560 = 4.1 in. (103 mm)776 − 560

Fig. 24b. Sloped roof plan for Example 5.



6. Total head at scupper overflow provision (see Fig. 8b) w/scupper set 3.5 in (88 mm) above roof surface:

Total head = hydraulic head (H) + height to scupper invert

Total head = 4.1 + 3.5 = 7.6 in. (190 mm)

7. Design rain load at low-point line (overflow scuppers) [Sect. 2.1.1.3.6.3]:

Design rain load (psf) = total head (max) × 5.2 ≥ 30 psf (1.5 kN/m2)

Design rain load (max) = 7.6 × 5.2 = 39.5 psf (1.9 kN/m2)

8. The sloped roof should support a maximum depth of water of 7.6 in. (190 mm) at the low-point line ofthe roof and a maximum average depth of water, of 3.8 in. (95 mm) or 19.8 psf (1 kN/m2) across the valleyof the sloped roof. The roof framing designer should check the roof for instability in the roof valley due toponding based on the design rain loads. Above the valley of this sloped roof, applicable code permitting, theroof beams and joists may qualify for reductions in the minimum design live loads, see Section 2.1.1.1.2.

Example 6: A proposed building to be located in St. Louis, MO, has a roof 200 ft (61 m) by 400 ft (122 m)and it has six roof drains (at mid-bay). Overflow drains are placed adjacent to the primary drains and set 3 in.(75 mm) above the roof surface. The roof slopes 1⁄4 in./ft (2%) as shown in Figure 25. Size the primary andoverflow roof drains, denoting the required hydraulic head above the overflow roof drains and the total headand the design rain load to be used by the roof framing designer.

a) Rainfall intensity (i) Figure 14 i = 3.3 in./hr (84 mm/hr)

b) Total number of drains (primary and overflow) needed: (Section 2.1.1.3.5.4 Part 1.a)

n = A = 200 × 400 = 5.3 ≤ 6 (using six 6 in. dia [150 mm] drains)15,000 15,000

c) Flow rate needed per drain (primary and overflow) (Section 2.1.1.3.5.4 Part 1.b):

Q = 0.0104 × i × A = 0.0104 × 3.3 × 200 × 400n 6

Q = 458 gpm (1730 dm3/min)

Fig. 25. Roof plan for Example 6.



d) Size of the primary and overflow drains (as equivalent) and their hydraulic heads:

Select a 6 in. dia. drain (Table 5); Q = 540 ≥ 458 gpm (150 mm dia. drain;

Q = 2040 ≥ 1730 dm3/min)

Hydraulic head (Table 8 by interpolation) = 3 + (1⁄2) (458 − 380)(540 − 380)

Hydraulic head = 3.25 in. (83 mm)

e) Total head at overflow drains (Fig. 8a):

Total head = hydr. head (max) + height to overflow drain inlet

Total head (max.) = 3.25 + 3.0 = 6.25 in. (159 mm)

f) Design rain load at low point of roof (Section 2.1.1.3.6.3):

Design rain load (psf) = total head (max) × 5.2 ≥ 30 psf (1.5 kN/m2)

Design rain load (max.) = 6.25 × 5.2 = 32.5 psf (1.6 kN/m2)

g) In the low areas of the roof, at the roof drains, the design rain load (max.) will be 32.5 psf (1.6 kN/m2)but it will rapidly reduce with the roof slope of 1⁄4 in./ft (2%) away from the low areas and possibly qualifyfor live load reductions, see Section 2.1.1.1.2. The roof designer should check the roof for instability inthe low areas due to ponding based on the design rain loads.

Example 7: A proposed building to be constructed of tilt-up walls and a plywood diaphragm roof is to belocated in Santa Cruz, CA. The roof is 150 ft (46 m) by 500 ft (152 m) and it has five roof drains near theperimeter (at mid-bay) and five overflow scuppers in the parapet walls set 4 in. (100 mm) above the low-pointof the roof and 2 in. (50 mm) above the adjacent roof surface. The roof slopes, 1⁄4 in./ft (2%), from a high-pointline along one side of the building to roof drain cricket areas along the opposite side of the building (seeFig. 26). Size the (primary) roof drains and overflow scuppers, denoting the required hydraulic heads at thedrains and scuppers and the total head and design rain load to be used by the roof framing designer.

a) Rainfall intensity (Fig. 13): i = 2 in./hr (50 mm/hr)

Fig. 26. Roof plan for Example 7.



b) Total number of drains and scuppers needed (Section 2.1.1.3.5.4):

n = A (using five 6 in. dia. drains and five 8 in. wide scuppers [150 mm dia. drains and200 mm wide scuppers])15,000

c) Flow rate needed per drain and scupper (Section 2.1.1.3.5.4):

Q = 0.0104 × i × A = 0.0104 × 2 × 150 × 500n 5

Q = 312 gpm (1180 dm3/min)

d) Size of (primary) drains and their hydraulic head:

Select a 6 in. dia. drain (Table 5); Q = 540 ≥ 312 gpm (150 mm dia. drain, Q = 2040 ≥ 1180 dm3/min)

Hydraulic head at drains (Table 8 by interpolation) Hydraulic head at drains = 2.5 + (1/2) (312 − 270)380 − 270

Hydraulic head at drains = 2.7 in. (68 mm)

e) Size of overflow scuppers, equivalent to the roof drains (Sec. 2.1.1.1, Recommendation 2.1.1.1.15),and their total head (similar to Fig. 8b):

Select an equivalent scupper from Table 6 with flow Q ≥ 540 gpm (2040 dm3/min):A ‘‘channel’’ scupper,4 in. (100 mm) high by 24 in. (610 mm) wide: scupper flow capacity Q = 560 ≥ 540 gpm (2120 ≥2040 dm3/min).

According to Section 2.1.1.1, Recommendation 2.1.1.1.12, the scupper height h should be 1 in. (25 mm)higher than the (estimated) water buildup H. Checking Table 6 for h = 4-1 = 3 in. (75 mm) for the neededflow rates:

Q = 360 ≥ 312 gpm (Q = 1360 ≥ 1180 dm3/min)

f) Hydraulic head (H) at scupper (Section 2.1.1.3.6.2 and Table 6):

Hydraulic head (H) = 2 + 312 − 200 = 2.7 in. (68 mm)360 − 200

g) Total head at low point of roof = hydraulic head (H) + height to scupper from the roof’s low-point.

Total head at low-point (max.) = 2.7 in. + 4 in. = 6.7 in. (168 mm)

Total head adjacent to scuppers = hydr. head (H) + height to scupper from adjacent roof surface.

Total head at scuppers = 2.7 in. + 2 in. = 4.7 in. (119 mm)

h) Design rain load at roof drains, low-point of roof (Sect. 2.1.1.3.6.3):

Design rain load (psf) = total head (max.) × 5.2 ≥ 30 psf (1.5 kN/m2)

Design rain load (max.) at drains = 6.7 × 5.2 = 34.8 psf (1.7 kN/m2)

i) Design rain load at scuppers (Section 2.1.1.3.6.5)

Design rain load = total head (max) × 5.2 psf (Note: minimum rain load does not apply becausescupper is not at roof’s low-point.)

Design rain load at scuppers = 4.7 × 5.2 = 24.4 psf (1.2 kN/m2)

j) In the low areas at the roof drains, the design rain load will be 34.8 psf (1.7 kN/m2), but it will rapidlydecrease with the roof slope of 1⁄4 in./ft (2%) away from the low cricket areas and possibly qualify for liveload reductions, see Section 2.1.1.1.2. The roof designer should check the roof for instability in the lowareas due to ponding, based on the design rain loads.



E.3 Job Aids—Snow and Rain Loads and Drainage

NOTE: Reference to Section, Figure and Table numbers is to Section 2.0. Most referenced Figuresfrom Section 2.0 are reproduced in this Job Aid.

Part 1. Roof Snow Loads (New Construction)



Part 2. Flat-Roofs and Low-Sloped Roofs (Ref Sect 2.1.1.2.5/2.1.1.2.6)



Part 3. Sloped-Roofs (Ref Sect 2.1.1.2.7/2.1.1.2.8.1)



Part 4. Sloped-Roofs (Continued)

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



Part 5. Curved Roofs (Ref Sect 2.1.1.2.8.2)

Fig. 2. Snow loads for curved roofs.



Part 6. Valley-Roofs (Ref Sect 2.1.1.2.8.3)

Note: The above snow methodology is also applicable to multiple gable and barrel vault roofs.



Part 7. Lower-Roofs (Ref Sect 2.1.1.2.9)

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



Part 8. Lower Detached RoofsFrom Adjacent Structure or Terrain Feature

(Ref Sect 2.1.1.2.9.2)

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



Part 9. Sloped Roofs Sliding Snow Onto Lower Roofs(Ref Sect 2.1.1.2.9.2)

Fig. 6. Sliding snow load for lower roofs



Part 10. Roof Projections and Parapets(Ref Sect 2.1.1.2.9.4)



Part 11. Roof Rain Loads (New Contruction)



Part 12. Roof Drains and Scuppers (Ref Sect 2.1.1.3.5)



Part 13. Roof Drains andScuppers (Continued)(Ref Sect 2.1.1.3.5)



Part 14. Overflow Relief Provisions(Ref Sect 2.1.1.3.5.3)



Part 15. Overflow Relief Provisions (Continued)(Ref Sect 2.1.1.3.5.3)



Part 16. Sloped Roofs (Ref Sect 2.1.1.3.7)

(Job Aids — May 1998)



Documents

Factory Mutual Data Sheet 1-54 Roof Loading