Snow Loading Analysis Calculator

ASCE 7-05 Code for Buildings with Flat or Low Slope Roofs (<= 5 deg. or 1 in./ft.)
for Balanced Snow, Drift, and Rain-on-Snow Surcharge Loadings

Input Data

Building Classification [?]   Table 1-1, page 3
Ground Snow Load, pg psf Figure 7-1, pages 84-85 and Table 7-1, page 92
Length of High Roof, Lu ft. Length of Roof Upwind of the Snow Drift
Length of Low Roof, LL ft. Length of Roof Downwind of the Snow Drift
Horiz. Dist. from Eave to Ridge, W ft. Horizontal Distance from Eave to Ridge
Type of Roof   Type of Roof = Monoslope, Gable, or Hip
Obstruction Height, ho ft. High Roof - Low Roof Elevations
Roof Slope in./ft. S = Rise per foot of Run
Exposure Factor, Ce [?]   Table 7-2, page 92
Thermal Factor, Ct [?]   Table 7-3, page 93


Roof Angle, q deg. q = ATAN(S/12)
Importance Factor, I   Table 7-4, page 93
Snow Density, g pcf g = 0.13*pg+14 <= 30  (Eqn. 7-3, page 83)
Flat Roof Snow Load, pf psf pf = 0.7*Ce*Ct*I*pg  (Eqn. 7-1, page 81)
pf(min) psf pf(min) = pg*I  for pg <= 20 ,  pf(min) = 20*I  for pg > 20
pf(use) psf pf(use) = maximum of: pf  or  pf(min)  (Section 7.3, page 81)
Balanced Snow Load Ht., hb ft. hb = pf(use)/g  (Section 7.1, page 81)
Clear Height, hc ft. hc = ho-hb >= 0  (Section 7.1, page 81)
Leeward Drift Height, hdL ft. hdL = 0.43*Lu^1/3*(pg+10)^1/4-1.5,  with Lu>=25'  (Figure 7-9)
Windward Drift Height, hdw ft. hdw = 0.75*(0.43*LL^1/3*(pg+10)^1/4-1.5),  with LL>=25'
Design Drift Height, hd ft. hd = minimum of: (maximum of: (hdL  or  hdw))  or  hc
Ratio, hc/hb   If hc/hb >= 0.2 , then snow drifts are required to be applied
Drift Length, w ft. If hd <= hc:  w = 4*hd ,  if hd> hc:  w = 4*hd^2/hc  (Sect. 7.7.1)
Drift Length, w(max) ft. w(max) = minimum of:  8*hc  or  LL
Drift Length, w(use) ft. w(use) = minimum of:  w  or  w(max)
Wt. of Drift at High End, pd psf pd =hd*g  (maximum value)
Rain-on-Snow Surch., prs psf prs = 5.0 psf  when 0 < pg <=20 and q < W/50)  (Sect. 7.10)
Balanced Snow Load, pf(bal) psf pf(bal) = maximum of: pf+prs  or  pf(min)
*Total Snow Load, p(total) psf p(total) = pf(bal)+pd

Configuration of Snow Drift on Lower Roof

(Length of High Roof)             
Surcharge Load
Due to Drifting
   Rain-on-Snow Surch.
Balanced Snow Load
(Length of Low Roof)

Determine Value, pd, Along Snow Drift Load:

Distance, x ft.  
pd @ x psf  

Snow Drift Load

Occupancy Category of Buildings and Other Structures for Flood, Wind, Snow, Earthquake, and Ice Loads
Nature of Occupancy
Occupancy Category
Buildings and structures that represent a low hazard to human life in the event of failure including, but not limited to:
- Agriculture facilities
- Certain temporary facilities
- Minor storage facilities
Buildings and other structures except those listed in Categories I, III and IV II
Buildings and other structures that represent a substantial hazard to human life in the event of failure including, but not limited to:
- Buildings and other structures where more than 300 people congregate in one area
- Buildings and other structures with day-care facilities with capacity greater than 150
- Elementary or secondary school facilities with capacity greater than 250
- Colleges & adult education facilities with a capacity greater than 500
- Health care facilities with a capacity greater than 50 resident patients but not having surgery or emergency treatment facilities
- Jails and detention facilities
Buildings and other structures, not includes in Occupancy Category IV, with potential to cause substantial economic impact and/or mass disruption of day-to-day civilian life in event of failure, including, but not limited to:
- Power generating stations, water treatment facilities, sewage treatment facilities, and telecommunication centers
- Buildings and structures not included in Category IV containing sufficient quantities of toxic, explosive, or other hazardous materials dangerous to the public if released
Buildings and other structures designated as essential facilities including, but not limited to:
- Hospitals and health care facilities having surgery or emergency treatment facilities
- Fire, rescue and police stations and emergency vehicle garages
- Designated earthquake, hurricane or other emergency shelters
- Designated emergency preparedness, communication, and operation centers and other facilities required for emergency response
- Power-generating stations and other public utility facilities required in an emergency
- Ancillary structures required foroperation of Category IV structures during an emergency
- Aviation control towers, air traffic control centers and emergency aircraft hangars
- Water storage facilities and pump structures required to maintain water pressure for fire suppression
- Buildings and other structures having critical national defense functions
- Buildings and structures containing extremelyhazardous materials where quantity of material exceeds a threshhold quantity established by authority having jurisdiction
Terrain Category Fully Exposed Partially Exposed Sheltered
B (see Sect.6.5.6) 0.9 1.0 1.2
C (see Sect.6.5.6) 0.9 1.0 1.1
D (see Sect.6.5.6) 0.8 0.9 1.0
Above treeline in windswept mountain areas. 0.7 0.8 N.A.
In Alaska, areas where trees do not exist within 2 mile radius of site. 0.7 0.8 N.A.

The terrain category and roof exposure condition chosen shall be representative of anticipated conditions during life of structure. An exposure factor shall be determined for each roof of structure.

Partially Exposed: All roofs except as indicated below.
Fully Exposed: Roofs exposed on all sides with no shelter* afforded by terrain, higher structures or trees. Roofs that contain several large pieces of mechanical equipment, parapets which extend above height of balanced snow load (hb), or other obstructions are not in this category.
Sheltered: Roofs located tight in among conifers that qualify as obstruction.

*Obstructions within distance of 10ho provide "shelter", where ho is height of obstruction above roof level. If only obstructions are a few deciduous trees which are leafless in winter, "fully exposed" category shall be used except for terrain Category "A". Note that these are heights above roof. Heights used to establish the Terrain Category in Section 6.5.3 are heights above ground.

Thermal Condition* Ct
All structures except as indicated below: 1.0
Structures kept just above freezing and others with cold, ventilated roofs in which the thermal resistance (R-value) between the ventilated space and heated space exceeds 250 F*h*ft^2/Btu (4.4 K*m^2/W). 1.1
Unheated structures and structures intentionally kept below freezing. 1.2
Continuously heated greenhouses** with roof having thermal resistance (R-values) less than 2.00 F*h*ft^2/Btu (0.4 K*m^2*W). 0.85

* These conditions shall be representative of the anticipated conditions during winters for life
of structure.
** Green houses with constantly maintained interior temperature of 50oF (10oC) or more at any
point 3 feet above floor level during winters and having either maintenance attendant on duty
at all times or temperature alarm system to provide warning in event of heating failure.

I 0.8
II 1.0
III 1.1
IV 1.2
*See Section 1.5 and Table 1-1.
Minimum values of "pf" shall apply to monoslope roofs with slopes < 15 degrees, and gable and hip roofs with slopes less than the larger of 2.38o and (70/W)+0.5, where "W" = horizontal distance from eave to ridge, in feet.
The Clear Height,"hc", is determined based on the assumption that the upper roof is blown clear of snow in the vicinity of the drift. This is a reasonable assumption when the upper roof is nearly flat.
Leeward drift results from snow blown off a high roof onto a lower roof. Typically, the leeward drift height, "hdL", is greater than the windward drift height, "hdw", unless the length of the lower roof is considerably longer than the length of the upper roof.
Windward drift results from snow blown against a projection or wall below a high roof. Typically, the windward drift height, "hdw", is less than the leeward drift height, "hdL", unless the length of the lower roof is considerably longer than the length of the upper roof.
Per ASCE 7-05, Section 7.10:
For locations where 0< pg <= 20 psf, all roofs with a slope < W/50 (with W in ft.) shall have a 5 psf rain-on-snow surcharge. This rain-on-snow augmented design load applies only to the balanced load case and need not be used in combination with drift, sliding, unbalanced, or partial loads.
The maximum value of the total snow load at the high end shall be the superimposed values of drift, balanced snow load and rain-on-snow surcharge (if applicable).
Note: Per Section 7.10, the rain-on-snow surcharge need NOT be combined with the snow drift.


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Disclaimer: This calculator is not intended to be used for the design of actual structures, but only for schematic (preliminary) understanding of structural design principals. For the design of an actual structure, a competent professional should be consulted.

‘Calculations courtesy of Alex Tomanovich, PE ’