About Snow Load Calculator (ASCE 7-22)
The snow load calculator determines roof design snow loads per ASCE 7-22 Chapter 7. Starting from the mapped ground snow load pg, it applies the exposure factor Ce, thermal factor Ct, and importance factor Is to get the flat roof load pf = 0.7 Ce Ct Is pg, then reduces it by the slope factor Cs for the balanced sloped roof load ps = Cs pf. For low-slope roofs under 15 degrees it also checks the code minimum pm and reports which load governs.
An optional drift mode adds the leeward drift surcharge that forms against a roof step, parapet, or rooftop projection: the snow density, balanced depth hb, drift height hd from the upwind fetch, drift width w, and the peak surcharge pd = hd times gamma. Results are reported in psf with kPa conversions alongside, and a chart shows how the balanced load falls off with roof slope.
How It Works
- Enter the ground snow load pg from the ASCE 7 maps or your local jurisdiction, then select the exposure factor Ce for terrain and sheltering, the thermal factor Ct for the roof heat condition, and the risk category, which sets the importance factor Is.
- The calculator computes the flat roof load pf = 0.7 Ce Ct Is pg and, for slopes below 15 degrees, the minimum load pm = Is pg when pg is 20 psf or less, otherwise 20 Is.
- Enter the roof slope and surface type. The slope factor Cs is read from the simplified linear ASCE 7 Figure 7.4-1 curves (warm slippery 1.0 to 5 deg, warm other to 30 deg, cold slippery to 10 deg, cold other to 45 deg, all reaching 0 at 70 deg) and the balanced load is ps = Cs pf. The design load is the larger of ps and pm where the minimum applies.
- In drift mode, enter the upwind fetch lu and the obstruction height. The tool computes gamma = 0.13 pg + 14 (max 30 pcf), hb = ps/gamma, hd = 0.43 lu^(1/3) (pg+10)^(1/4) - 1.5, the drift width w, and the surcharge pd = hd gamma, capping the drift at the clear height hc when it would overtop the projection.
Worked Example
A heated office building (risk category II, Is = 1.0) in a pg = 30 psf region has a fully exposed roof with Ce = 1.0 and Ct = 1.0. The flat roof load is pf = 0.7 x 1.0 x 1.0 x 1.0 x 30 = 21 psf. At a 10 degree slope on a non-slippery membrane the warm-roof curve gives Cs = 1.0, so ps = 21 psf. Because the slope is under 15 degrees the minimum pm = 20 x 1.0 = 20 psf is checked, and ps = 21 psf governs (about 1.01 kPa). Against a 6 ft roof step with a 100 ft upwind fetch, the snow density is gamma = 0.13 x 30 + 14 = 17.9 pcf, hb = 21/17.9 = 1.17 ft, and hd = 0.43 x 100^(1/3) x 40^(1/4) - 1.5 = 3.52 ft, giving a drift surcharge pd = 3.52 x 17.9 = 63 psf over a width w = 4 x 3.52 = 14.1 ft.
Formulas
- Flat roof snow load
pf = 0.7 * Ce * Ct * Is * pg- Minimum snow load (slope < 15 deg)
pm = Is * pg (pg <= 20 psf); pm = 20 * Is (pg > 20 psf)- Sloped roof (balanced) snow load
ps = Cs * pf- Snow density
gamma = 0.13 * pg + 14 <= 30- Leeward drift height
hd = 0.43 * lu^(1/3) * (pg + 10)^(1/4) - 1.5- Drift surcharge and width
pd = hd * gamma; w = 4*hd (hd <= hc); w = 4*hd^2/hc <= 8*hc (hd > hc)
Standards & References
- ASCE 7-22 Chapter 7 -- Snow Loads
- ASCE 7-22 Table 1.5-2 -- Importance factors
- ASCE 7-22 Figure 7.6-1 -- Drift loads on lower roofs
- IBC 2024 Section 1608 -- Snow loads
Frequently Asked Questions
What is the flat roof snow load formula in ASCE 7?
ASCE 7 computes the flat roof snow load as pf = 0.7 Ce Ct Is pg. The 0.7 factor converts the mapped ground snow load to a roof load, Ce accounts for wind exposure of the site and roof, Ct for heat loss through the roof, and Is for the consequence of failure via the risk category. All balanced roof loads derive from this value.
When does the minimum snow load pm govern?
The minimum load applies to low-slope roofs, generally monoslope, hip, and gable roofs with slopes under 15 degrees. It equals Is times pg when pg is 20 psf or less, and 20 Is otherwise. It represents a uniform load case for light-snow regions and governs whenever it exceeds the sloped balanced load ps, which this calculator flags automatically.
What is the slope factor Cs and why does the roof surface matter?
Cs reduces the balanced load on steeper roofs because snow slides off. Slippery surfaces such as metal, slate, and smooth membranes shed snow at much lower slopes, so their Cs curve starts dropping at 5 degrees (warm roof) or 10 degrees (cold roof), while other surfaces hold snow up to 30 or 45 degrees. Every curve reaches Cs = 0 at 70 degrees. Unobstructed slippery surfaces only qualify if snow can actually slide clear of the eave.
What is a snow drift surcharge and when must I design for it?
Wind transports snow from an upwind roof and deposits it against steps, parapets, and rooftop units, forming a triangular drift that can be several times the balanced load. ASCE 7 requires the leeward drift check whenever the clear height hc above the balanced snow is at least 20 percent of the balanced depth hb. The drift height depends on the upwind fetch lu and pg, and the peak surcharge is pd = hd times the snow density.
How do I convert snow loads from psf to kPa?
Multiply pounds per square foot by 0.04788 to get kilopascals, so a 21 psf roof load is about 1.01 kPa and a 40 psf load is about 1.92 kPa. The calculator reports both unit systems side by side so results can be checked against SI-based codes and load-combination software.
What exposure and thermal factors should I use?
Ce depends on the terrain category and how sheltered the roof is: fully exposed roofs in open terrain can use values as low as 0.7, typical partially exposed roofs use 1.0, and sheltered roofs in wooded terrain go up to 1.2. Ct is 1.0 for heated structures, 1.1 for structures kept just above freezing, 1.2 for unheated and open-air structures, and 0.85 for continuously heated greenhouses, which melt snow off the roof.