Calculate roof snow load with our free ASCE 7 calculator. Get flat and sloped roof loads, snow depth, total roof weight, and code factors instantly.
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Snow load matters more than most roof checks. A roof that handles rain fine can fail under two feet of wet snow. This snow load calculator applies ASCE 7 formulas to turn your ground snow load into a roof snow load. It factors in exposure, thermal condition, roof slope, and surface type. You’ll see flat roof load, sloped roof load, snow depth, and total weight on the roof. The tool follows ASCE 7-22 and 7-16, including the importance factor rules for older editions. Builders use it for framing takeoffs. Homeowners use it to check whether a roof can handle a heavy winter.
Pick your unit system. Imperial uses psf. Metric uses kN/m².
Choose a location preset or enter ground snow load by hand. Presets cover 20 U.S. and Canadian cities.
Select a code edition. ASCE 7-22 drops the importance factor. ASCE 7-16 keeps it.
Enter roof slope. Use rise:12, degrees, or percent.
Pick an exposure category. Options run from fully exposed to sheltered, plus above treeline.
Set the thermal condition. Heated, cold ventilated, unheated, below freezing, or greenhouse.
Choose the roof surface. Non-slippery covers asphalt and shingle. Slippery covers metal and slate.
Add building length and width if you want total roof load. Then read the result panel.
The core formula converts ground snow load to flat roof snow load, then adjusts for slope. ASCE 7 uses three factors: exposure, thermal, and importance.
Formula: Flat Roof Snow Load (Pf) = 0.7 × Ce × Ct × Is × Pg
Formula: Sloped Roof Snow Load (Ps) = Cs × (Pf + Rain Surcharge)
Formula: Minimum Pf (when Pg > 20 psf) = 20 × Is
Formula: Slope Factor (Cs) for non-slippery = 1.0 up to 30°, then (60 − slope) ÷ 40
Formula: Slope Factor (Cs) for slippery = 1.0 up to 15°, then (60 − slope) ÷ 45
Formula: Snow Depth = Ps ÷ Snow Density
Exposure factor (Ce) accounts for wind removing snow from the roof. A fully exposed roof in open terrain gets a lower factor than a sheltered roof in a city. Thermal factor (Ct) accounts for heat escaping through the roof. A heated building melts snow faster, so its factor is lower than an unheated barn. Importance factor (Is) applies only under ASCE 7-16 and raises the load for high-occupancy or essential buildings. ASCE 7-22 removed it and uses separate ground snow maps by risk category.
The slope factor drops the load as the roof gets steeper. Non-slippery roofs keep full load up to 30°. Slippery roofs lose load above 15°. Both reach zero at 60°, where snow can’t stick. If ground snow load is 20 psf or less and the roof is nearly flat, the tool adds a 5 psf rain-on-snow surcharge. Snow density is assumed at 300 kg/m³, which matches settled snow, not fresh powder. Depth comes from dividing the sloped load by that density.
A 40-foot by 30-foot building in New York City. Ground snow load 30 psf. Roof slope 6:12. Exposure B, partially exposed. Heated structure. ASCE 7-22. Non-slippery roof surface.
Step 1 – Slope angle. 6:12 = atan(6 ÷ 12) = 26.57°.
Step 2 – Slope factor. 26.57° is below 30°, so Cs = 1.0.
Step 3 – Exposure factor. B, partially exposed = 1.0.
Step 4 – Thermal factor. Heated = 1.0.
Step 5 – Importance factor. ASCE 7-22 removed it, so Is = 1.0.
Step 6 – Flat roof snow load. Pf = 0.7 × 1.0 × 1.0 × 1.0 × 30 = 21 psf.
Step 7 – Minimum check. Pg > 20 psf, so minimum Pf = 20 psf. 21 psf passes.
Step 8 – Rain surcharge. Pg > 20 psf, so no rain surcharge.
Step 9 – Sloped roof snow load. Ps = 1.0 × 21 = 21 psf.
Step 10 – Snow depth. 21 psf ÷ 18.73 psf/ft = 1.12 ft, or 0.34 m.
Step 11 – Total roof load. Plan area 40 × 30 = 1,200 ft². Total load = 21 × 1,200 = 25,200 lbs.
Interpretation: The roof needs to carry 21 psf of snow. That’s about 13.5 inches of settled snow across the whole roof, or 25,200 pounds total. If the roof were slippery metal, Cs would still be 1.0 at 26.57° because the slippery threshold is 15°. If the roof were steeper than 30°, the load would drop.
The weight of snow on the ground in a given region. ASCE 7 maps list values by location. New York City is 30 psf. Buffalo is 50 psf. Anchorage is 60 psf. It’s the starting point for roof snow load.
Multiply ground snow load by 0.7, then by exposure, thermal, and importance factors. That gives flat roof load. For sloped roofs, multiply by the slope factor. A 30 psf ground load with all factors at 1.0 gives 21 psf.
Flat roof load ignores pitch. Sloped roof load applies the slope factor, which reduces load above 30° on non-slippery roofs or above 15° on slippery roofs. At 60°, the sloped load reaches zero.
Ce accounts for wind removing snow. Fully exposed roofs in open terrain get 0.9 or lower. Sheltered roofs in cities get 1.2. The calculator loads these values from ASCE 7 exposure categories.
Ct accounts for heat melting snow. Heated buildings use 1.0. Cold ventilated roofs use 1.1. Unheated structures use 1.2. Below-freezing buildings use 1.3. Greenhouses use 0.85.
Cs reduces snow load as pitch rises. Non-slippery roofs keep full load up to 30°, then drop to zero at 60°. Slippery roofs keep full load up to 15°, then drop to zero at 60°.
About 1.12 feet, or 13.5 inches, using a settled snow density of 300 kg/m³. Fresh powder would be deeper. Wet snow would be shallower. The calculator uses 300 kg/m³ as a middle-ground figure.
Ground snow load, exposure, thermal condition, importance, slope, and surface type all matter. Roof geometry also plays a role. Valleys and dormers collect more snow. Drifting loads aren’t modeled.
Use it when comparing locations, roof pitches, or surface types. Manual math works for one calculation. The tool saves time when you need to check several scenarios or avoid factor table errors.
Mixing units across inputs. Forgetting the minimum Pf when ground snow load exceeds 20 psf. Using the wrong slope factor threshold for a slippery roof. And ignoring rain-on-snow surcharge on low-slope roofs in mild climates.
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