How to Calculate Roof Snow Load (ASCE 7-22, Step by Step)

Roof snow load is the design weight of snow your roof structure must carry. In the United States it is set by Chapter 7 of ASCE/SEI 7, the standard nearly every building code adopts. The good news: the core calculation is only a few steps, and this guide walks through each one.

Step 1: Find your ground snow load (Pg)

Everything starts with the ground snow load, Pg, in pounds per square foot (psf). It is the weight of snow that accumulates on open ground at your site, and it is fixed by your location. Get it from the ASCE 7 Hazard Tool or your local building department; many jurisdictions publish an adopted county or town value. Our per-state pages give a planning range, but the permit value is the one your jurisdiction adopts.

Step 2: Convert to the flat-roof load (Pf)

ASCE 7-22 Equation 7.3-1 converts ground snow to the flat-roof snow load: Pf = 0.7 × Ce × Ct × Is × Pg. The 0.7 base factor accounts for snow that blows or melts off a roof. Ce is the exposure factor (how wind-exposed the roof is), Ct is the thermal factor (how warm the roof is underneath), and Is is the importance factor (how critical the building is).

Step 3: Apply the slope factor (Cs) for a pitched roof

Sloped roofs shed snow, so multiply Pf by the slope factor Cs to get the sloped balanced load Ps = Cs × Pf. Cs is 1.0 up to a breakpoint slope and then decreases linearly to zero at 70°. Warm and slippery roofs shed sooner, so their breakpoint is lower.

Step 4: Check the minimum and rain-on-snow

Low-slope roofs (under 15°) must also carry a minimum load: Is × Pg if Pg ≤ 20 psf, otherwise 20 × Is. And in mild-winter areas (Pg ≤ 20) a nearly flat roof gets a 5 psf rain-on-snow surcharge. Your design load is the largest governing case.

Plug your numbers into the RoofHelm calculator and it does all four steps and shows every factor. For gable, stepped and multi-level roofs, also check the unbalanced, drift and sliding cases, any of which can govern.