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Roof Snow Load Calculator

Convert ground snow load to a flat-roof design snow load using the ASCE 7 factors, and get the total load on your roof.

Ground snow load (pg)

Exposure factor (Ce)

Thermal factor (Ct)

Importance factor (Is)

Roof plan area (optional)

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How it works

pf = 0.7 × Ce × Ct × Is × pg · Total load = pf × roof plan area

The ASCE 7 standard converts the ground snow load (pg) from your local map into a flat-roof snow load (pf) with a base factor of 0.7 and three adjustments: exposure (Ce) for how wind-swept the site is, thermal (Ct) for how warm the roof runs, and importance (Is) for how critical the building is. The result is the design snow load per square foot; multiply by the roof’s plan area for the total weight it must carry. Steep or slippery roofs shed some of this through a separate slope factor not applied here.

Worked example

With a 30 psf ground load, a partially exposed (Ce = 1.0), heated (Ct = 1.0), normal-importance (Is = 1.0) building, pf = 0.7 × 1.0 × 1.0 × 1.0 × 30 = 21 psf. Over a 1,500 sq ft roof that is 31,500 lb, or about 15.75 tons.

Sources: ASCE 7 Snow Loads (Wikipedia)

Frequently asked questions

What is the difference between ground and roof snow load?

Ground snow load (pg) is the weight of snow measured on the ground from historical records. Roof snow load (pf) is what actually rests on the roof after adjusting for wind, roof temperature and building importance — usually a bit less than the ground value on a flat, heated roof.

Where do I get my ground snow load?

Ground snow load comes from the ASCE 7 snow map or your local building department, and varies enormously by region and elevation — from near zero in the south to well over 100 psf in mountain areas. Always use the mapped value for your exact site.

Does roof slope reduce the snow load?

Yes. Steeper and slicker roofs shed snow, so ASCE 7 applies a slope factor (Cs) to get the sloped-roof load, and drifting can add load in valleys and against walls. This calculator gives the flat-roof load; a structural engineer applies slope and drift for the final design.

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This calculator is for educational purposes. Double-check important results before acting on them.