Roof Snow Load Calculator

Ground snow load vs. roof snow load

The starting point for any snow load calculation is the ground snow load (pg) — the weight of snow, in pounds per square foot (psf), that accumulates on flat open ground at your site. This value comes from your local building department or the ASCE 7 ground snow load maps, and it ranges from near 0 psf in the Deep South to 70 psf or more in mountainous and northern regions. The roof snow load is what actually rests on the roof after adjusting for wind exposure, heat loss through the roof, roof slope, and how critical the building is. Because wind and sliding remove some accumulation, the sloped-roof load is usually lower than the ground snow load.

Always use the site-specific ground snow load adopted by your jurisdiction — regional maps can vary block by block in mountainous terrain.

The ASCE 7 flat-roof formula and the 0.7 factor

ASCE 7 computes the flat-roof snow load as pf = 0.7 × Ce × Ct × I × pg. The basic 0.7 factor reflects the reality that a roof almost never holds the full ground snow load — wind scours some of it off before it settles. The exposure factor (Ce) captures how wind-swept the site is, the thermal factor (Ct) accounts for heat escaping through the roof and melting the underside of the snowpack, and the importance factor (I) increases the design load for essential facilities like hospitals and emergency shelters that must stay standing.

Slope factor: why steeper roofs carry less snow

The sloped-roof load is ps = Cs × pf, where Cs is the slope factor. Snow slides off steep roofs before it can fully accumulate, so Cs drops as pitch increases. In the simplified model used here, roofs up to about 30 degrees keep Cs = 1.0, then Cs decreases linearly to 0 at roughly 70 degrees. Real ASCE 7 tables treat slippery surfaces — metal and membrane roofs — differently from non-slippery shingle roofs, letting their Cs begin dropping at a lower angle because they shed snow more readily. This calculator keeps a single linear curve for simplicity, so treat slippery-surface results as conservative.

Drift and unbalanced loads are not included

This tool estimates only the balanced, uniform snow load across the roof. It does not calculate snow drift at walls, parapets, and roof steps; unbalanced loads on gable and hip roofs; sliding-snow surcharge from adjacent higher roofs; or rain-on-snow surcharge. These localized loads can be more than double the balanced load and are often what causes structural failures. Drift and unbalanced cases must be evaluated by a licensed structural engineer — never skip them on a real project.