Snow load is the weight of accumulated snow a roof must carry, measured in pounds per square foot (psf). This calculator gives you two ways to estimate it — a quick depth-and-density method and an ASCE-style method using your region's ground snow load — and then converts the result into a total weight over your roof. It is a planning aid for homeowners, builders, and DIYers, not a substitute for engineered design.
Snow density: why depth alone isn't enough
A foot of light, fluffy powder and a foot of wet, packed snow weigh very different amounts. That is why this tool asks for snow type, not just depth. Fresh dry snow adds roughly 0.5 psf per inch, settled snow about 0.9, packed snow 1.25, wet snow 2.1, and an ice layer up to 5.0 psf per inch. Old snowpacks are usually a mix — they compress and absorb meltwater over time, so a two-week-old drift can weigh several times what it did when it fell. When in doubt, pick the heavier type; snow load failures almost always come from underestimating density.
Why slope and exposure matter
Two roofs in the same town can carry very different loads. A steep metal roof sheds snow quickly, so its design load is reduced by the slope factor Cs — above about 30° on a warm roof the load starts dropping, reaching zero near 70°. A shallow roof holds nearly everything that falls. Exposure matters too: a house standing alone in an open, windy field loses snow to the wind (a lower exposure factor), while one tucked among trees and taller buildings collects drifting snow (a higher factor). Heat loss plays a role as well — a well-heated house melts snow from below, while an unheated barn keeps the full load.
Ground snow load vs. roof snow load
Building codes publish a ground snow load (pg) for each location — the weight of snow on flat, open ground. The roof rarely carries that full amount. The ASCE method starts by multiplying pg by 0.7 to convert ground to roof, then adjusts up or down for exposure, heat, building importance, and slope. The result, ps, is the load the structure is actually designed for. If you only know your ground snow load, use the ASCE Factors tab; if you can see and measure the snow, use the From Depth tab and compare.
What this calculator does not model
Real snow loading is messier than any single formula. This tool deliberately leaves out several effects that require engineering judgment: drifting against walls and parapets, sliding snow piling onto lower roofs, unbalanced loads on gable and hip roofs, rain-on-snow surcharge, and code minimum load requirements. Any of these can dramatically increase the local load. Use the result as a first-pass estimate and a way to build intuition — then have a licensed structural engineer and your local building department confirm anything that affects safety.