Roof snow load is a structural-safety calculation, not a material takeoff: how much weight of snow a roof must carry, in pounds per square foot. It starts from the local ground snow load and adjusts for the roof's exposure, temperature, slope, and importance. This guide explains the factors in plain terms — though final structural design always belongs to a licensed engineer.
Snow load is the downward weight that accumulated snow imposes on a roof, expressed in pounds per square foot (psf). It is one of the design loads a roof structure must resist, alongside the roof's own weight (dead load) and occupancy or wind loads. Underestimating it risks collapse; over-building it wastes material — which is why codes prescribe a specific method.
Everything starts with the ground snow load (pg) — the weight of snow on the ground for your specific location, derived from decades of weather data and published in code maps (and local amendments). It ranges from near zero in the south to 100+ psf in high mountains and snowbelt regions. You cannot guess this; look it up for the exact site, because it varies sharply over short distances in mountainous areas.
pg comes from the official code map or the local building department for the exact site — not a regional average. Mountain towns often have site-specific values higher than the mapped figure.
The ASCE 7 standard converts ground load to a design roof load. The balanced flat-roof snow load is:
Three factors fine-tune the load for the specific building:
| Factor | What it accounts for |
|---|---|
| Ce — Exposure | Wind exposure of the site (sheltered roofs hold more snow; ~0.7–1.2) |
| Ct — Thermal | Whether the building is heated (a warm roof melts snow; ~1.0–1.2) |
| Is — Importance | Building criticality (hospitals/assembly carry a higher factor than sheds) |
A heated, wind-exposed house has lower factors than an unheated, sheltered structure of high importance — the same ground load can produce quite different roof loads.
Steep roofs shed snow, so the balanced load is reduced by a slope factor Cs that decreases as pitch increases (a very steep, slippery roof can approach zero retained snow). Sloped-roof load ps = Cs × pf. But snow sliding off an upper roof onto a lower one adds load there, so sliding cuts both ways.
Ground load pg = 40 psf, exposed heated home: Ce = 1.0, Ct = 1.0, Is = 1.0. Flat-roof pf = 0.7 × 1.0 × 1.0 × 1.0 × 40 = 28 psf. A 6/12 roof might apply Cs ≈ 1.0 (little reduction); a 12/12 metal roof sheds much more. These numbers are illustrative — your jurisdiction's values govern.
The balanced load is rarely the worst case. Wind piles snow into drifts against walls, parapets, and at roof steps, where the local load can be two to three times the balanced figure. Valleys, dormers, and the lee side of gable and hip roofs collect unbalanced loads. These concentrated loads often govern the structural design even though they cover only part of the roof.
A snow-load estimate informs planning, but sizing rafters, trusses, and connections to carry it is the work of a licensed structural engineer using the full code provisions for your site. Treat the calculator's output as a starting figure, not a design.
Use the snow load calculator to apply the ASCE 7 factors to a ground snow load, then have a licensed engineer confirm the structural design — especially where drifting or unbalanced loads may govern.