By Zencalcy Editorial TeamLast reviewed About us
Roof snow load starts with the ground snow load for your location and is reduced for exposure, thermal effects and pitch. Wet snow weighs about 20 lb per cubic foot, so a foot of wet snow on a 1,500 square foot roof is over 22 tons — the reason roofs fail after rain-on-snow events.
Roof snow load and total weight on the structure.
pf = 0.7 × Ce × Ct × pg ps = Cs × pf
This is the ASCE 7 flat-roof snow load with a slope factor Cs of 1.0 up to 30° and 0.8 above. Exposure Ce is 1.2 for sheltered, 1.0 for normal and 0.8 for windy sites; thermal Ct is 1.0 for a cold roof and 1.2 for a warm roof. Check with the local building department — this is an estimate, not a design.
| Symbol | Meaning |
|---|---|
| pg | Ground snow load. |
| Ce, Ct | Exposure and thermal factors. |
| ps | Sloped roof snow load. |
pf = 0.7 × 1.0 × 1.0 × 40 = 28 psf; at 26.57° the slope factor is 1.0, so the roof carries 28 psf — 42,000 lb over 1,500 sq ft.
Continue with the Roof Pitch Calculator, Rafter Length Calculator, Roof Truss Calculator, or the Roofing Calculator — or browse more roofing and all construction calculators.
Fresh dry snow is 5–10 lb/ft³, settled snow 15–20, and wet snow or ice 40–60. A cubic foot is a 1 ft square one foot deep.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026