Ground load becomes roof load
Building codes publish a ground snow load (psf) for every location; roofs are designed for 0.7 × that ground load, adjusted for slope, exposure and heated vs unheated structures. The calculator applies the standard 0.7 flat-roof factor — sloped roofs shed more, and this is the conservative design number.
The warnings this calculation backs up
Most residential roofs in snow country are designed for 30–50 psf ground loads. Sliding snow from an upper roof onto a lower one, drifts against parapets, and unheated garages (no melt from heat loss) all create local loads above the average — that's what the code's adjustment factors exist for. When in doubt, an engineer's letter beats a calculator.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- National Roofing Contractors Association — installation standards for steep- and low-slope roof systems.
- FEMA Building Science — wind and snow load guidance for residential roofs.
- American Society of Civil Engineers — ASCE 7, the source of the ground snow load maps used in roof design.
Figures, formulas and reference tables last verified: 19 September 2026.