Roof Snow Load Calculator
"It's a lot of snow" becomes a number: pounds per square foot, against the limit your roof was built for. The sound of creaking has a unit, it's here.
Reference limits: modern residential trusses are typically designed for 20–30 psf snow load in most of the US (snow-belt counties 40–70 psf). A flat garage/carport is often 20–25 psf. Check your county's building code design value, it's the number that beats "the eaves are sagging."
How the math works
- Load (psf) = depth(in) ÷ 12 × density(lb/ft³). 18 in of wet snow = 18/12 × 12 = 18 psf. Drift adds 15% (pitched) / 40% (flat).
- Pitch unloading: at 4:12 and steeper, sliding sheds 25–60% of the load from the roof structure, the tool shows both with-shed and worst-case values. Warm roofs shed, cold roofs stick.
- Verdict bands: green < 70% of 25 psf; caution 70–100%; danger > design limit. Flat roofs get judged at face value, no shed credit.
- Warning signs (act regardless of the number): cracking/creaking, sagging ridge line, doors that suddenly stick, crushed drywall corners. Evacuate first, shovel from the ground/ladder after.
How many pounds per square foot is snow on a roof?
Fresh powder ≈ 4 psf per foot of depth; typical wet snow ≈ 12 psf per foot; ice is 30 psf per foot. 18 in of wet snow is ~18 psf, already at the margin of a 20 psf flat-roof design.
At what depth should I shovel my roof?
Roof shoveling is for the flat sections (porch, garage, addition) at ~6–8 in of wet snow; pitched roofs shed through the eaves, clear the gutters and the lower 3 ft, never climb onto a loaded roof. Icicle dams on the eave line are the same problem in a different outfit.
Is a flat roof more dangerous in snow?
Yes, flat and low-slope roofs keep every pound (no shed credit), often with lower design loads, and the failure happens at the drain/drift line. The 2–4 in ponding on a flat roof after melt-refreeze is the 30 psf ice crust the calculator prices.