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Snow Load Calculator

When should I shovel snow off my roof? What is the weight of the snow covering my house? Find the answers to these questions with our snow load calculator!

From your local code map. 20–40 psf is typical in the northern US.
Clear
Design roof snow load16.584 psf794.1 Pa — 81 kg/m²
Flat-roof snow load21 psf0.7 × 1 × 1 × 30
Slope factor Cs0.7897snow sheds at 26.57°
Exposure factor Ce1
Thermal factor Ct1
Roof angle26.565°6 in 12
Total snow weight on the roof19,901 lb9.95 US tons over 1,200 sq ft of footprint
Load acts on the footprint1,200 sq ftnot the 1,342 sq ft of sloped surface — snow falls vertically
Equivalent depth of fresh snow81 cmat about 100 kg/m³ for fresh snow
Equivalent depth of packed snow20.2 cmat about 400 kg/m³ once settled
Rain-on-snow and driftingnot includedvalleys, parapets and lower roofs collect drifts that can double the local load

The method

pf = 0.7 × Ce × Ct × Is × pg, reduced by the slope factor Cs

Snow falls vertically

Which means the load acts on the roof’s horizontal projection, not on its sloped area. A steep roof has more surface but catches no more snow — and if it is steep and slippery, it holds less, because the snow slides off. That is what the slope factor accounts for.

These are the balanced-load figures. The loads that actually collapse roofs are usually unbalanced: drifts against a parapet or a taller wall, snow sliding from an upper roof onto a lower one, or an ice dam holding meltwater at the eaves. Those can be several times the balanced figure over a small area, and they need proper design.

Note that a heated building gets a lower factor than an unheated one, because heat loss through the roof melts the underside of the snowpack. A well-insulated modern roof is closer to the cold case than an old draughty one.