What is roof snow load?
Roof snow load is the downward weight that accumulated snow places on a roof, expressed in pounds per square foot (psf). Knowing it helps homeowners, builders and engineers judge whether a roof is at risk of overload before deciding to shovel or reinforce. This calculator offers two approaches: a quick estimate from measured snow depth and density, and the ASCE 7 (US standard) flat-roof formula based on ground snow load.
How to use it
For a fast field estimate, pick Snow depth & density, enter how many inches of snow are on the roof and an approximate density. Fresh light snow is about 7 lb/ft³, settled snow ~15, wet or packed snow ~30, and solid ice up to 57. For code-based design, choose ASCE 7 ground snow load and enter your ground snow load pg (from local snow maps) plus the exposure (Ce), thermal (Ct) and importance (Is) factors for your building.
The formula explained
The depth method uses $$\text{Load} = \frac{\text{depth}}{12} \times \text{density}$$ Dividing inches by 12 converts depth to feet, and multiplying by density (lb per cubic foot) yields a load per square foot. The ASCE 7 method uses $$p_f = 0.7 \cdot C_e \cdot C_t \cdot I_s \cdot p_g$$ where 0.7 is the basic exposure factor for a flat roof and the other coefficients adjust for wind exposure, heating and building criticality.
Worked example
Suppose you measure 12 inches of settled snow at a density of 15 lb/ft³. $$\text{Load} = \frac{12}{12} \times 15 = 1 \times 15 = \textbf{15 psf}$$ Over a 100 ft² section of roof, that is 1,500 lb of snow.
FAQ
Is this an official engineering tool? No — it provides estimates. The ASCE 7 method follows the US standard form, but final design must use a licensed engineer and local code values.
How much snow load can a roof hold? Many residential roofs are designed for roughly 20–40 psf, but this varies widely. Check your local building code.
Why does wet snow matter so much? Density, not depth, drives weight. A foot of wet snow can weigh several times more than a foot of fresh powder.
