Estimate the weight of snow on your roof in pounds per square foot from the depth and snow type — a planning estimate, not a structural design figure.
psf = (depth_in ÷ 12) × snow_density
Snow load is the depth in feet times the snow's density. Fresh snow is light (about 7 lb/ft³); settled or wet snow is far heavier (15–30 lb/ft³).
Loads above about 20 psf warrant attention on many residential roofs. This is a rough estimate — not a substitute for an engineer.
Twelve inches of settled snow has a density around 15 lb per cubic foot. One foot deep is therefore about 15 lb per square foot. On a 1,500 square foot roof that's 22,500 lb — over 11 tons sitting on the structure.
That 15 psf is comfortably under the ~20 psf many residential roofs are designed for, but wet spring snow or an ice layer can be two to four times heavier. The same depth of ice (around 57 lb per cubic foot) would be 57 psf — well into dangerous territory. This is exactly why this is a planning estimate, not a design tool: real load calculations follow ASCE 7 and your local code, and structural decisions need a licensed engineer.
This is a planning estimate using typical snow densities. Structural design loads follow ASCE 7 and local code — consult a licensed engineer for anything load-bearing.
The balanced load is the snow sitting evenly across the roof, but it is rarely the worst case. Wind redistributes snow into drifts against walls, parapets, and roof steps, where the local load can be two to three times the balanced figure. Valleys and the lee side of gable and hip roofs collect unbalanced loads. These concentrated loads often govern the structural design even though they cover only part of the roof.
Settled snow is about 15 lb per cubic foot, so 12″ of it is roughly 15 lb per square foot.
Ground snow load is the weight of snow on the ground for a specific location, taken from code maps and derived from decades of weather data. It is the starting point for the roof load calculation and varies sharply over short distances in mountainous areas.
Steeper roofs shed snow, so a slope factor reduces the balanced load as pitch increases. A very steep, slippery roof retains little snow, but snow sliding onto a lower roof adds load there.
No. It informs planning, but sizing rafters, trusses, and connections to carry the load is the work of a licensed structural engineer using the full code provisions for your site, especially where drifting governs.