How Much Does a Foot of Snow Weigh on a Roof?

A roof does not care how deep the snow is. It cares how much that snow weighs, and weight depends on density, which varies wildly between dry powder and wet, settled snow.

Density is everything

Fresh powder can be as light as 3–5 pounds per cubic foot, while old, wet, compacted snow approaches 20–30 pcf. ASCE 7 estimates settled snow density as γ = 0.13 × Pg + 14 psf-per-foot, capped at 30 pcf, so higher ground snow loads come with denser snow.

Turning depth into load

Multiply density by depth to get load. A foot of powder at 5 pcf is only 5 psf, but a foot of wet snow at 20 pcf is 20 psf. Two feet of dense snow plus a rain event can quickly approach many roofs' design capacity.

Knowing your number

Use the calculator to get your design roof snow load in psf, then watch real accumulation. If you can estimate the depth and whether it is light or wet, you can judge when you are approaching the design value and whether removal is warranted.