How it works
Snow weight depends far more on how dense the snow is than on how deep it is. The Insurance Institute for Business & Home Safety (IBHS) gives simple rules: 10 to 12 inches of fresh snow weighs about 5 pounds per square foot, 3 to 5 inches of old, packed snow weighs about the same, and 1 inch of ice weighs about 5 pounds per square foot.
This calculator uses those rules as densities of 5 lb per cubic foot for fresh snow, 15 for packed snow and 57.2 for ice, and adds up the layers. Enter each layer separately, because a roof often holds old packed snow under new snowfall.
Use the roof’s ground footprint, not the sloped surface, since snow loads are measured on the horizontal area. IBHS estimates that a typical roof can carry about 20 pounds per square foot of snow before stress becomes a concern. Older roofs, flat roofs and roofs with earlier damage may carry less. Drifts near parapets, valleys and lower roofs can be several times deeper than the average.
Formula
Load (lb/sq ft) = (fresh in × 5 + packed in × 15 + ice in × 57.2) ÷ 12; total = load × roof length × width
Questions people ask
How much does a foot of snow weigh on a roof?
About 5 pounds per square foot for fresh, fluffy snow, and up to about 15 or more for settled, packed snow. On a 1,200 sq ft roof, a foot of fresh snow weighs about 6,000 pounds.
How much snow can a roof hold?
IBHS estimates a typical roof can handle about 20 pounds per square foot: roughly 4 feet of fresh snow or about 16 inches of packed snow. Roofs in snowy regions are often designed for much more. Your local building department can tell you the design snow load.
What are warning signs of too much snow on a roof?
Sagging roof lines, new cracks in walls or ceilings, doors upstairs that suddenly stick, and creaking or popping sounds. Leave the building and call a professional if you see them.
Is wet snow heavier?
Yes. Heavy, wet spring snow can weigh 20 pounds per cubic foot or more, four times fresh powder. If the snow is wet, enter it as packed snow for a closer estimate.