Structural tool
Roof Snow Load Calculator
Enter your ground snow load and roof pitch to get the design sloped-roof snow load, following the ASCE 7 formula that building departments use.
Sloped roof snow load is ps = 0.7 × Ce × Ct × I × Cs × pg. A 30 psf ground load on a low-slope warm roof gives a 21 psf flat-roof load. Roofs steeper than 30° begin shedding, and above 70° carry no design snow load.
Calculate snow load
Ground snow load comes from your local building department or the ASCE hazard tool.
Sloped roof load
21.0 psf
pounds per square foot
Reading the ASCE 7 formula
The 0.7 coefficient reflects the fact that roofs rarely hold as much snow as the ground beside them, because wind and thermal loss remove some of it. Ce adjusts for how exposed the site is, Ct for whether the building is heated, and I for how serious a failure would be.
The slope factor Cs is the part homeowners find counterintuitive. Below about 30° a roof holds essentially all its snow. Between 30° and 70° the design load falls off linearly as snow slides. Above 70° there is no design snow load at all.
What the number is for
Snow load determines rafter or truss sizing, and it is why a garage in Buffalo is framed differently from an identical garage in Atlanta. If you are adding heavy roofing such as tile, slate, or a second layer of shingles, the added dead load stacks on top of this snow load, and that combination is what an engineer checks.
Drifting is a separate calculation this tool does not cover. Snow piles deeper against walls, behind parapets and on the lower roof of a two-level building, and those drift loads can be several times the balanced load. Any roof with a step in it needs a full ASCE drift analysis.