Enter a ground snow load and roof factors to estimate flat and sloped roof snow load. This is a planning estimate, not a structural design value.
How to Use the Roof Snow Load Calculator
Enter the ground snow load for the site, the roof's slope in degrees, and the exposure, thermal, and importance factors — all default to 1.0, the typical case. Check the slippery-surface box for unobstructed metal roofing, which sheds snow at a lower slope than shingles or a roof with vents and chimneys in the way.
This tool produces a simplified estimate for planning purposes. It is not a substitute for a structural engineer's calculation and should never be the sole basis for a building permit, a truss design, or any load-bearing decision. Ground snow load especially is a location-specific code value adopted by your local jurisdiction — look it up from your building department or the ASCE 7 Hazard Tool rather than trusting the default shown here.
The ASCE 7 Method, Simplified
The formulas below follow the framework in ASCE 7, the standard most US building codes reference for snow load design.
flat-roof snow load: Pf = 0.7 × Ce × Ct × Is × Pg
sloped-roof snow load: Ps = Cs × PfPg is the ground snow load. Ce, Ct, and Is adjust for terrain exposure, whether the building is heated, and the structure's risk category — a hospital or fire station gets a higher importance factor than a garden shed. Cs, the slope factor, is where this calculator simplifies most: the real ASCE 7 charts vary by exact roof surface and thermal condition, while this tool approximates them with a linear falloff between a full-load threshold slope and 70°, past which snow is assumed to shed entirely.
Why Steeper Roofs Carry Less Snow
Snow sheds off a steep roof largely under its own weight, the same reason a steep driveway clears faster than a flat one after a storm. A slippery metal roof starts losing snow past about 5°; a rougher shingle roof tends to hold on until somewhere past 30°. That's the entire justification for the slippery-surface toggle here — it shifts where the falloff begins, nothing more.