Snow Load Calculator
The snow load your roof actually has to carry — not just the snow on the ground. ASCE 7 flat-roof and sloped-roof snow load from the ground snow and the exposure, thermal and importance factors.
Ground Snow Is Not Roof Snow
The snow load a roof must carry is not the same as the snow sitting on the ground beside it. Wind scours some off, a warm roof melts some away, and a steep roof sheds it — but a sheltered, cold, low-slope roof can hold nearly all of it. ASCE 7 turns the mapped ground snow load into a design roof load with a handful of factors.
Flat-Roof Snow Load
The 0.7 is the basic ground-to-roof conversion. Ce reflects how exposed the roof is to wind (exposed roofs lose snow, sheltered ones keep it), Ct the thermal condition (a heated roof melts snow; an unheated one does not), and Is the importance of the building by risk category.
The Slope Reduction
On a sloped roof, the design load is ps = Cs · pf. The slope factor Cs stays at 1.0 up to a threshold, then falls toward zero as the roof steepens — sooner for slippery surfaces like metal and membrane, later for shingles and gravel, and sooner for warm roofs than cold ones. A low-slope roof also has a minimum load floor that can govern.
Frequently Asked Questions
Where do I get the ground snow load?
From the ASCE 7 ground snow maps or your local building department. It varies enormously – near zero in the deep South, 20 to 50 psf across the northern US, and far higher in mountains.
Why does an unheated roof get more snow?
A heated building loses warmth through the roof and melts the underside of the snowpack, so it sheds faster. An unheated or well-insulated cold roof keeps the snow, so its thermal factor is higher.
Does this cover snow drifts?
No. This is the balanced load. Drifts against walls and parapets, snow sliding off a higher roof, and rain-on-snow can all produce larger local loads that need separate checks.
Related calculators
- Wind Load Calculator — the other major environmental load.
- Beam Load Calculator — carry the snow load into the rafters and beams.
- R-Value Calculator — a warmer roof sheds snow but loses heat.
- All engineering calculators — the full library.