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ASCE 7 starts from the mapped ground snow load pg and reduces it to a flat-roof value with the 0.7 base factor, then adjusts for how exposed the roof is (Ce), whether the building is heated (Ct) and its importance (Is). Slope, drifting and sliding snow are separate checks on top of this.
- pg = 40 psf, factors = 1.0.
- pf = 0.7 × 40 = 28 psf flat-roof snow.
Reference tables
Typical ASCE 7 factors
| Factor | Range |
|---|---|
| Ce (exposure) | 0.7 (windy) – 1.2 (sheltered) |
| Ct (thermal) | 1.0 (heated) – 1.2 (unheated) |
| Is (importance) | 0.8 – 1.2 by risk category |
A number the tables do not give: a heated, partially exposed roof with 40 psf ground snow carries about 28 psf on a flat roof before drift.
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FAQ
What is the 0.7 factor?
The base flat-roof conversion in ASCE 7 — snow on a roof is generally less than on the ground due to wind and thermal effects.
Do I add drift?
Yes, separately. Drifts against walls and at roof steps can far exceed the balanced load and are their own calculation.
Where do I get ground snow load?
From the ASCE 7 ground-snow maps or the local jurisdiction, which may set a site-specific value.
Does roof slope reduce it?
Steeper, slippery roofs shed snow, so a slope factor Cs can reduce the sloped-roof load — but check sliding onto lower roofs.
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