How it works
Ground snow load is the design value the code publishes for your site. Wind scours snow off roofs, and heat from the building melts some of it, so a flat roof carries less than the ground. The 0.7 factor and the exposure factor Ce and thermal factor Ct capture that. The importance factor Is raises the load for buildings where failure is more serious.
On a slope, snow slides off. The slope factor reduces the load on steep roofs, sooner for slippery surfaces and for warm roofs. Low-slope roofs get a minimum load so that a low ground snow load never gives a load that is too small.
Worked example
A heated building (Ct 1.0), risk category II, in partially exposed terrain B with a 50 psf ground snow load and a 6/12 shingle roof:
- pf = 0.7 × Ce 1 × Ct 1 × Is 1 × pg 50 = 35 psf
- Slope = arctan(6 ÷ 12) = 26.6°; Cs = 1; ps = 1 × 35 = 35 psf
- Design snow load = 35 psf
| Input | Value |
|---|---|
| Ground snow load pg | 50 psf |
| Exposure factor Ce | Terrain B, partially exposed (1.0) |
| Thermal factor Ct | Heated building (1.0) |
| Risk category (importance factor Is) | II: most buildings (1.0) |
| Roof pitch: rise per 12 in of run | 6 in |
| Roof surface | Any surface (shingles, wood, rough) |
| Result | Value |
|---|---|
| Flat roof snow load pf | 35 psf |
| Roof slope angle | 26.6 ° |
| Slope factor Cs | 1 |
| Sloped roof snow load ps = Cs × pf | 35 psf |
| Minimum snow load pm (roofs under 15°) | 0 psf |
| Balanced snow load to design for | 35 psf |
Assumptions and limits
- Follows ASCE 7-16, Chapter 7, for balanced snow loads on simple gable, hip, and monoslope roofs. ASCE 7-22 changed the ground snow load basis and some factors, and your code may use an older or newer edition.
- Ground snow load pg is an input. This tool does not look up site values.
- Cs breakpoints are those of ASCE 7-16 Figure 7.4-1, with a straight-line fall to zero at 70°. A roof counts as "warm" when Ct is 1.0 or less and "cold" when Ct is above 1.0.
- Not a structural design. Trusses and rafters must be designed for the governing load combinations by a qualified professional.
Common questions
What is the difference between ground snow load and roof snow load?
Ground snow load is the code value for your site. Roof snow load is derived from it with the exposure, thermal, importance, and slope factors, and is usually less on a heated flat roof because wind and heat remove some of the snow.
Where do I find my ground snow load?
Ask your local building department, or use the ASCE 7 Hazard Tool for your address. Local authorities can publish their own values, so always confirm which one your permit requires.
Why is there a minimum snow load?
A single storm can put more snow on a low-slope roof than the reduced formula predicts in low-snow areas, so the code sets a floor of the ground snow load times Is up to 20 psf.
What is a cold roof?
One where Ct is above 1.0: an unheated building, a cold ventilated roof with a well-insulated ceiling below, or a freezer. Less melt means more snow stays and slides less.
Sources
- ASCE/SEI 7-16, Chapter 7: Eq. 7.3-1 pf = 0.7 Ce Ct Is pg; Table 7.3-1 (Ce); Table 7.3-2 (Ct); Table 1.5-2 (Is); Section 7.4 and Figure 7.4-1 (Cs breakpoints 5°, 10°, 30°, 45°, 70°); Section 7.3.4 (minimum snow load, slopes under 15°).
- Tables as reproduced in published summaries; confirm against the edition adopted where you build.
- Read them: ASCE 7-16 balanced snow load summary (structed.org)
Updated 2026-09-30