Roof Snow Load Calculator
Calculate the design roof snow load from the ground snow load, roof pitch, exposure and thermal factors using the ASCE 7 method, plus the total snow weight on the roof.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Design roof snow load
- 21 psf
- Slope factor Cs 1 at 6/12 (26.6 degrees), non-slippery surface
- Flat roof snow load
- 21 psf
- 0.7 x Ce 1 x Ct 1 x Is 1 x 30 psf ground snow
- Total snow weight on the roof
- 15.75 tons
- 31,500 lb over 1,500 sq ft of horizontal roof area
- Equivalent snow depth
- 12.6 in
- Settled snow at 20 lb per cubic foot; fresh powder is lighter, wet snow and ice far heavier
- Important
- Planning estimate
- Uniform load only. Drifts, sliding snow, unbalanced loads and rain-on-snow add to this; a structural engineer and the local building code govern.
Roof Snow Load formula
Flat roof load pf = 0.7 x Ce x Ct x Is x pg (Is = 1.0 for houses)
Sloped roof load ps = Cs x pf
Cs = 1.0 up to 30 degrees (5 degrees for slippery roofs), then straight line to 0 at 70 degrees
Total weight = ps x horizontal roof area; Depth = ps / 20 lb per cu ftWorked example
A fully exposed, heated house with a 50 psf ground snow load and a standing-seam metal roof at 8/12 (33.7 degrees): pf = 0.7 x 0.9 x 1.0 x 1.0 x 50 = 31.5 psf. Metal is slippery, so Cs = 1 - (33.7 - 5) / 65 = 0.56, and ps = 31.5 x 0.56 = 17.6 psf.
The same house with asphalt shingles: Cs = 1 - (33.7 - 30) / 40 = 0.91, so ps = 31.5 x 0.91 = 28.6 psf. On a 2,000 sq ft horizontal roof area that is 57,200 lb, or 28.6 tons, equal to about 17.2 in of settled snow at 20 lb per cubic foot.
Quick reference
| Pitch | Angle | Cs shingles | Load shingles | Cs slippery | Load slippery |
|---|---|---|---|---|---|
| Flat | 0 deg | 1.00 | 21.0 psf | 1.00 | 21.0 psf |
| 3/12 | 14.0 deg | 1.00 | 21.0 psf | 0.86 | 18.1 psf |
| 4/12 | 18.4 deg | 1.00 | 21.0 psf | 0.79 | 16.7 psf |
| 6/12 | 26.6 deg | 1.00 | 21.0 psf | 0.67 | 14.0 psf |
| 8/12 | 33.7 deg | 0.91 | 19.1 psf | 0.56 | 11.7 psf |
| 10/12 | 39.8 deg | 0.75 | 15.9 psf | 0.46 | 9.8 psf |
| 12/12 | 45.0 deg | 0.63 | 13.1 psf | 0.38 | 8.1 psf |
How the ASCE 7 snow load works
Every building code snow calculation starts with the ground snow load, pg, which your building department publishes or you read from the ASCE 7 maps. The flat roof load is 0.7 x Ce x Ct x Is x pg: the exposure factor Ce is lower for windswept sites and higher for sheltered ones, the thermal factor Ct rises for unheated buildings that never melt snow off the roof, and the importance factor Is is 1.0 for houses and most ordinary buildings.
Sloped roofs shed snow, so the flat roof load is multiplied by a slope factor Cs. Roofs covered with shingles or tile keep their full load up to 30 degrees (about 7/12) and taper to zero at 70 degrees. Slippery, unobstructed surfaces such as standing-seam metal start shedding at just 5 degrees on a heated building. Snow guards, chimneys and parapets turn a slippery roof into an obstructed one.
What this estimate leaves out
This is the uniform balanced load only. ASCE 7 also requires checking unbalanced loads on gable and hip roofs, drifts against parapets, taller walls and rooftop equipment, snow that slides from a higher roof onto a lower one, and rain falling on snow. Low-slope roofs under about 15 degrees also have a minimum load of the smaller of pg or 20 psf. Any of these can govern the design of a particular rafter or beam.
Use the number here to compare roof options, to talk with your building department, and to judge whether a heavy winter is approaching design conditions. Roof framing, trusses and any decision to clear a roof should follow the local code and a structural engineer's review, not this calculator.
Key facts
- ASCE 7 reduces the ground snow load by 30% for a flat roof (the 0.7 factor) because wind removes some snow and roofs shed heat.
- Settled snow weighs about 20 lb per cubic foot, fresh powder 5 to 7 lb, and solid ice 57 lb, so 1 ft of settled snow is about 20 psf.
- Ground snow loads run from 0 psf along the Gulf Coast to 40 to 70 psf across the upper Midwest and New England and over 100 psf in mountain areas.
- The IRC prescriptive rafter and truss tables are based on ground snow loads of 30, 50 and 70 psf; higher loads require engineered design.
- An unheated building carries 20% more design snow (Ct 1.2) because no heat escapes through the roof to melt it.
Frequently asked questions
How do I calculate snow load on a roof?
Multiply the ground snow load by 0.7 and by the exposure, thermal and importance factors, then by the slope factor for the pitch. A 30 psf ground load on a heated, partially exposed 6/12 shingle roof gives 21 psf.
How much does snow weigh on a roof?
Settled snow weighs about 20 lb per cubic foot, so each foot of depth adds 20 psf. A 21 psf design load equals 12.6 in of settled snow, or 15.75 tons over a 1,500 sq ft roof.
Does a steeper roof reduce snow load?
Yes above 30 degrees for shingle roofs and above 5 degrees for slippery metal roofs on heated buildings. A 12/12 shingle roof carries 63% of the flat roof load; a 12/12 metal roof carries 38%.
What ground snow load should I use?
The value adopted by your local building department, which may differ from the national map for elevation or local records. Typical values are 20 to 30 psf in the mid-Atlantic, 40 to 70 psf in the upper Midwest and New England, and 100 psf or more in mountain towns.
Sources and references
Next steps for this project
The usual order for a roofing & siding project. See all roofing & siding calculators.
Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.