Snow Load Calculator
Estimate roof snow load from the ground snow load and roof area: get the design load in psf and the total pounds your roof must support.
How this calculator works
Snow sitting on a roof has real weight, and that weight has to be carried by the structure below. This calculator starts from the ground snow load for your area, then reduces it to a roof design load, because a roof sheds some snow that flat ground would keep.
The simplified approach multiplies the ground snow load by 0.7 to get the flat-roof design load. Once you have that load per square foot, multiplying by the roof area gives the total pounds the roof must support.
Formula: Simplified flat-roof design snow load = 0.7 × ground snow load (ASCE 7, with exposure, thermal, and importance factors all taken as 1.0). Total load = roof area × design load.
Worked example
A 1,000 sq ft roof in an area with 30 psf ground snow load:
- Design load: 0.7 × 30 = 21 psf
- Total load: 1,000 × 21 = 21,000 lb
Reference
Ground snow to roof design load (×0.7):
| Ground snow | Roof design |
|---|---|
| 20 psf | 14 psf |
| 30 psf | 21 psf |
| 40 psf | 28 psf |
| 50 psf | 35 psf |
How to use
Enter your area’s ground snow load in pounds per square foot and your roof area in square feet. The calculator returns the design load in psf and the total pounds the roof carries. Treat the result as a planning figure and confirm the real design values with your local building department.
This is a planning estimate, not an engineered design. Structural sizing is safety-critical and code-governed — confirm all members, connections, and footings with a licensed structural engineer and your local building code.
Frequently asked questions
What is ground snow load and where do I find it?
Ground snow load is the weight of snow expected on the flat ground in your area, measured in pounds per square foot. It comes from local climate records and building-code maps for your region. Your local building department or a structural engineer can give you the correct value for your exact location before you rely on any estimate.
Why multiply ground snow load by 0.7?
A sloped roof holds less snow than the open ground because snow slides and blows off, so codes apply a reduction. The simplified ASCE 7 flat-roof factor of 0.7 turns the ground load into a roof design load. Exposure, thermal, and importance factors can raise or lower this further, so 0.7 is a planning starting point, not a final number.
How do I get the total load on my roof?
Multiply the roof design load in pounds per square foot by the roof area in square feet. A design load of 21 psf over a 1,000 square foot roof is 21 times 1,000, which is 21,000 pounds. That total is the snow weight the roof structure has to carry, on top of the roof's own weight.
Is this estimate enough to size my roof structure?
No. This gives a rough design load for planning and conversation only. Real roof design also accounts for drifting, sliding, unbalanced loads, roof shape, and the dead weight of the roof itself. Member sizing, connections, and footings are safety-critical and must be confirmed by a licensed structural engineer against your local code.
Estimates only. Verify quantities with your supplier before purchasing.