Snow Load Calculator
A snow load calculator applies 0.7 × ground snow load: 30 psf on the ground gives a 21 psf roof load, or 21,000 lb on a 1,000 sq ft roof.
Estimating roof snow load
A snow load calculator converts the ground snow load for your area into the load pressing on a roof. Enter the roof area in square feet and the ground snow load in pounds per square foot (psf). The tool multiplies the ground load by 0.7 to get the roof load, then multiplies by the area to get total pounds.
Think of a thick, wet blanket draped over the roof. The calculator tells you how many pounds that blanket weighs per square foot, and across the whole roof.
The formula in plain words
- Roof snow load (psf) = 0.7 × ground snow load.
- Total load (lb) = roof snow load × roof area.
The 0.7 factor comes from a simplified use of the design standard, with the exposure, thermal, and importance factors all taken as 1.0. It is a starting point and not a design value.
Worked example: a small house roof
A roof has 1,000 sq ft of projected area. The ground snow load is 30 psf.
- Roof load: 0.7 × 30 = 21 psf
- Total: 21 × 1,000 = 21,000 lb
That is 10.5 tons of snow, roughly the weight of five or six cars parked on the roof.
Worked example: a larger roof in heavy snow country
A roof has 1,800 sq ft of projected area. The ground snow load is 50 psf.
- Roof load: 0.7 × 50 = 35 psf
- Total: 35 × 1,800 = 63,000 lb
Roof load by ground snow load
| Ground snow load | Roof snow load at 0.7 |
|---|---|
| 10 psf | 7 psf |
| 20 psf | 14 psf |
| 30 psf | 21 psf |
| 40 psf | 28 psf |
| 60 psf | 42 psf |
Per square foot the load looks small. Over a big roof it adds up quickly, which is why the total pounds figure is worth reading.
What the tool leaves out
Several real-world effects can push the load above or below this simple number:
- Drifting. Wind piles snow against walls, dormers, and higher roofs, and the drift can be much heavier than the flat load.
- Slope. Steeper roofs shed snow, and some surfaces such as metal shed it faster. This tool applies no slope reduction.
- Heat loss. A warm roof melts snow, and a cold roof holds it.
- Rain on snow. Rain soaks into the snowpack and adds weight.
- Unbalanced loads. Snow on one side of a ridge but not the other loads the structure unevenly.
Because of these, roof design is done with the building code and an engineer, not with a simple multiplier.
How to get the numbers
Find the roof’s projected area by multiplying the footprint’s length and width, including overhangs. That is the plan view, so a steep pitch does not enlarge it.
Ask your building department for the ground snow load. It is often listed on the permit checklist. If your site is at a high elevation, the value can be higher than a nearby town at lower altitude.
Common mistakes
- Using the sloped roof area. The tool wants the area seen from above.
- Guessing the ground snow load. It varies from town to town, so get it from the building department.
- Treating the result as the allowed limit. It is a demand, and the roof’s capacity depends on rafter size, spacing, and span.
- Clearing snow yourself without care. Roof work in winter is dangerous, so use a roof rake from the ground or hire a professional.
- Ignoring signs of overload. Sagging, new cracks, or sticking doors under heavy snow are reasons to call a professional.
Related calculators
Load on a beam or joist can be explored with the beam load calculator. Decks carry snow too, so check the deck load calculator. Wind pressure is covered by the wind load calculator. For any structure that must carry snow, consult a licensed engineer.
Frequently asked questions
How do I estimate roof snow load?
Take the ground snow load for your location and multiply by 0.7 for a simple flat-roof design load. With a 30 psf ground load, the roof load is 21 psf. Multiply by the roof area for total pounds.
Where do I find the ground snow load?
Your local building department publishes it, and it is based on maps in the design standard the code adopts. It varies by town and elevation. Ask the building department for your site's value.
Is this enough to design a roof?
No. It is a simplified estimate. Real design loads account for roof slope, exposure, heat loss through the roof, building importance, drifting, and unbalanced snow. A structural engineer or the local code sets the number for a permit.
What area should I enter?
Enter the flat, projected area, which is the footprint of the roof as seen from above. Do not use the longer sloped surface.
How much does snow weigh?
It varies a lot. Light powder weighs far less than wet, heavy snow or ice, and rain on snow adds more. That is why design uses a code value and not the depth you see.
Estimates only. Verify quantities with your supplier before purchasing.