Snow Load Calculator
Roof snow load, total weight, and watch verdict from depth, ratio, and area.
What it does
Snow load turns depth into weight through water content: an inch of water equivalent weighs 5.2 lb per square foot, and the snow ratio tells how many inches of snow make that inch. Fluffy 20:1 powder weighs half the typical 10:1 snow at the same depth.
The tool above takes snow depth with ratio and roof area in either system. It returns load per area with total roof weight and a watch verdict beside them.
The default of 24 in at 10:1 over 1,000 ft2 gives 12.48 psf and 12,480 lb (6.24 tons): moderate, monitor drifts and thaw. The metric mirror of 61 cm over 93 m2 gives 61 kg/m2 and 5,673 kg.
The math follows Weather Service snow load guidance with winter weather education matching the 10:1 rule. Both melt snow to water inches first, then multiply by water weight.
How to use
- Pick imperial (in, ft2) or metric (cm, m2).
- Measure settled snow depth off drifts.
- Enter the ratio: 10 typical, lower for wet snow.
- Enter the roof footprint area.
- Read load first, then total weight for the structure.
Formula
- ratio
- Snow inches per water inch; 10:1 typical.
- 5.2
- Pounds per square foot per water inch.
- area
- Roof footprint carrying the snow.
Flat roof math throughout; pitch sheds some load by slope while valleys collect drifts. Rain on snow adds water directly on top of these numbers.
Worked example
Example: 24 in at 10:1 over 1,000 ft2
Imperial defaults: depth 24 in, ratio 10, area 1000 ft2.
- Water: 24 / 10 = 2.4 in of water.
- Load: 2.4 x 5.2 = 12.48 psf.
- Total: 12.48 x 1,000 = 12,480 lb (6.24 tons).
- Verdict: 10 to 20 reads moderate, monitor drifts and thaw.
- Tool shows 12.48 psf, 12,480 lb total, the moderate verdict.
Reference table
| Snow | Load | Verdict |
|---|---|---|
| 12 in | 6.24 psf | Light: routine winter load |
| 24 in | 12.48 psf | Moderate: monitor drifts and thaw |
| 24 in wet 5:1 | 24.96 psf | Heavy: consider clearing |
Limits
- Planning math only; codes and engineers set real limits.
- Drifts pile multiples against parapets and valleys.
- Rain on snow adds weight past these numbers.
- Flat roof assumed; pitch sheds unevenly.
Common mistakes
- Weighing fluffy and wet snow by depth alone.
- Forgetting drifts multiply the flat average.
- Ignoring rain falling into the pack.
- Raking from ladders instead of the ground.
- Waiting for sagging doors before acting.
Try it yourself
Quick checks. Answers sit tucked under each question.
12 in at 10:1?
1.2 in water: 6.24 psf, light load.
24 in of wet 5:1 snow?
4.8 in water: 24.96 psf, heavy, consider clearing.
Metric 61 cm over 93 m2 at 10:1?
61 kg/m2, 5,673 kg total: moderate.
Questions
How much does snow weigh?
About 5.2 lb per square foot per water inch. A foot of 10:1 snow is near 6.24 psf.
What is the 10:1 ratio?
Ten inches of snow holding one inch of water. Wet snow runs near 5:1, powder past 20:1.
How much snow is dangerous?
It depends on water content and the roof design. Past 20 psf, monitor closely and check local guidance.
Does pitch matter?
Yes. Steep metal sheds snow while low slopes and valleys collect it.
Rain on snow?
Adds weight fast: an inch of rain is 5.2 psf on top of the pack.
How do I clear safely?
Rakes from the ground, balanced passes, never ladders in storms. Hire out steep roofs.