Snow Load Calculator
Evaluate whether snow accumulation on your roof is within safe limits. This calculator combines actual snow depth and density measurements with building code ground snow load values to determine the design snow load in pounds per square foot.
Snow Conditions
Building Details
Snow Load Analysis
Snow Density Reference
How to Calculate Snow Load
Snow load calculation uses two approaches and takes the more conservative (higher) value as the design load.
- Depth-Based Load: Multiply the snow depth (in feet) by the density (in pcf). Fresh snow density is ~5 pcf, settled snow is ~15 pcf, and wet snow is ~40 pcf. A safety factor of 1.25 is applied.Depth Load = Depth (ft) × Density (pcf) × 1.25
- Code-Based Load: The ground snow load (from local building code) is multiplied by a roof factor that accounts for pitch. Flatter roofs retain more snow.Code Load = Ground Snow Load × 0.7 (for pitches under 12:12)
- Design Load: The higher of the two values is the design load. If it exceeds 50 psf, the roof may be at risk and snow removal should be considered.
Example Snow Load Calculation
2 ft of settled snow on a 4:12 pitch roof
- Depth-based load: 2 ft × 15 pcf (settled) × 1.25 safety factor = 37.5 psf
- Code-based load: 30 psf ground snow × 0.7 roof factor = 21 psf
- Design load (higher): 37.5 psf — within safe range for standard roof design (50 psf)
- Equivalent wet snow: 2 ft × 40 pcf × 1.25 = 100 psf — this would exceed typical design limits and require immediate removal
Common Mistakes in Snow Load Estimation
Using average snow depth instead of maximum drift
Snow drifts at valleys, roof-to-wall intersections, and around obstructions can be 3-5 times deeper than the average roof depth. Check the worst-case drift location, not the average across the whole roof.
Forgetting the roof factor on steep slopes
Steep roofs shed snow naturally, reducing the effective load. But a sudden thaw followed by freeze can create ice dams that hold snow in place. Always use the code roof factor, not manual judgment.
Ignoring unbalanced snow loading
Wind can deposit more snow on one side of a roof. Building codes often require designing for 1.5 times the balanced load on the leeward side while the windward side gets reduced load. This unbalanced condition can be more dangerous than uniform loading.
Frequently Asked Questions
What is the standard snow load for my area?
Ground snow loads vary by region from 10 psf (warm climates) to 100+ psf (mountain areas). Check your local building department for the code-map value specific to your address and elevation.
What roof pitch is best for snow?
Steeper roofs (6:12 and above) shed snow naturally. A 6:12 pitch has a 0.7 roof factor versus 1.0 for flat roofs. Pitches over 12:12 use an additional reduction factor but may accumulate drifting at wall intersections.
How much snow can a typical roof hold?
Most residential roofs in snow country are designed for 30-50 psf. One foot of dry snow (15 pcf) = 15 psf. One foot of wet snow (40 pcf) = 40 psf. Two feet of wet snow = 80 psf, which exceeds most residential designs.