Snow Load Calculator
Estimate roof snow pressure, total snow weight, capacity utilization, and the maximum snow depth implied by a known allowable roof snow load.
Roof and snow inputs
Live results
Current load compared with allowable load
Calculation breakdown
| Measure | Value | Unit | How it is used |
|---|---|---|---|
| Roof pitch angle | 26.57 | degrees | Converts plan area to sloped surface area |
| Snow density | 15.61 | lb/ft³ | Converts snow depth into pressure |
| Snow depth | 1.50 | ft | Thickness in the pressure calculation |
| Projected roof area | 1,200.00 | ft² | Length × width |
| Sloped roof surface area | 1,341.64 | ft² | Projected area ÷ cos(pitch) |
| Snow load | 23.42 | psf | Depth × density |
How to use this snow load calculator
What this calculator does. It estimates the pressure created by a uniform layer of snow on a roof, the approximate total snow weight over the sloped roof surface, and how that pressure compares with an allowable roof snow load that you enter. It also reverses the same density relationship to estimate a maximum snow thickness and corresponding total weight. The result is a planning estimate, not a structural inspection, code determination, or permission to remain inside a building that shows distress.
When to use it. Use it to compare recent snowfall with a known design limit, to understand how wetter snow changes roof loading, to prepare a conversation with a structural engineer or building official, or to decide whether conditions deserve closer professional attention. The FEMA Snow Load Safety Guide explains why actual roof performance depends on more than depth alone and why removal work must be planned safely.
How to calculate. The calculator opens with a complete demonstration: a 40 ft by 30 ft roof, a 6-in-12 pitch, 18 inches of settled snow, and a 30 psf allowable load. Its example workbook is immediately available. To use your own values:
- Replace Roof length and Roof width with the horizontal plan dimensions of the roof area being evaluated.
- Enter Roof pitch as inches of rise per 12 inches of run, then enter the representative Snow cover thickness.
- Select the closest Snow type. When uncertain, a denser category gives a more conservative weight estimate.
- Enter an Allowable roof snow load obtained from reliable building documents, a qualified professional, or the authority having jurisdiction.
- Read the live results, comparison chart, and calculation table. Choose Download Excel to save the current typed model. Reset clears the demonstration and results; export remains disabled until every required value is complete and valid again.
Input guide. Roof length and Roof width are required positive decimal values in feet; 40 ft and 30 ft are realistic examples. They determine projected area, so doubling either dimension doubles total snow weight but does not change pressure in psf. Do not use the sloped rafter length here. Roof pitch is required from 0 through 24 inches of rise per 12 inches of run; 6 means 6:12. A steeper pitch increases the calculated sloped surface area under this geometric model. Snow cover thickness is required, accepts zero or a positive decimal in inches, and should represent a cautious roof measurement rather than a sheltered ground reading. Snow type is required and supplies density: fresh, damp fresh, settled, wind-packed, very wet, or ice. Density can vary within one roof, so avoid assuming that old, wet, or refrozen snow behaves like fresh powder. Allowable roof snow load is a required positive value in pounds per square foot. The example is 30 psf, but this is not a universal safe limit; using an unverified generic number is the most serious interpretation error.
Output guide. Snow load on roof is the estimated pressure in psf and is driven by depth and density. Total snow weight is that pressure multiplied by sloped roof surface area. Roof surface area is projected area divided by the cosine of the pitch angle. Capacity utilization compares current pressure with the entered allowable load; 100% means they are equal, while a value above 100% means the estimate exceeds the entered limit. Remaining load margin is allowable minus current pressure and becomes negative above the entered limit. Maximum snow thickness is the depth of the selected snow type that corresponds to the entered allowable pressure. Maximum snow weight applies that allowable pressure to the same roof surface. The pills repeat the current snow load, total weight, and utilization. The table shows the pitch angle, density, depth in feet, projected area, surface area, and pressure. The chart compares two compatible pressure values – current and allowable – in psf.
Worked example. For the startup values, 18 inches equals 1.50 ft. Settled snow density is 15.61 lb/ft³, so the snow load is 1.50 × 15.61 = 23.42 psf. A 6:12 pitch has an angle of arctan(6/12) = 26.57°. The 1,200 ft² projected area becomes 1,200 ÷ cos(26.57°) = 1,341.64 ft² of sloped surface. Multiplying by 23.415 psf gives about 31,413 lb. Compared with 30 psf allowable, utilization is 78.05%, leaving 6.59 psf of pressure margin. At the same density, 30 ÷ 15.61 × 12 gives a maximum depth of 23.06 inches.
How the model works
snow load (psf) = snow depth (ft) × snow density (lb/ft³)
roof surface area = length × width ÷ cos(arctan(pitch ÷ 12))
total snow weight = snow load × roof surface area
The model uses bulk-density categories to translate depth into pressure. The National Weather Service defines snow density as mass per unit volume and explains snow-water-equivalent observations in its snow water equivalent guidance. The selected density is an approximation; a layered snowpack can contain light snow, wet snow, and ice at the same time.
Design limits and safety
Building design snow loads are not determined by a single nationwide number. Modern structural practice considers mapped ground snow load and building-specific factors. ASCE/SEI 7-22 covers snow and other minimum design loads, while the ASCE 7 Hazard Tool provides location-based hazard information. Local adoption, roof shape, exposure, thermal conditions, occupancy importance, drifting, sliding, unbalanced loading, and existing structural condition still require professional judgment.
Act on warning signs, not only the number. Sagging, new cracks, unusual creaking or popping, jammed doors, leaks, and visible deformation can indicate distress. Ice is especially dense. When collapse is a concern, leave the area and contact emergency services or a qualified structural professional; do not climb onto a potentially unstable roof.