Enter Roof and Snow Data
Formula Used
The calculator uses this preliminary roof snow process:
Flat roof snow load: pf = 0.7 × Ce × Ct × I × Pg
Sloped roof snow load: ps = Cs × pf
Minimum adjusted load: ps,min = max(ps, selected minimum roof load)
Service snow load: S = ps,min × unbalanced multiplier + rain surcharge + average drift surcharge
Average drift surcharge: qd,avg = 0.5 × snow density × drift height
Factored load: Wu = dead load factor × D + snow load factor × S
These equations give a planning estimate. Final design may require code-specific drift, sliding, ponding, and load combination checks.
How to Use This Calculator
- Select the unit system used by your project notes.
- Enter roof length and width as projected plan dimensions.
- Add ground snow load from your local snow map or authority.
- Enter exposure, thermal, and importance factors.
- Add roof slope and choose the roof surface type.
- Use drift options near parapets, step roofs, higher walls, or equipment screens.
- Press calculate and review the service and factored results.
- Download the CSV or PDF report for project records.
Example Data Table
| Input | Example Value | Purpose |
|---|---|---|
| Ground snow load Pg | 35 psf | Base regional snow demand |
| Exposure factor Ce | 1.00 | Accounts for wind exposure |
| Thermal factor Ct | 1.00 | Accounts for roof heat loss |
| Importance factor I | 1.00 | Accounts for building risk |
| Roof slope | 20 degrees | Adjusts snow retention |
| Drift height | 1.2 ft | Models local drift surcharge |
Roof Snow Load Planning Guide
Why Roof Snow Load Matters
Snow looks light when it falls. It can become heavy after wind, melt, rain, refreeze, or long cold storage. A roof must carry that weight safely. The load is not always even. Drifts can form behind parapets, higher walls, roof steps, solar racks, skylights, and mechanical screens. These zones often control framing demand.
Main Load Factors
The starting point is the ground snow load. It comes from local maps or adopted rules. The calculator then applies exposure, thermal, and importance factors. Exposure adjusts for wind removal or wind collection. Thermal behavior adjusts for warm or cold roof conditions. Importance adjusts the demand for higher risk buildings. These factors create the flat roof snow load.
Slope and Surface Effects
Sloped roofs can shed snow. Smooth metal, glass, or slippery roofing may shed faster than rough shingles. A low slope usually keeps most snow in place. A steep slope may reduce the balanced load. Do not assume that slope removes all risk. Valleys, dormers, gutters, snow guards, and lower roofs can collect sliding snow.
Drift and Rain Concerns
Wind can move snow across a roof. It drops snow where air slows down. That creates a triangular drift shape. This tool estimates an average drift surcharge from snow density and drift height. Rain on snow can also add load. It matters on low slope roofs where water cannot drain quickly. Add a surcharge when required by your local rules.
Using the Results
Review both service and factored results. Service loads help with general roof demand. Factored loads help compare against strength design values. The peak drift load helps flag local framing areas. Use the total force for rough support reactions. Always confirm final loads with the adopted building code and a qualified professional when the project affects safety.
Frequently Asked Questions
What is roof snow load?
Roof snow load is the weight of snow that a roof must support. It is usually expressed as psf or kPa. It depends on ground snow load, roof shape, exposure, thermal behavior, slope, and drift conditions.
How is ground snow load different from roof snow load?
Ground snow load is the mapped snow weight on open ground. Roof snow load is the adjusted load on the roof. The adjustment uses factors for exposure, thermal conditions, importance, slope, and special accumulation patterns.
Why does the calculator use a 0.7 factor?
The 0.7 factor is commonly used in flat roof snow load calculations. It converts mapped ground snow demand into a basic flat roof demand before other factors are applied. Local rules may modify this approach.
What is a slope factor?
A slope factor reduces roof snow load when snow can slide or shed. The reduction depends on the roof angle and surface. Smooth surfaces usually shed snow more easily than rough surfaces.
Should I include snow drift?
Include drift when the roof has parapets, higher adjacent roofs, equipment screens, roof steps, or other obstructions. Drift can create a local load much higher than the balanced roof load.
What snow density should I enter?
Fresh snow may be light. Wet or compacted snow is much heavier. Use a density required by your code, engineer, weather data, or project specification. Conservative values are better for early planning.
What is rain-on-snow surcharge?
Rain-on-snow surcharge represents extra weight from rainwater held in or above snow. It is often important on low slope roofs. Use this input when required by drainage checks or local design provisions.
Can this tool design roof beams?
No. It estimates roof snow demand. Beam, joist, truss, deck, connection, and support design need material properties, spans, load paths, deflection limits, and code checks.
Why are service and factored loads different?
Service load is the expected unfactored demand used for many checks. Factored load increases demand with load factors. Strength design often compares factored loads with factored resistance.
Can I use metric units?
Yes. Select metric units and enter meters, kPa, and kN/m³. The calculator converts values internally and reports both common unit systems in the result panel.
Is this calculator enough for permit design?
Usually no. Permit work may require adopted code maps, local amendments, drift cases, sliding snow checks, load combinations, and sealed drawings. Use this tool for planning and early review.