Standard and code basis
Roof snow loads in Canada are governed by the National Building Code of Canada 2020, Division B, Clause 4.1.6.2. The specified snow load (S) combines ground snow accumulation with rain-on-snow and accounts for roof geometry, exposure, and building importance:
S = Is × (Ss × Cb × Cw × Cs × Ca + Sr)
Ground snow load (Ss) and associated rain load (Sr) data come from NBCC 2020 Appendix C — the authoritative Canadian climatic data table. This calculator includes published Ss and Sr values for over 30 Canadian cities.
The five NBCC factors — what each one means
| Factor | Name | Values used | What it accounts for |
|---|---|---|---|
| Is | Importance factor for snow | 1.0 (normal) · 1.15 (high) · 1.25 (post-disaster) | Higher-importance buildings (hospitals, arenas, emergency facilities) must be designed for a larger snow load to reduce the risk of failure. |
| Ss | Ground snow load (kPa) | From NBCC Appendix C for your city | The 50-year return period ground snow load at the building site — the statistical basis of Canadian structural snow design. |
| Cb | Basic roof load factor | 0.8 (fixed) | Accounts for the fact that roofs accumulate less snow than the ground (wind scour, heat loss from the building). NBCC fixes this at 0.8 for all standard roofs. |
| Cw | Wind exposure factor | 1.0 sheltered · 0.75 normal · 0.5 open | Wind removes snow from exposed roofs. A building in an open field (Cw = 0.5) accumulates less snow than one sheltered by adjacent buildings (Cw = 1.0). |
| Cs | Slope factor | 1.0 if slope ≤ threshold; (70 − slope)/40 above threshold; 0 above 70° | Steep roofs shed snow by sliding. Normal roofs begin reducing at 30°; slippery surfaces (metal, plastic) begin at 15°. Above 70°, no snow load is assumed. |
| Sr | Associated rain load (kPa) | From NBCC Appendix C for your city | Rain falling on accumulated snow increases the load. Added at full value — not factored — because rain-on-snow is a direct static load. |
The calculation — step by step
- Look up Ss and Sr for your city from the NBCC 2020 Appendix C data embedded in the calculator. If your city is not listed, you can enter a manual Ss value from Appendix C or a site-specific study.
- Determine Isbased on your building's importance category (NBCC Table 4.1.6.1): Normal = 1.0, High = 1.15, Post-disaster = 1.25.
- Determine Cb, Cw, Cs: Cb is always 0.8. Cw is chosen from sheltered/normal/open. Cs is calculated from the roof slope and roof surface type.
- Apply the NBCC formula:
S = Is × (Ss × Cb × Cw × Cs + Sr)
The result is the specified roof snow load in kPa. - Calculate total load on roof:
Total load (kN) = S × roof area (m²)
Also converted to pounds for reference.
Reading your result
- Specified load (kPa): The factored design roof snow load. This is the value used in structural permit submissions and rafter span tables.
- Total load (kN / lbs): Total force on the roof — useful for checking post and foundation loads below the roof structure.
- Individual factors (Is, Cb, Cw, Cs): Shown so you can verify each step against the NBCC formula.
Assumptions and limitations
- Calculates the balanced (uniform) snow load only. Unbalanced loads, drift loads (Clause 4.1.6.3), and sliding snow accumulation at lower roofs are not calculated and may govern the design.
- The accumulation factor Ca is set to 1.0 (no special accumulation). Roofs with parapets, valleys, or adjacent higher roofs require Ca > 1.0 — these configurations need engineer review.
- Ground snow load (Ss) data is from NBCC 2020 Appendix C. For locations not in the Appendix C table, a site-specific study by a geotechnical engineer or climate specialist is required.
- Does not account for ponding (rain + snow melt on flat roofs), which can create a progressive load instability on flexible roof structures.
Ready to calculate?
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