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Updated June 2026Full formula and methodology: How this calculator works →
What is roof snow load?
Roof snow load is the structural load imposed on a roof by accumulated snow and ice. It is one of the dominant design loads for Canadian buildings — in many regions, snow load governs the design of the roof structure more than any other load, including wind.
Snow load varies dramatically across Canada. A building in Vancouver (Ss = 1.9 kPa) faces very different snow loads than the same building in Quebec City (Ss = 2.8 kPa) or Whitehorse (Ss = 1.6 kPa but with extreme cold and ice). The NBCC provides ground snow load data for hundreds of Canadian locations, and the roof snow load formula converts ground snow load to the load the roof must actually carry.
Roof snow loads also vary within a single structure. Flat sections accumulate more than sloped sections. Areas adjacent to higher walls or parapets receive drifted snow. These localized loads can be 2–3 times the basic roof snow load and require specific attention in structural design — use the beam load calculator to size the rafters and roof beams carrying these loads.
NBCC 2020 snow load formula
The specified roof snow load is calculated from NBCC 2020, Clause 4.1.6.2:
S = Is × [Ss × (Cb × Cw × Cs × Ca) + Sr]
Where each term has a specific physical meaning:
- S = specified roof snow load (kPa)
- Is = importance factor (0.8 to 1.3 depending on building category)
- Ss = ground snow load from NBCC Appendix C (kPa)
- Cb = basic roof snow load factor (0.8 for most roofs)
- Cw = wind exposure factor (1.0 sheltered, 0.75 open, 0.5 windswept)
- Cs = slope factor (1.0 for flat; reduces for steep roofs)
- Ca = accumulation factor (1.0 for simple roofs; >1.0 at drift locations)
- Sr = associated rain load (kPa)
Roof factors explained
Cb = 0.8 (basic roof factor): The roof snow load is always less than the ground snow load because wind partially clears roofs and heat from the building melts snow from below. Cb = 0.8 is the standard value for most heated roofs.
Cw (wind factor): A building in an open field exposed to sweeping winds gets a Cw of 0.75 or even 0.5. A building sheltered by trees or adjacent structures uses Cw = 1.0. Using the reduced Cw requires confidence that the site will remain wind-exposed — future landscaping or construction could change this.
Cs (slope factor): For roofs with slope angle α: Cs = 1.0 for α ≤ 30°; reduces linearly to 0.0 at α = 70°. Metal and glass roofs with good slipperiness use a lower threshold — Cs begins reducing at 15° instead of 30°.
Ca (accumulation factor): Ca = 1.0 for simple, unobstructed roofs. At the base of a higher adjacent wall, parapets, or level changes, Ca can be as high as 2.0 or more — creating a drift load zone that requires heavier structure beneath the drift.
Worked examples
Example 1 — Flat commercial roof in Ottawa (Ontario):
- Ss = 2.0 kPa, Sr = 0.3 kPa, Is = 1.0 (normal), Cb = 0.8, Cw = 1.0, Cs = 1.0, Ca = 1.0
- S = 1.0 × [2.0 × (0.8 × 1.0 × 1.0 × 1.0) + 0.3] = 1.0 × [1.6 + 0.3] = 1.9 kPa
Example 2 — Sloped roof (35°) on a chalet in Quebec City:
- Ss = 2.8 kPa, Sr = 0.4 kPa, Is = 1.0, Cb = 0.8, Cw = 1.0, Cs = 0.875 (slope reduction), Ca = 1.0
- S = 1.0 × [2.8 × (0.8 × 1.0 × 0.875 × 1.0) + 0.4] = 1.0 × [1.96 + 0.4] = 2.36 kPa
Ground snow loads by Canadian city (NBCC 2020)
| City | Province | Ss (kPa) | Sr (kPa) | Flat roof load (kPa)* |
|---|---|---|---|---|
| St. John's | NL | 2.8 | 0.8 | 3.0 |
| Quebec City | QC | 2.8 | 0.4 | 2.6 |
| Montreal | QC | 2.1 | 0.4 | 2.1 |
| Ottawa | ON | 2.0 | 0.3 | 1.9 |
| Halifax | NS | 1.8 | 0.6 | 2.0 |
| Vancouver | BC | 1.9 | 0.5 | 2.0 |
| Edmonton | AB | 1.5 | 0.2 | 1.4 |
| Toronto | ON | 1.4 | 0.4 | 1.5 |
| Calgary | AB | 1.3 | 0.2 | 1.2 |
| Winnipeg | MB | 1.3 | 0.2 | 1.2 |
| Saskatoon | SK | 0.9 | 0.1 | 0.8 |
| Victoria | BC | 0.7 | 0.5 | 1.1 |
* Flat roof load calculated using Is=1.0, Cb=0.8, Cw=1.0, Cs=1.0, Ca=1.0
Frequently asked questions
What is the snow load on a roof in Canada?
How much snow is too much for a roof?
Does a steeper roof have less snow load?
What does the importance factor Is affect?
What is the rain-on-snow load (Sr) in the NBCC formula?
How do I know if my roof is overloaded with snow?
Do snow drifts increase the roof load above the NBCC calculated value?
What is the difference between ground snow load (Ss) and roof snow load?
Code reference
Based on NBCC 2020, Division B, Clause 4.1.6.2 (snow loads on roofs). Ground snow load data from NBCC 2020 Appendix C (climatic data for Canadian locations). Importance categories per NBCC Table 4.1.6.1. Always consult a licensed structural engineer for building permit applications.
What to do with your roof snow load result
Your calculated roof snow load (Ss) is the factored design load your roof structure must resist under NBCC 2020. Here is how to apply it:
- Compare to your existing structure: If your roof has 2×6 rafters at 24" O.C. and your Ss exceeds 1.5 kPa, have a structural engineer confirm the framing can carry the load. This matters most for homes built before 1980 in high-snow zones — Ottawa (Ss = 2.4 kPa), Quebec City (Ss = 3.2 kPa), or anywhere with Ss > 2.0 kPa.
- Use it to determine rafter span limits: CSA O86 span tables require the governing snow load for your location. For Ss = 1.9 kPa (Toronto), 2×8 SPF rafters at 24" O.C. span approximately 4.0m. For Ss = 3.0 kPa (Ottawa design), the same rafter spans only about 3.2m — a difference that changes your framing plan.
- Include it in any permit submission: All structural permit drawings in Canada must state the governing snow load from NBCC Appendix C for your location. Your result from this calculator matches the NBCC Appendix C published values used by engineers and building officials.
- Check for drift and unbalanced loads separately: If your roof has skylights, parapets, or elevation changes (split-level homes, additions), additional drift loads per NBCC Clause 4.1.6.3 may apply on top of the balanced load this calculator produces. Drift loads can double the local roof load in some configurations — flag this for your structural engineer.