On this page
Updated June 2026What 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.
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?
Specified roof snow loads across Canada range from approximately 0.5 kPa in sheltered low-snow areas to over 2.5 kPa in high-snowfall regions like Quebec City (Ss = 2.8 kPa) and St. John's, NL (Ss = 2.8 kPa). After applying NBCC roof factors, a typical flat roof in Ottawa (Ss = 2.0 kPa) carries approximately 1.6 kPa of specified roof snow load.
How much snow is too much for a roof?
Most residential roofs are designed for 1.0–2.0 kPa of specified snow load. One kPa equals approximately 100 kg/m². Warning signs of potential overload include cracking sounds from the structure, doors or windows that suddenly stick or won't close properly, visible sagging of the roof deck or ridge, or cracks appearing in interior walls or ceilings.
Does a steeper roof have less snow load?
Yes. The roof slope factor Cs reduces snow load on steeper roofs. For roofs with slopes over 30°, Cs drops below 1.0 — snow slides off before accumulating to full depth. For slopes over 70°, Cs = 0 (no snow load). Unheated roofs with good slipperiness (metal, glass) can also have Cs reduced further under the NBCC.
What does the importance factor Is affect?
The importance factor Is scales the entire snow load based on the consequences of failure. For normal residential buildings, Is = 1.0. For post-disaster buildings (hospitals, fire stations), Is = 1.3, meaning the roof must be designed for 30% more snow load. For low human occupancy buildings like agricultural storage, Is = 0.8.
What is the rain-on-snow load (Sr) in the NBCC formula?
The rain load Sr accounts for rain falling on existing snow — a common scenario during freeze-thaw cycles in southern Canada. It adds a fixed load based on local rain statistics. In Vancouver, Sr = 0.5 kPa (significant); in Winnipeg, Sr = 0.2 kPa (minor). The Sr term is always added to the snow load — it does not get reduced by the Cb or Cs factors.
How do I know if my roof is overloaded with snow?
A rough check: measure the snow depth on your roof and multiply by the snow density (fresh snow ≈ 1 kN/m³, packed snow ≈ 3 kN/m³, wet snow ≈ 4 kN/m³). If the calculated load approaches your building's design load, consider removing snow — especially from flat roofs and roofs with complex geometry where drifting can concentrate load.
Do snow drifts increase the roof load above the NBCC calculated value?
Yes. The accumulation factor Ca in the NBCC formula accounts for drift loading at walls, parapets, and level changes. At the base of a higher adjacent wall or parapet, Ca can be significantly greater than 1.0, creating local drift loads much higher than the basic roof snow load. These localized areas require special structural attention.
What is the difference between ground snow load (Ss) and roof snow load?
Ss is the ground snow load — the statistical snow depth on the ground at a given location, based on weather records. Roof snow load is always less than ground snow load because wind clears snow from roofs, heat from conditioned spaces melts snow from below, and roof slopes allow shedding. The NBCC basic roof factor Cb = 0.8 reflects this relationship for most roofs.
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.