How to Calculate Snow Load on a Roof in Canada (NBCC 2020)

What is snow load and why it matters in Canada

Snow load is the downward force that accumulated snow exerts on a roof structure, measured in kiloPascals (kPa). In Canada, roof structures must be designed to withstand the snow loads specified by the National Building Code of Canada (NBCC 2020) for their specific location — not a national average, and not the values used in the United States.

The difference between cities is dramatic. A roof in Lethbridge, Alberta faces a ground snow load of 0.8 kPa, while a roof in St. John's, Newfoundland faces 2.8 kPa — more than three times as much. A structure designed to US building codes (ASCE 7) may be significantly undersized for Canadian conditions, particularly in Atlantic Canada and Quebec.

Roof collapses from snow overloading happen every winter in Canada. The most vulnerable structures are flat-roofed commercial buildings, older homes in heavy-snowfall regions, and any roof that has been re-roofed with heavier materials without a structural reassessment.

Ground snow load (Ss) vs rain-on-snow (Sr)

NBCC 2020 Table C-2 gives two snow values for every city in Canada:

• Ss — Ground snow load: The 1-in-50 annual probability snow load at ground level, in kPa. This is based on decades of snow measurement records and represents the load your roof must be designed to resist.
• Sr — Rain-on-snow load: An additional load that accounts for rain falling on snowpack, causing it to become denser and heavier. Sr is higher in coastal cities (0.5–0.8 kPa in Halifax and St. John's) and lower in dry prairie cities (0.1 kPa in Saskatoon and Regina).

Both values are used together in the NBCC design formula — you cannot use Ss alone and ignore Sr.

Roof shape factor: how slope and geometry reduce the design load

The roof shape factor (Cb) accounts for the fact that not all the ground snow accumulates on a roof — wind carries some of it away, and steep slopes shed it. NBCC 2020 Clause 4.1.6.2 sets the following:

There are also three additional factors in the full formula: Cw (wind exposure coefficient, default 1.0 for most buildings), Cs (slope coefficient, 1.0 for sloped roofs unless unobstructed), and Ca (accumulation factor for complex roof shapes, valleys, and skylights). For a simple residential roof, all three are 1.0.

The NBCC 2020 snow load formula

Per NBCC 2020 Clause 4.1.6.2, the specified snow load on a roof is:

Where Is is the importance factor (1.0 for normal residential buildings, 1.15 for post-disaster buildings). For a typical house, this simplifies to:

Worked example — Ottawa, flat roof: Ss = 2.0 kPa, Sr = 0.3 kPa. S = (0.8 × 2.0) + 0.3 = 1.6 + 0.3 = 1.9 kPa. This means the roof structure must resist 1.9 kPa of snow load — roughly 190 kg per square metre.

Worked example — Lethbridge, flat roof: Ss = 0.8 kPa, Sr = 0.1 kPa. S = (0.8 × 0.8) + 0.1 = 0.64 + 0.1 = 0.74 kPa. Lethbridge's combination of low snowfall and low rain-on-snow makes it one of Canada's lightest snow load cities.

NBCC 2020 ground snow loads by city

The table below shows Ss (ground snow load), Sr (rain-on-snow), and the calculated specified roof snow load S for a flat roof (Cb = 0.8, all other factors = 1.0) at each city.

Source: NBCC 2020 Appendix C, Table C-2. S calculated as (0.8 × Ss) + Sr with Is = Cb = Cw = Cs = Ca = 1.0.

How to compare calculated load to your roof's capacity

A roof's structural capacity in kPa is determined by its span, lumber size, grade, and spacing — it is not printed on the building anywhere. The information comes from the original structural drawings, the building permit, or an assessment by a structural engineer.

For a standard light-frame wood residential roof built to code in Canada, the typical design capacity is 1.0–1.5 kPa. If your calculated snow load S is within that range or below it, the roof was designed to handle it. If S exceeds it — for example, a St. John's flat roof at 3.0 kPa — a structural engineer must verify the roof is adequate or specify upgrades.

Signs a roof may be overloaded: visible sagging of rafters or decking, doors and windows that have become difficult to open, cracking sounds from the attic during heavy snowfall. If you observe any of these, remove snow immediately using a roof rake from ground level and contact a structural engineer.

You should also consider drift loading — snow that accumulates in drifts against parapets, at roof step changes, or below mechanical equipment. Drift loads can be two to three times the calculated flat roof load and are the most common cause of localized roof failure.