In a freezing climate a deck has to hold up the snow that lands on it. The Canadian building code treats an accessible exterior platform much like a small roof for the purpose of load, and the resulting numbers shape how beams and joists are sized.
The occupancy minimum
Under Part 9 of the National Building Code, balconies, decks and other accessible exterior platforms that serve a single dwelling unit are designed to carry the specified roof snow load or an occupancy load of 1.9 kPa, whichever is greater. In practice this means a deck in a low-snow area is still designed for the 1.9 kPa people-and-furniture load, while a deck in a high-snow area is governed instead by the snow figure.
How the specified snow load is calculated
The specified snow load under Part 9 follows a formula that combines the ground snow load with a roof factor and an associated rain load:
The basic roof factor Cb is 0.45 where the entire width of the roof does not exceed 4.3 m, and 0.55 otherwise. The specified snow load is in no case taken as less than 1 kPa. The ground snow load Ss and rain load Sr are location-specific values published for each area in the code's supplementary data.
Ground snow load varies widely by location
The single most important input is the local ground snow load, and it changes dramatically across the country. The figures below are published design snow loads for decks and balconies in selected British Columbia communities, calculated as 0.55·Ss + Sr, to show the range a designer can encounter within one province.
| Location | Design snow load (kPa) |
|---|---|
| Victoria | 1.0 |
| Vancouver | 1.2 |
| Kamloops | 1.2 |
| Cranbrook | 1.9 |
| Prince George | 2.1 |
| Terrace | 3.6 |
| Whistler | 6.1 |
A deck in Victoria is governed by the 1.9 kPa occupancy minimum, while a deck in Whistler must be designed for several times that load. The same drawing cannot serve both places.
Loads the standard tables do not cover
Concentrated and prolonged loads change the calculation. A hot tub, large planters, or heavy fixed furniture can exceed the snow and occupancy loads the prescriptive tables assume, and these situations may require an engineered design rather than a standard span table.
From load to member sizing
Once the governing load is known, framing members are selected from the span tables that correspond to that load, or by engineered calculation where the prescriptive tables do not apply. Higher design loads mean larger members, closer joist spacing, or both. Wood framing exposed to repeated wetting is in a wet service condition, which is not always captured by the basic prescriptive solutions and may need additional design attention.
General reference values. Obtain the official ground snow load for your site from your local building department before sizing members.
References
- British Columbia, BC Building Code, Section 9.4 Structural Requirements.
- BC Housing, Building Safe and Durable Decks and Balconies (PDF).
- National Research Council Canada, National Building Code of Canada 2020.