National baseline
NBC Part 4 Division B Section 4.1.6 covers snow, ice, and rain loads on roofs, with ground snow load data in Appendix C climatic data tables.
Canadian building code question
What snow load requirements apply to buildings in Canada?
The NBC requires roofs to be designed for snow loads based on the site's ground snow load, modified by factors for roof geometry, exposure, accumulation, drift, sliding, and rain-on-snow surcharge as specified in Part 4 Division B.
Snow loads are a primary structural design consideration for most Canadian buildings. The National Building Code provides ground snow load data for every location in Canada and sets out the methodology for converting ground snow loads to roof design loads, accounting for roof shape, exposure, drifting, sliding, and the combination of snow with rain.
What to check first
- Ground snow loads (Ss) and associated rain loads (Sr) are published in the NBC climatic data tables for specific locations across Canada.
- Roof snow loads are calculated by modifying the ground snow load with factors for basic roof snow load, wind exposure, slope, shape, accumulation, and importance.
- Additional loads for drift accumulation at parapets, roof steps, and adjacent higher roofs must be considered in the structural design.
Jurisdiction notes
Provincial variations
Provinces adopt the NBC snow load provisions with the national climatic data. Some municipalities in high-snow regions may have supplementary snow load data.
Site-specific considerations
Elevation, terrain exposure, roof geometry, and the presence of adjacent taller structures all affect the design snow load.
Work through it in this order
- Determine the ground snow load (Ss) and associated rain load (Sr) from the NBC climatic data for the building location.
- Calculate the basic roof snow load factor (Cb) based on the building's width and exposure.
- Apply slope, shape, wind exposure, and importance factors to determine the specified roof snow load.
- Calculate drift loads at parapets, roof steps, and locations adjacent to higher roofs.
- Verify unbalanced snow load and sliding snow load conditions for sloped roofs.
Common questions
Where do I find the ground snow load for my location?
Ground snow loads are published in the NBC climatic data tables (Appendix C) by location name. Natural Resources Canada also provides an online climatic data tool for site-specific values.
Do flat roofs have higher snow loads than sloped roofs?
Flat and low-slope roofs generally retain more snow than steeply sloped roofs. The NBC applies slope reduction factors for roofs above certain slopes, but flat roofs may also accumulate drift loads at parapets.
Is rain-on-snow load always required?
The associated rain load (Sr) is always included in the snow load combination. In locations with high rainfall during the snow season, this surcharge can be a significant portion of the total roof load.