How is the design snow load calculated in Ontario?
Part 4 calculates roof snow load with the formula S = Is[Ss(Cb Cw Cs Ca) + Sr], using the ground snow load (Ss) and rain load (Sr) for the building's location and factors for roof shape, wind exposure, slope and accumulation.
Each variable adjusts the location-specific ground snow load for the realities of the roof: Is is the importance factor (higher for post-disaster buildings, lower for low-occupancy buildings), Ss and Sr are the 1-in-50-year ground snow and associated rain loads taken from the climatic data tables for the municipality, Cb is the basic roof snow load factor, Cw accounts for wind exposure, Cs for roof slope (steep roofs shed snow), and Ca for accumulation effects like drifting against a taller wall or sliding snow from an upper roof. Drift loading at roof steps and parapets is frequently the governing case and a common source of review comments on permit drawings.
The code starts with how much snow your town gets in a bad winter, then adjusts for your roof: steep roofs shed snow, sheltered roofs keep it, and spots next to walls collect deep drifts. That final number is what the structure must carry.
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This page is provided for general information. Building code requirements depend on your specific project, occupancy and municipality, and code editions change. Always verify requirements against the current Ontario Building Code and confirm with your local building department.