How do people underestimate snow load when building a garage roof in Ottawa?

Underestimating snow load on a garage roof is a mistake that can have catastrophic consequences in Ottawa, where winter snowfall regularly accumulates to levels that put serious structural demands on any roof. The Ontario Building Code sets specific snow load requirements for Ottawa, and builders who ignore or undersize these requirements are gambling with structural safety.

Ottawa's ground snow load under the Ontario Building Code is 2.4 kPa (approximately 50 pounds per square foot), which is among the highest values in populated southern Ontario. This is the weight of snow that the code assumes will accumulate on the ground during a design winter event. The roof snow load is calculated from this ground value using factors for roof slope, wind exposure, building shape, and — critically — whether the roof geometry creates areas where snow drifts and accumulates beyond the uniform load. For a typical garage roof, the design roof snow load after all factors are applied often works out to 1.5 to 2.0 kPa (roughly 30 to 40 pounds per square foot), with higher localized loads where drifting occurs.

Where Homeowners and Builders Get It Wrong

The most common mistake is using undersized roof framing — rafters or trusses that are adequate for the dead load (the weight of the roofing materials and sheathing) but not adequately sized for Ottawa's full snow load. This happens when builders use span tables from regions with lower snow loads, when they space rafters at 24 inches on centre where 16 inches is required for the span, or when they use 2x6 rafters where the span and load demand 2x8 or 2x10. Engineered roof trusses designed specifically for your garage dimensions and Ottawa's snow load are the safest approach, and they are actually cost-competitive with site-built rafters because they are manufactured efficiently and install quickly.

The second mistake is ignoring drift loads. When a garage is built near a house or other taller structure, wind carries snow off the taller roof and deposits it on the lower garage roof. This creates a drift zone where snow accumulates to significantly greater depth than the uniform load. The Ontario Building Code requires that these drift loads be calculated and accounted for in the structural design. A garage roof designed for uniform snow load alone can be dangerously underdesigned in the drift zone. In Ottawa, where roof-to-roof drifting is common in suburban neighbourhoods with attached or closely spaced garages, drift loads can exceed 4 to 5 kPa in localized areas — more than double the uniform design load.

The third mistake is not accounting for rain-on-snow events. Ottawa's late-fall and early-spring weather frequently produces rain falling on an existing snowpack. The rain saturates the snow, dramatically increasing its weight. Dry fresh snow weighs roughly 5 to 10 pounds per cubic foot, but saturated snow and slush can weigh 30 to 40 pounds per cubic foot — a fourfold increase. A roof designed with minimal margin above the code minimum snow load can be pushed to its limits during a rain-on-snow event, particularly if the roof drainage is blocked by ice dams.

Low-slope garage roofs are particularly vulnerable. Some homeowners choose a nearly flat roof for aesthetic reasons or to maximize height under zoning restrictions. Low-slope roofs shed snow poorly — the snow sits and accumulates rather than sliding off, and ice dams form at the edges preventing drainage. The Ontario Building Code assigns higher snow load factors to roofs with slopes below 30 degrees (approximately 7:12 pitch) because more snow stays on the roof. A garage with a 2:12 or 3:12 pitch in Ottawa must be designed for essentially the full ground snow load, plus drift and rain-on-snow considerations.

The consequences of underdesigned roof framing range from visible sagging and deflection (rafters bowing under load, which stresses roofing materials and causes leaks) to partial or complete roof collapse. Ottawa has experienced garage roof collapses during heavy snow years, and they almost always involve structures that were either built without permits, built with undersized framing, or built to standards from a less demanding climate zone. A collapsed garage roof does not just destroy the garage — it destroys everything inside it and can injure or kill anyone who happens to be there.

The cost of properly engineered roof trusses or adequately sized rafters for Ottawa's snow load adds $1,000 to $3,000 to a standard two-car garage build compared to the minimum framing that might pass in a milder climate. That is an insignificant premium for structural safety over the life of the building.

Connect with builders on Ottawa Garages who use engineered trusses and design for Ottawa's full snow load requirements including drift and rain-on-snow scenarios.

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