How deep do footings need to be for a detached garage in Ottawa because of frost?

In Ottawa, your detached garage footings must extend to a minimum depth of 1.2 metres (4 feet) below finished grade to get below the frost line, though many builders in the region go to 1.5 metres (5 feet) for added safety. This requirement comes from the Ontario Building Code and reflects Ottawa's continental climate, where ground temperatures can plunge well below freezing during prolonged cold snaps that reach -25°C to -30°C or colder.

The reason depth matters so much is frost heaving. When water in the soil freezes, it expands and pushes upward with tremendous force. If your footings sit above the frost line, that upward pressure will crack your foundation, shift your walls, and jam your garage doors out of alignment. Repairs from frost heave damage are expensive and disruptive, so getting the footings right the first time is not optional — it is essential.

Footing size and reinforcement also matter beyond just depth. For a standard two-car detached garage, footings are typically 16 to 20 inches wide and 8 inches thick, poured with steel rebar for tensile strength. The footing sits at the bottom of a foundation wall that rises from below grade up to at least 6 inches above finished grade to protect the wood framing from moisture and splash-back. In areas of Ottawa with high water tables or clay-heavy soil — common in neighbourhoods like Orleans, Riverside South, and parts of Gloucester — your builder may recommend a perimeter drainage system (weeping tile) around the footings to manage hydrostatic pressure.

There are a few foundation options to consider. A full perimeter foundation with a poured concrete slab gives you the most solid and long-lasting result, and is the standard approach for heated or finished garages. A floating slab (also called a monolithic slab) combines the footing and floor into one pour, which can work for smaller unheated garages on well-drained soil, though it still needs to account for frost protection at the edges. Some builders use frost-protected shallow foundations (FPSF), which use rigid insulation around the perimeter to keep frost from penetrating beneath the slab, allowing shallower footings. FPSF designs must be engineered specifically for Ottawa's climate zone and are less common for garages but can save on excavation costs.

The City of Ottawa building permit process includes a footing inspection before you can pour concrete, so the inspector will verify your depth meets code. You can reach the City's building services by calling 3-1-1 to confirm requirements for your specific lot.

Getting the foundation right is the single most important step in a detached garage build. Ottawa Garages can connect you with builders who understand local soil and frost conditions and build foundations that last decades.

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• Homeupgraders
• RenoMotion Inc.
• Transitions Renovations
• Dreamwood Construction & Renovations
• Nic’s D.U.C.T Works Inc