A contractor in Windsor pours footings at 1,050–1,200 mm. The same contractor building the same deck in Thunder Bay digs to 2,100 mm — nearly a metre deeper, three times the concrete. The reason is frost. Ontario spans from Lake Erie to James Bay: a climate range wider than most countries. Understanding what drives frost depth, how the Ontario Building Code handles it, and why your municipality's requirement can differ from the town next door is the difference between a deck that stays level for thirty years and one that starts heaving after the second winter.
Water expands roughly 9 percent when it freezes. In unsaturated coarse sand, most of the pore space is air and the volume change is manageable. In saturated silt or clay, the pore space is nearly all water — and when that freezes from the surface downward, it forms ice lenses: horizontal layers of pure ice that grow by drawing additional water upward from the water table by capillary action. The result is ground that doesn't just expand 9 percent — it can heave 50 to 150 mm over a single winter.
If the bottom of your footing sits inside the freeze zone, the ice lenses lift the footing. Come spring thaw, the footing settles — but rarely to exactly the same position. Repeat this five winters and the far post of your deck is 30–40 mm higher than the ledger connection, railings are loose and unsafe, the ledger lag screws have cracked their plates and started prying the rim joist away from the house, and the inspector who looks at it tells you the whole thing needs to come down and the footings need to be redug to code depth. The only reliable solution is to place the footing bearing surface below the depth of seasonal frost penetration. The code mandates this — and it works.
Division B of the Ontario Building Code 2024, Article 9.12.2.2., governs minimum foundation depths in Part 9 (residential and small buildings). The core requirement is Table 9.12.2.2., which is organized by soil type and whether the foundation contains a heated space. For coarse grained soils (sand, gravel) with no heated space — which is what a deck footing is — the requirement is a flat: "below the depth of frost penetration."
Notice what the code does not do: it does not tell you what that depth is in millimetres. The OBC leaves that to the municipalities, who determine it from climatic data published in Supplementary Standard SB-1 (Volume 2 of the OBC). SB-1 Table 2 provides January design temperatures (the 2.5% and 1% values — the cold experienced in the worst 2.5% and 1% of hours), degree-days below 18°C, and other climatic parameters for hundreds of Ontario locations. Engineers and building officials use that data — along with methods from the Canadian Foundation Engineering Manual (CFEM) — to calculate frost penetration depths, which then appear in municipal building guides and bylaws as the enforceable minimum footing depth for your area.
The table also contains two important notes. Sentence 9.12.2.2.(5): the depth "is permitted to be decreased where experience with local soil conditions shows that lesser depths are satisfactory." That gives building officials the discretion to allow shallower footings in specific well-drained sandy areas. Sentence 9.12.2.2.(7): the frost depth rule does not apply to unattached decks not more than 1 storey, not more than 55 m², with floor joists within 600 mm of grade, no roof, and not attached to another structure. That exception is the reason you sometimes see floating deck blocks sitting on compacted gravel — they're code-compliant for the specific scope of small unattached decks, but the moment the deck gets attached to the house, the exception disappears and the full frost depth applies.
The Supplementary Standard SB-1 Table 2 provides climatic design data for over 200 Ontario locations. The columns most relevant to frost depth are the January design temperature (2.5% column — the temperature exceeded in only the coldest 2.5% of January hours) and the annual degree-days below 18°C (a measure of cumulative cold over the whole year). Windsor comes in at -16°C January 2.5% with 3,400 degree-days; Ottawa at -25°C with 4,440 degree-days; Kapuskasing at -34°C with 6,250 degree-days; Moosonee at -36°C with 6,800 degree-days.
To estimate frost penetration depth from these parameters, engineers typically use the Stefan equation or tables from the CFEM: frost depth scales roughly with the square root of the freezing index (degree-days below 0°C), modified by the thermal properties of the soil. Sandy well-drained soil conducts heat quickly and freezes deep fast; water-saturated clay freezes slowly to a moderate depth but heaves significantly. The result, translated into building practice, gives the frost-depth zone bands that Ontario is divided into: roughly 1,200 mm for southern Ontario, 1,500 mm for central and eastern, 1,800 mm for the near north, and 2,100 mm for Thunder Bay and the far north.
Two towns 50 kilometres apart can have different minimum footing depths for reasons that have nothing to do with the latitude. Elevation matters: Caledon (425 m above sea level in the Niagara Escarpment) has a colder January design temperature than Toronto at 90 m, even though they're in the same general region. The Georgian Bay snowbelt creates moisture and temperature conditions that push frost penetration deeper in Owen Sound and Collingwood than in comparable-latitude locations to the east. The Ottawa Valley is sheltered from Lake effects but exposed to Arctic air, giving it a substantially colder design temperature than Kingston despite being at a similar latitude. Champlain Sea clay in the Ottawa area — a legacy of the shallow sea that covered the valley after the last ice age — saturates easily and can heave aggressively, leading to conservative local requirements.
Municipal bylaws are also updated unevenly. Ottawa's deck building guide was updated to reflect 1,800 mm for exposed piers after 2012; the previous version showed 1,500 mm, and contractors working from old guides sometimes use the wrong number. If you're in a municipality that recently annexed a smaller town or township, the depth requirement may have been harmonized (or may not have been). The safe approach: before submitting a permit drawing, call your local building department and ask for the current minimum footing depth for the structure type you're building. Quote the OBC clause and ask specifically whether the municipality has published a local standard. This takes five minutes and prevents re-digging.
OBC Table 9.12.2.2. treats heated and unheated foundations differently. A heated basement in coarse grained well-drained soil has a minimum depth of 1.2 m (good soil drainage to not less than frost penetration depth). The heated space continuously warms the soil immediately beneath the footings, reducing the effective frost depth — so the code allows a reduced minimum provided drainage is adequate. A deck pier is the opposite: no heated space, completely exposed to ambient temperature on all sides, with cold air circulating under the deck above it. Table 9.12.2.2. puts it in the "no heated space" column, which requires bearing below frost penetration depth with no 1.2 m minimum exception. This is why a deck footing in Ottawa needs to go deeper than the house basement footing in the same location — the deck pier is fully exposed, the basement footer benefits from geothermal heating and the house's own heat loss to the ground.
Far southwest (Windsor, Leamington, Chatham, Sarnia): The mildest winter climate in Ontario. SB-1 January 2.5% values of -15°C to -16°C with around 3,400–3,600 degree-days. Regional minimum of 1,200 mm, widely applied across Essex, Kent, and Lambton counties. Windsor is moderated by Lake Erie and regularly cited as Ontario's most temperate winter zone.
Southern Ontario (London, Kitchener-Waterloo, Guelph, GTA, Hamilton, Niagara): January 2.5% values from -16°C to -21°C, degree-days from 3,400 to 4,200. The 1,200 mm standard applies across this zone. Niagara Falls and St. Catharines are at the mild end; Halton Hills and Caledon push toward the central Ontario range. The lake-effect moderation from Lake Ontario and Lake Huron keeps the GTA milder than its latitude would suggest.
Central Ontario and cottage country (Barrie, Orillia, Parry Sound, Bracebridge, Huntsville, Collingwood, Owen Sound): January 2.5% values drop to -24°C to -26°C, degree-days rise to 4,200–4,900. The regional baseline moves to 1,500 mm. Muskoka in particular has significant freeze-thaw variability due to lake effects and elevation. Cottage properties in this zone that sit vacant all winter face the full frost load with no geothermal warming from the building — size exposed footings conservatively.
Eastern Ontario (Ottawa, Kingston, Peterborough, Pembroke, Belleville): Ottawa anchors this zone at 1,800 mm with its -25°C January design temperature and 4,440 degree-days. Kingston and Peterborough are somewhat milder (-22°C to -23°C) and typically use 1,500 mm. Pembroke and Renfrew County, exposed to Ottawa Valley cold air, push toward 1,500–1,800 mm. Bancroft and Hastings County are cold (-28°C to -31°C) and should use 1,500 mm minimum with local confirmation.
Near north (North Bay, Sudbury, Sault Ste. Marie, Elliot Lake, Haileybury): January 2.5% temperatures of -28°C to -34°C, degree-days above 5,000. Regional baseline moves to 1,800 mm. Greater Sudbury officially uses 1,800 mm. The Sault's SB-1 data is comparable to Sudbury and the same standard applies. Note that the snowbelt effects around Sault Ste. Marie create variable soil saturation — wet years can push frost deeper.
Far north (Timmins, Thunder Bay, Cochrane, Kapuskasing, Kenora, Dryden, Fort Frances): January 2.5% temperatures below -31°C, degree-days above 5,600. Thunder Bay specifies 2,100 mm. Timmins at -34°C is conservative at 1,800 mm but some contractors and inspectors use 2,100 mm — verify locally. Cochrane, Kapuskasing, and the James Bay coastal communities (Moosonee, Attawapiskat) are in the extreme frost zone where seasonal freeze penetrates well beyond 2 m. For any work in these areas, a conversation with the local building authority is non-optional.
Frost depth governs how deep you dig — it has nothing to do with how wide the footing needs to be. Width (diameter for a cylindrical pier, breadth for a pad footing) is governed by the load per post and the allowable soil bearing pressure from OBC 9.4.2. The two calculations are independent and both must be satisfied simultaneously. A 1,800 mm deep footing with an undersized diameter will sink under load in summer; a properly-sized footing at 900 mm depth will heave in winter. You need both dimensions right, and the Deck Footing Depth calculator on this site handles the sizing side.
Common footing types in Ontario residential construction: Sonotube piers (concrete-filled cardboard tubes, the standard residential solution), poured pier-and-footing pads (for heavier loads where the required bearing area exceeds Sonotube sizes), Bigfoot plastic bell bases (a one-pour shortcut for a flared base), and helical screw piers (the engineered option that bypasses the dig entirely, practical on clay in winter or on tight lots with no equipment access). All of these, except helical piers specifically engineered to bypass frost, still require the bearing surface to be at or below frost depth.
If your municipality is not in this tool's list, or the regional baseline is flagged and you want confirmation, the process is: 1. Call or email your local Chief Building Official or building permit office and ask: "What is your required minimum footing depth for an exposed deck pier / foundation footing?" Most inspectors know this number cold and will give it to you in 60 seconds. 2. If the municipality is newly amalgamated or has no published standard, ask them to point you to the relevant building standard — it may be inherited from the previous township. 3. Look up SB-1 Table 2 data for the nearest named location in OBC Volume 2 and apply the CMHC regional baseline for that degree-day range. 4. When in doubt, add 150–300 mm to the nearest regional baseline and call it done. Going slightly deeper costs a small amount of concrete and absolutely eliminates the risk. Going too shallow costs a full re-dig, a re-inspection, and potentially a failed final permit if the structure has already been framed.
This tool is a pre-permit reference, not a permit document. Every foundation in Ontario requires a building permit and must comply with the OBC and the authority having jurisdiction. Municipal minimum footing depths are set locally and may have changed since this tool was last verified (2026-06-01). Always confirm with your local building inspector before submitting permit drawings or pouring concrete. Verified against OBC 2024 Division B Section 9.12.2 and SB-1 Table 2 (Volume 2) by ToolFluency research team. Reviewed by a Red Seal Carpenter.
Free Ontario frost depth lookup tool. Find the minimum footing depth for your municipality — Toronto (1200 mm), Ottawa (1800 mm), Thunder Bay (2100 mm) and 85+ more. Based on OBC 9.12.2 and SB-1 climatic data.
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