Ontario Frost Depth by Municipality

Frost Depth in Ontario: Why Your Footing Depth Varies by 900 mm Across the Province

A contractor in Windsor pours footings at 1,200 mm. The same contractor building the same deck in Thunder Bay digs to 2,400 mm — 1,200 mm deeper, twice the concrete in every pier. 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.

What is frost heave — and why does 9% matter?

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.

What does the Ontario Building Code actually say about frost depth?

According to Division B of the Ontario Building Code 2024 (as amended January 16, 2025), Article 9.12.2.2., the minimum depth of foundations below finished ground level must conform to Table 9.12.2.2. The table works on three variables: soil type, whether the foundation contains a heated space, and soil drainage. On rock or well-drained coarse-grained soil (clean sand, gravel), the table sets no depth limit. On poorly drained coarse-grained soil or on silt with no heated space, the footing must bear below the depth of frost penetration. On clay or soils not clearly defined — which describes most Ontario building lots — an unheated foundation must go at least 1.2 m and not less than the frost depth. In practice, a deck pier is an unheated foundation on unproven soil, so building departments usually apply the below-frost-penetration requirement — that is where your municipal minimum comes from. The calculator above starts from that safe default (unheated, clay or not sure, poor drainage) and lets you change the soil, drainage and heated inputs to see exactly which table cell applies.

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) permits the depth to be decreased where experience with local soil conditions shows lesser depths perform, or where the foundation is designed for the lesser depth. 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.

How is frost depth calculated from SB-1 climatic data?

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, 2,100 mm for the far north, and a conservative 2,400 mm for Thunder Bay.

Why do footing depth requirements vary between Ontario municipalities?

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 is a good example: older City deck guidance cited 1,800 mm for exposed piers, while the City’s current sample deck plan dimensions footings at 1,500 mm, so both numbers are in circulation. 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.

Why does a deck pier go deeper than a heated basement footing?

OBC Table 9.12.2.2. treats heated and unheated foundations differently because a heated basement continuously feeds warmth into the soil beneath its footings, cutting the effective frost penetration. The clay row makes the split explicit: a foundation with a heated basement or crawl space needs 1.2 m; the same foundation with no heated space needs 1.2 m but not less than the depth of frost penetration — in Ottawa that pushes an exposed pier to 1,800 mm while the heated house footing beside it can stop at 1.2 m. Sentence 9.12.2.2.(2) adds a trap worth knowing: a foundation insulated in a way that reduces heat flow to the soil beneath the footings counts as containing no heated space and must go to the full unheated depth. A deck pier is the extreme case — no heat source at all, cold air circulating under the deck above it — so it always gets the full local frost depth.

Regional frost depth zones in Ontario

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): We list Ottawa at a conservative 1,800 mm (confirm with the City; its current sample deck plan shows 1,500 mm) given 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 Highlands are cold (-28°C January design temperature, SB-1) and should use 1,500 mm minimum with local confirmation.

Near north (North Bay, Sudbury, Sault Ste. Marie, Elliot Lake, Haileybury): SB-1 January 2.5% design temperatures run about -25°C at Sault Ste. Marie to -32°C at Haileybury (North Bay and Sudbury both sit at -28°C), with degree-days around 5,000. Regional baseline moves to 1,800 mm. Greater Sudbury officially uses 1,800 mm, and Sault Ste. Marie building practice applies the same 1,800 mm standard — verify locally, as Lake Superior moderates the Sault's published climate data relative to inland towns. Snowbelt effects around the Sault also 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: 2,400 mm (conservative; confirm with the City). 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 by Ontario municipality (all 85)

Local frost depth used for unheated foundations (deck piers, porches, garages, cottages on clay or unknown soil), measured from finished grade to the bottom of the footing. A heated basement on clay needs 1.2 m (Table 9.12.2.2), and rock or well-drained sand and gravel can need less, so use the calculator above for your soil. Town not listed? Use the deepest figure for your region and confirm it with your building department.

"Regional baseline" = CMHC/NRC frost-zone figure checked against SB-1 climatic data. Figures compiled June 2026; always confirm with your local building department.

Footing types and how depth interacts with sizing

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 maximum allowable bearing pressure from OBC Table 9.4.4.1 — 150 kPa for dense sand, gravel or stiff clay, 75 kPa for firm clay, 40 kPa for soft clay. 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.

What to do if you can't find your municipality's published number

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. Use the calculator's "Not listed? Pick your region" option, which applies the deepest figure we list for that region, or 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 lookup is one of ToolFluency's Ontario Building Code tools — pair it with the Deck Footing Depth calculator to size the tube once you know how deep to dig, and the Deck Ledger Attachment tool for the house connection. If you build decks and foundations for a living, the Ontario Contractor Toolkit collects the OBC, WSIB and business tools in one place — and Crew handles the timesheets, job costing and invoicing side of the same jobs.

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 verification. Always confirm with your local building inspector before submitting permit drawings or pouring concrete. Code citations verified against the Ontario Building Code 2024 (O. Reg. 163/24, as amended January 16, 2025), Division B Article 9.12.2.2. and Table 9.12.2.2., and Supplementary Standard SB-1 Table 2 (Volume 2), by a Red Seal Carpenter (Ontario CofQ) — July 6, 2026; soil / drainage / heated logic re-checked October 2, 2026. Municipal figures compiled June 1, 2026.