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Frost Depth in Ontario: What the Building Code Says

Frost depth in Ontario: the 2024 Building Code sets no single number, uses local experience, and requires 1.2 m in clay. Heated vs unheated, decks, garages.
minimum depth in clay soils
1.2 m
minimum depth in clay soils
frost depth set by local experience
Local
frost depth set by local experience
deck and shed exemption size
55 m²
deck and shed exemption size
joist height limit for the exemption
600 mm
joist height limit for the exemption

Last updated

There is no single frost depth in the Ontario Building Code. The code says the depth of frost penetration "shall be established on the basis of local experience" (OBC Div. B 1.1.3.2.OBC Div. B 1.1.3.2. Depth of Frost Penetration(1) Depth of frost penetration shall be established on the basis of local experience.), and the footing depth table uses that local value together with a fixed minimum of 1.2 m in clay or soils not clearly defined (OBC Div. B Table 9.12.2.2.OBC Div. B Table 9.12.2.2. Minimum Depths of Foundations — Forming Part of Sentence 9.12.2.2.(1)Table 9.12.2.2 — Minimum Depths of Foundations — Forming Part of Sentence 9.12.2.2.(1) Forming Part of Sentence 9.12.2.2.(1) Type of Soil | Min Depth of Foundation Containing Heated Basement or Crawl Space[1] With Good Soil Drainage | Min Depth of Foundation Containing Heated Basement or Crawl Space[1] With Poor Soil…). Many southern Ontario building departments work with about 1.2 m (4 ft), and colder regions often ask for more.

This page explains how the 2024 code handles frost for houses and other Part 9 buildings, where the exemptions are, and why heated basements are treated differently. Your building department sets the local frost depth and decides how the code applies to your site.

Frost depth rules at a glance

Item Rule in short Where to look
Frost depth value from local experience, not printed in the code 1.1.3.2.
Footing depth Table 9.12.2.2., by soil, drainage and heating 9.12.2.2.(1)
Insulated foundations treated as containing no heated space 9.12.2.2.(2)
Concrete steps, 1 or 2 risers can sit on the ground 9.12.2.2.(4)
Concrete steps, more than 2 risers follow the depth table 9.12.2.2.(3)
Shallower footings allowed by local experience or design 9.12.2.2.(5)
Small accessory buildings exempt within size and height limits 9.12.2.2.(6)
Small decks exempt within size and height limits 9.12.2.2.(7)
Attached unheated garages below frost level 9.35.3.2.(3)
Engineered foundations below frost damage, or designed for frost 4.2.4.4.

This is our summary of the articles, not a copy of the code. Read the full text before you set a footing depth.

Why frost depth matters

When water in the soil freezes, it expands and can lift a footing (frost heave). Soil can also freeze to the side of a foundation and pull it up, which the code calls adfreezing or "frost grip" (Div. A 1.4.1.2.OBC Div. A 1.4.1.2.(1) AdfreezingAdfreezing means the adhesion of soil to a foundation unit resulting from the freezing of soil water. (Also referred to as "frost grip.")). Putting the bearing surface below the frost line, or on soil that does not heave, avoids both problems.

The depth table in plain words

Table 9.12.2.2.OBC Div. B Table 9.12.2.2. Minimum Depths of Foundations — Forming Part of Sentence 9.12.2.2.(1)Table 9.12.2.2 — Minimum Depths of Foundations — Forming Part of Sentence 9.12.2.2.(1) Forming Part of Sentence 9.12.2.2.(1) Type of Soil | Min Depth of Foundation Containing Heated Basement or Crawl Space[1] With Good Soil Drainage | Min Depth of Foundation Containing Heated Basement or Crawl Space[1] With Poor Soil… has four soil rows and four columns: heated or unheated foundation, with good or poor soil drainage.

  • Rock: no minimum depth, heated or not.
  • Coarse-grained soils (sand and gravel): no minimum, except an unheated foundation on poorly drained soil goes below the depth of frost penetration.
  • Silt: no minimum under a heated basement or crawl space, but an unheated foundation goes below the depth of frost penetration.
  • Clay or soils not clearly defined: at least 1.2 m in every case, and an unheated foundation must also go at least as deep as frost penetration.

"Good soil drainage" in the table means good drainage to at least the depth of frost penetration. Silts and clays are the soils that heave, which is why they carry the strictest rules.

Heated vs unheated foundations

A heated basement or crawl space warms the soil under its footings, so frost cannot reach them as easily. That is why the heated columns of the table are more lenient.

The catch is insulation. A foundation insulated in a way that reduces heat flow to the soil beneath the footings counts as containing no heated space (9.12.2.2.(2)OBC Div. B 9.12.2.2. Minimum Depth of Foundations(1) Except as provided in Sentences (4) to (7), the minimum depth of foundations below finished ground level shall conform to Table 9.12.2.2. (2) Where a foundation is insulated in a manner that will reduce the heat flow to the soil beneath the footings, the foundation depth shall conform to that required for…). So a heavily insulated foundation, or one with insulation under the footings, must meet the unheated depths. The same idea shows up for slabs: insulation around a slab-on-ground must not stop building heat from reaching the soil under the perimeter where the exterior walls are not on footings below frost (9.25.2.3.(5)OBC Div. B 9.25.2.3. Installation of Thermal Insulation(1) Insulation shall be installed so that there is a reasonably uniform insulating value over the entire face of the insulated area. (2) Insulation shall be applied to the full width and length of the space between furring or framing. (3) Except where the insulation provides the principal resistance to air leakage…).

Exemptions: steps, sheds, decks

The depth table does not apply to:

  • Concrete steps with 1 or 2 risers, which can be laid on the ground (9.12.2.2.(4)OBC Div. B 9.12.2.2. Minimum Depth of Foundations(1) Except as provided in Sentences (4) to (7), the minimum depth of foundations below finished ground level shall conform to Table 9.12.2.2. (2) Where a foundation is insulated in a manner that will reduce the heat flow to the soil beneath the footings, the foundation depth shall conform to that required for…). Steps with more risers follow the table.
  • Accessory buildings that are not masonry or masonry veneer, not more than 1 storey, not more than 55 m², with the underside of the floor joists not more than 600 mm above finished ground (9.12.2.2.(6)OBC Div. B 9.12.2.2. Minimum Depth of Foundations(1) Except as provided in Sentences (4) to (7), the minimum depth of foundations below finished ground level shall conform to Table 9.12.2.2. (2) Where a foundation is insulated in a manner that will reduce the heat flow to the soil beneath the footings, the foundation depth shall conform to that required for…).
  • Decks and other exterior platforms of not more than 1 storey and 55 m², with joists not more than 600 mm above ground, not supporting a roof, and not attached to another structure unless it can be shown that differential movement will not harm that structure (9.12.2.2.(7)OBC Div. B 9.12.2.2. Minimum Depth of Foundations(1) Except as provided in Sentences (4) to (7), the minimum depth of foundations below finished ground level shall conform to Table 9.12.2.2. (2) Where a foundation is insulated in a manner that will reduce the heat flow to the soil beneath the footings, the foundation depth shall conform to that required for…).

A deck attached to the house usually does not qualify, so its piers go below frost. See the deck building code guide for pier and footing details.

Depths can also be reduced where experience with local soil conditions shows lesser depths work, or where the foundation is designed for them (9.12.2.2.(5)OBC Div. B 9.12.2.2. Minimum Depth of Foundations(1) Except as provided in Sentences (4) to (7), the minimum depth of foundations below finished ground level shall conform to Table 9.12.2.2. (2) Where a foundation is insulated in a manner that will reduce the heat flow to the soil beneath the footings, the foundation depth shall conform to that required for…). Frost-protected shallow foundations fall under that design route.

Attached garages

Foundations for attached unheated garages and carports must be below frost level, except as Section 9.12. allows (9.35.3.2.(3)OBC Div. B 9.35.3.2. Protection from Damage Due to Soil Movement(1) In clay-type soils subject to significant movement with a change in soil moisture content, the foundation depth of carports or garages connected to a dwelling unit directly or by a breezeway shall be approximately the same depth as the foundation of the main building. (2) Where slab-on-ground construction is used…). In clay soils that shrink and swell with moisture, an attached garage or carport foundation must be about as deep as the house foundation, and a garage slab needs a construction joint where it meets the house slab (9.35.3.2.(1) and (2)OBC Div. B 9.35.3.2. Protection from Damage Due to Soil Movement(1) In clay-type soils subject to significant movement with a change in soil moisture content, the foundation depth of carports or garages connected to a dwelling unit directly or by a breezeway shall be approximately the same depth as the foundation of the main building. (2) Where slab-on-ground construction is used…). Our attached garage guide covers the rest.

Frost and plumbing

Frost shows up in Part 7 too. An outside hose connection needs a frost-proof hydrant or a stop-and-waste valve inside the building near the wall (7.6.1.4.OBC Div. B 7.6.1.4. Protection for Exterior Water Supply(1) Pipes that pass through an exterior wall to supply water to the exterior of the building shall be provided with (a) a frost-proof hydrant, or (b) a stop-and-waste cock located inside the building and close to the wall.), and a vent pipe through a roof where it may be subject to frost closure must be enlarged to resist it (7.5.6.5.OBC Div. B 7.5.6.5. Terminals(1) Except as provided in Sentence (3), the upper end of every vent pipe that is not terminated in open air shall be connected to a venting system that terminates through a roof to open air. (2) The upper end of every vent pipe that is terminated in open air, other than a vent pipe that serves an oil interceptor or a…). Water service depth is usually set by the municipality, so check the local standard.

Common mistakes

  1. Looking for one Ontario number in the code. There is none: the local building department sets it.
  2. Assuming a heated basement always helps. Insulation that cuts heat to the soil under the footings makes it an unheated foundation.
  3. Using the deck exemption for an attached deck. The exemption is for freestanding decks, unless differential movement is shown not to harm the house.
  4. Shallow garage footings. Attached unheated garages go below frost, and in moving clay as deep as the house.
  5. Forgetting the 1.2 m clay minimum. Even a heated basement in clay needs 1.2 m.

How to look it up yourself

Open Division B, Part 9 of the 2024 Compendium. Section 9.12 is "Excavation" and holds the depth article. Search terms that appear in the code text:

  • "Minimum Depth of Foundations" (9.12.2.2.)
  • "depth of frost penetration" (the table and 1.1.3.2.)
  • "below frost level" (garages and slabs)
  • "adfreezing" (the defined term)

Read the article in the code viewer alongside the depth table.

BCIN exam angle

Footing depth by soil type, the heated vs unheated columns and the deck and shed exemptions are common House and Small Buildings questions. Study the House chapter on excavation, dampproofing, footings and foundations, then try the free quiz on the House exam page.

Based on the official 2024 Building Code Compendium on ontario.ca.

© King's Printer for Ontario, 2024. Reproduced with permission.

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Frequently asked questions

What is the frost depth in Ontario?
The Ontario Building Code does not publish one frost depth for the province. It says the depth of frost penetration is established on the basis of local experience (Division B, Article 1.1.3.2.). Many southern Ontario building departments work with about 1.2 m (4 ft), and colder regions often ask for more, so ask your municipality for the local value.
How deep do footings have to be in Ontario?
It depends on soil, drainage and whether the foundation encloses heated space. In clay or soils not clearly defined, foundations must be at least 1.2 m deep, and an unheated foundation in those soils must also go below the depth of frost penetration. Rock has no minimum (Division B, Table 9.12.2.2.).
Do deck footings have to be below the frost line in Ontario?
Usually yes. A deck of not more than 1 storey and 55 m², with joists not more than 600 mm above ground, no roof, and not attached to another structure (unless differential movement is shown not to harm that structure) is exempt from the depth table (Division B, Sentence 9.12.2.2.(7)).
Does an attached garage need frost-depth footings?
Yes. Foundations for attached unheated garages or carports must be below frost level, except as Section 9.12. allows, and in clay soils that move with moisture they must be about as deep as the house foundation (Division B, Article 9.35.3.2.).
Do concrete steps need footings below frost?
Steps with 1 or 2 risers can sit on the ground. Exterior concrete steps with more than 2 risers need foundations at the depth required by Table 9.12.2.2. (Division B, Sentences 9.12.2.2.(3) and (4)).