Frost Protection

Written for Canada. Plans are metric, but many crews and suppliers work in feet and inches, so this site gives both: millimetres and metres, with feet and inches beside them. Codes are the National Building Code of Canada 2020 (NBC 2020) Part 9 as your province adopts it. A number marked Confirm has not been checked against every province, so check it with your own authority before you act on it.

Reviewed by a working tradesman with 17+ years in the field.

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Frost heave lifts what sits on it. Keep footings below the frost line, and keep water away from the soil.

The big idea

Frost heave happens when soil freezes and expands, and the soil lifts whatever sits on it. A footing that is lifted unevenly cracks the wall above it, and a slab that heaves cracks and opens at its joints. Frost protection is about three things: footings below the frost line, water kept out of the soil, and insulation where the footing is shallow by design.

Three causes of frost heave

A conference paper from a US transportation research council says that, typically, three conditions are necessary for frost heave: freezing temperatures, frost-susceptible soil and available water. It is written about roads, but the physics is the same under a footing. That is why drainage, clean granular fill and good grading belong on a frost page.

• Frost-susceptible soil: silt and some clays pull water up as they freeze. Coarse gravel and sand drain freely and are far less of a problem.
• Water: grade the site so surface water runs away, keep clean free-draining fill beside the wall, put drain tile and stone at the footing, and keep roof water out of the foundation drain. See the Drain Tile, Gravel Beds and Drainage Corrections pages.
• Cold: you cannot change the weather, but you can keep the excavation from freezing during construction, and you can use insulation where the design calls for it.

What the code says

From Part 9, Section 9.12, as printed in the Vancouver Building Bylaw 2014 (a municipal reprint, possibly older than your province's edition):

• The bottom of excavations shall be kept from freezing throughout the entire construction period (9.12.1.3).
• Excavations for foundations shall extend to undisturbed soil (9.12.2.1).
• The minimum depth of foundations below finished ground level shall conform to Table 9.12.2.2 (9.12.2.2). The table sets depth by soil type, soil drainage, and whether there is a heated space. For rock it gives no limit. For clay and soils not clearly defined the readable entries show a minimum of 1.2 m (3 ft 11 in), and for some other soils the table calls for going below frost penetration. The text I read was jumbled, so I could not map every column reliably. Read the table in your own code edition.
• Where a foundation is insulated to reduce heat flow to the soil under the footings, the depth shall be that required for foundations with no heated space (9.12.2.2).
• Depths may be decreased where experience with local soil conditions shows that lesser depths are satisfactory (9.12.2.2), and the article exempts some small accessory buildings and some decks and platforms from the table. Check the exact conditions in your edition before relying on an exemption.

The code text I read gives no frost-depth figure. The depth of frost penetration is referred to in the table but not given there, so it comes from your building department, the geotechnical report or the drawings. Do not use a rule of thumb, or a depth from another region.

Example (numbers made up). Frost depth is measured from finished ground. If existing ground sits 0.3 m (1 ft) above finished ground and the footing bottom must be 1.5 m (4 ft 11 in) below finished ground, you dig 1.5 + 0.3 = 1.8 m (5 ft 11 in) from the existing ground. A hole that deep is a trench.

Frost-protected shallow foundations

These use insulation to keep the soil under the footing from freezing, so the footing can go shallower. Rigid foam is placed against the foundation and extends outward as a horizontal "wing". They are a designed system.

• A US Fine Homebuilding article (Martin Holladay, 2010, updated 2023) describes them as usually thick-edge slabs wrapped in vertical and horizontal rigid foam, with excavations as shallow as 12 or 16 in. (300 or 400 mm). It says the required foam depends on the climate and lists cases where they are a poor fit: steep sites, permafrost, and termite-heavy areas where foam below grade is questionable. It covers US codes and not Canada.
• An archived CMHC design guide for shallow frost-protected wood foundations says the method is for heated buildings and "will not adequately protect buildings which are unheated for prolonged periods". It is limited to non-permafrost regions, and the building must be completed, backfilled and heated before freezing weather. It is an older document, so treat it as background, not a current design source.
• The thickness, wing width and protection of the foam come from the design or an engineer. Whether this method is accepted where you build is a question for your building department.
• Protect the foam from damage and digging, keep it covered, and keep water off it. Do not change the insulation to save money.

Slabs and frost

• Compact the granular base and keep it free draining.
• Put insulation at slab edges where the design calls for it (see Foundation Insulation).
• Keep water away from the slab edge.

Signs of frost heave

• Seasonal lifting of walks or driveways.
• Cracks in slabs or walls that open in winter.
• Doors and windows that stick in winter.
• Uneven or lifted ground beside the building.
• Drain lines that freeze or back up.

Isolated footings, piers and posts can also be gripped on their sides by frozen soil. This is a known engineering concern, so ask an engineer about posts and piers rather than copying a depth.

Common mistakes

• Using the wrong frost depth
• Clay or silt backfill beside the wall
• No drainage at the footing
• Foam that is damaged or soaked
• An unheated addition on shallow footings with no protection
• Pouring on frozen ground