AntoineAsphalt.caHow Deep Do Deck Footings and Fence Posts Need to Be in Ontario?
Quick answer
Under the Ontario Building Code, deck footings must reach at least 1.2 m (about 4 feet) below finished grade, measured to the underside of the footing, to get below frost. The exceptions are footings on sound bedrock and qualifying low, detached floating decks. That rule is for decks, not fences; as one local benchmark, Oakville's standard division fence sets its posts in concrete at least 0.6 m below grade.
How deep do deck footings have to be in Ontario?
A 2025 municipal deck guide from Carleton Place, which references the 2024 Ontario Building Code (Sentence 9.12.2.2.(7)), sums up the rule:
- Footings for a regular (non-floating) deck must be at least 1.2 m (47¼ in.) below finished grade.
- The depth is measured to the underside of the footing.
The Building Code applies across Ontario. Hamilton, Burlington and Oakville may also have their own deck handouts, so check with your building department.
Why do deck footings need to go so deep?
The 1.2 m depth is meant to put the footing below frost. When wet soil freezes, it expands and can push shallow footings up. This is called frost heave, and it can make a deck shift.
Are there exceptions to the 1.2 m rule?
Yes:
- Bedrock. Footings can sit on sound bedrock that isn't affected by frost, at any depth.
- Floating decks. Low, detached decks that meet set limits can sit on footings at grade. See our guide on floating deck rules in Ontario.
- Other designs. Alternatives, such as insulating footings, must be designed by a qualified professional.
For screw-in steel posts, see helical piles vs concrete footings, and ask your building department what it accepts.
How big and thick does a deck footing need to be?
- Footings must be at least 100 mm (4 in.) thick.
- The footing's area depends on the beam and joist spans and the soil. One example from the guide: spans of 1.83 m × 1.83 m need a footing of 0.11 m² on soil that bears 75 kPa.
- Pre-cast deck blocks are typically 280 × 280 mm.
How deep should fence posts be in Ontario?
We found no Ontario rule that sets a depth for residential fence posts. Online rules of thumb vary and don't come from an authoritative source.
One local benchmark is Oakville's Division Fence By-law 2023-076. Its standard fence is a 1.2 m chain-link fence with 4 cm galvanized posts "set in 5 cm of concrete to at least 0.6 m below grade," spaced 3 m apart. That is the spec Oakville uses for cost-sharing between neighbours. It is not a building rule for all fences. For any other fence, ask your installer how deep they set the posts and why.
Before you dig for any footing or post, request a utility locate through Ontario One Call. It is free.
How deep footings and fence posts go (frost depth)
- Ground line (finished grade)
- Deck footing underside: about 1.2 m (3.9 ft) below grade
- Deck footing thickness: at least 100 mm (4 in.)
- Oakville's standard chain-link fence post: set at least 0.6 m (2 ft) below grade in concrete
Not to scale. Based on OBC as summarized by Carleton Place (2025) and Oakville By-law 2023-076.
Frequently asked questions
Does a ground-level deck need 4-foot footings?
Not if it meets Ontario's floating-deck limits: 55 m² or smaller, one storey, no roof, low to the ground and not attached to the house without a professional's sign-off.
How thick does a deck footing need to be?
At least 100 mm (4 in.). The area depends on the spans and the soil.
Can I use helical piles instead of concrete footings?
See our helical piles vs concrete footings guide, and ask your building department what it accepts for your deck.
Do fence posts have to go below the frost line?
We found no Ontario rule that requires it. Oakville's standard chain-link fence uses posts set at least 0.6 m below grade in concrete.
Related guides & services
See how footings fit into our deck building projects.
Building Code rules here are summarized from a municipal guide. Confirm what applies to your project with your local building department before you dig.
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