How deep do deck footings need to be in Ontario?
Nearly every failed deck we are called to look at failed underneath. Here is the number that decides whether yours lasts.
When somebody calls because their deck has gone wonky, we can usually guess the cause before we arrive. It is rarely the boards. It is what is holding them up.
The number you need
Footings have to bear below the frost line, which across most of southern Ontario means roughly four feet, or about 1.2 metres. Your municipality sets the figure for your address and it does vary, so we confirm it rather than assume.
Below that depth, the ground does not freeze, so it does not expand. Above it, the ground swells every winter and settles every spring. A footing that stops short of the frost line gets lifted a little each year, and the deck comes with it.
Why frost heave wrecks a deck slowly
Frost heave rarely destroys anything in one season, which is exactly what makes it deceptive. The first winter you notice nothing. By the third, one corner sits proud of the others. By the fifth, the ledger where the deck meets the house is under strain, the stairs no longer meet the landing squarely, and the railing has movement in it.
By then the fix is not a repair. It is taking the deck apart to get at footings that were never deep enough.
A deck on shallow blocks is not cheaper. It is a deck you pay for twice.
How we actually do it
- Locate services first. Anything that involves digging in Ontario starts with a locate request, and it is free.
- Dig to the required depth, then confirm the hole is clean and the base is undisturbed soil, not backfill.
- Set cardboard tube forms, brace them plumb, and pour concrete in one go.
- Set the saddle or bracket into the wet concrete so the post is anchored rather than just resting on top.
- Book the footing inspection where the municipality requires one, before anything gets covered.
What about deck blocks?
Precast deck blocks sitting on grade have their place: a small, freestanding, ground-level platform that is not attached to the house and is easy to re-level. A floating garden platform is a fair use.
The moment a deck is attached to your house, blocks become a bad idea. The house is on a proper foundation and does not move with frost. A deck on blocks does. Fastening one to the other means every winter the connection between them is being pulled at, and that connection is what holds people up.
Rock, clay, and the local ground
Not every lot digs the same. Up against the escarpment in Burlington we sometimes hit rock, which changes the approach and the cost. Parts of Mississauga near the lake are heavy clay that holds water and moves. Sandy ground around Wasaga behaves differently again, and Muskoka is often rock close to the surface.
None of that is a problem, but it is a reason to look at the actual property before quoting a number. Anyone who prices deck footings over the phone without asking what the ground is like is guessing.
Common questions
Do I need a permit for a deck in Ontario?
Generally yes if the deck is more than about 60 cm above grade or attached to the house, though the exact threshold varies by municipality. We confirm the requirement for your address and handle the permit and inspection.
How many footings does a deck need?
It depends on the span, the beam size, and the joist layout rather than the deck's square footage. A properly designed frame decides the spacing, not the other way around.
Can you add footings under an existing deck?
Sometimes, if the framing is sound and there is access underneath. It is worth assessing before assuming the whole deck has to go, especially if the structure above is in good shape.
Is a floating deck a bad idea?
Not on its own. A freestanding, low, unattached platform on blocks is a legitimate build. The problem is bolting that approach to a house that sits on a real foundation.
Where we do it
