Deck Footing Depth in Ontario, and What the Code Really Says

Ask around and you will be told that deck footings in Ontario go down four feet. That is close enough to be useful and wrong in ways that cost money, because the number comes from a table most people have never read.

Here is what the Ontario Building Code actually says, which municipalities publish their own figure, and where the real traps are.

Where the 1.2 m comes from

The rule lives in Article 9.12.2.2 of the Building Code, Minimum Depth of Foundations. It points at a table, and the table sets depth by soil type, not by a single province-wide number.

• On rock, or on well-drained coarse-grained soil, the table sets no depth limit at all.
• On silt, the footing goes below the depth of frost penetration.
• On clay, or soil that is not clearly defined, the figure is 1.2 m, or the depth of frost penetration, whichever is deeper.

That last line is the one everybody quotes and nearly everybody shortens. It is a floor, not a ceiling. Most of the GTA sits on clay or on fill nobody has classified, which is why 1.2 m is the working number here, and it is also why an inspector can ask for deeper on a particular lot without changing the rule.

One practical consequence worth knowing: 1200 mm is 47¼ inches, not 48. Richmond Hill and Clarington publish 1200 mm. Oakville and Milton publish 4 ft 0 in, which is 1219 mm. Build to four feet and you clear all four.

What each municipality publishes

We went looking for a published deck footing depth in fourteen GTA and Durham municipalities. Four publish one. Most do not.

Checked September 2026 against each municipality’s own deck guides, checklists and sample drawings. Where we could not read a document we have said so rather than filled the gap in.

A city that publishes no figure is not a city with no requirement. It means the Code governs and the depth goes on your drawings for the plans examiner to check. Oshawa is the clearest case: it publishes a thorough deck construction guide covering pier diameters and soil bearing, and deliberately puts no depth figure in it.

When a deck can skip deep footings

The Code exempts some decks from the depth rule. All five conditions have to be true at once:

• Not more than one storey
• Not more than 55 m² in area
• Finished ground to the underside of the floor joists not more than 600 mm
• Not supporting a roof
• Not attached to another structure, unless you can show the movement will not damage it

Miss any one of them and the depth rule applies to the whole deck.

The 55 m² trap

This is the one we see most often, and it is an expensive mix-up.

55 m² is the Building Code area limit for skipping frost-depth footings. 10 m² is the municipal area limit for skipping a building permit. They are two different rules, from two different documents, with two different consequences, and they get merged constantly.

A 30 m² floating deck at 500 mm, detached from the house, is under the Code’s 55 m² and may sit on deck blocks. It is also over the 10 m² permit threshold in most municipalities, so it still needs a permit. No permit does not mean no footings, and shallow footings do not mean no permit.

The permit thresholds we confirmed:

• Toronto: no permit for an uncovered platform not more than 60 cm above adjacent grade. Toronto sets no area cap, which makes it the odd one out.
• Mississauga, Markham, Richmond Hill, Burlington: detached, not more than 600 mm above grade, and not more than 10 m².
• Oakville: over 10 m² and over 61 cm needs a permit, and any deck attached to the house needs one whatever its size.
• Milton: freestanding over 10 m², or any deck attached to a building.
• Whitby and Oshawa: measured on height alone, over 2 ft.

Note the measuring point moves. Permit thresholds are usually taken to the deck surface. The Code’s 600 mm is to the underside of the joists. On a deck with 2x8 joists that is most of a foot of difference, and it decides which rule you are under.

Undisturbed soil, and why it matters more than depth

Every municipality that publishes a depth publishes it with the same phrase attached: to undisturbed soil. Richmond Hill, Oakville and Clarington all say it outright.

Depth on its own proves nothing. A footing four feet down in backfill against a foundation wall, or in the trench where the services went in, is sitting on ground that will keep settling. That is a worse footing than three feet on native clay, and it is the failure we get called out to most often on decks somebody else built.

Milton adds a rule worth repeating: any deck footing within 4 ft of the house foundation must go down to the level of the existing foundation. Close to the house you are almost always in backfill, and matching the house footing is the only way to be on the same ground it is.

Pier diameter is not one number either

The published minimums vary, and so does the soil bearing capacity they assume:

• Oakville and Milton: minimum 8 in pier for a 4x4 post, 10 in for a 6x6, sized on 2000 lb/ft² bearing.
• Oshawa: 4x4 posts up to 5 ft of deck height on minimum 8 in sonotubes, 6x6 above that on minimum 10 in, assuming 2090 lb/ft² unless an inspector says otherwise.
• Clarington: a table keyed to bearing capacity, with piers from 250 mm up to 600 mm as the soil gets softer.

There is no single GTA-wide bearing capacity, which is why a pier schedule copied from one town’s sample drawing does not automatically pass in the next.

Screw piles

Helical and screw piles are accepted, and they are genuinely good in tight yards and where digging would wreck a lawn. What they are not is a prescriptive shortcut.

Every municipality we found that addresses them requires an engineer:

• Milton: a professional engineer must determine depth, spacing and size based on soil condition.
• Oakville: manufacturer information must be submitted with the permit application, and the depth is set by an engineer.
• Oshawa: an engineered design and a commitment to general review form.

No municipality publishes a screw pile depth you can just build to. If someone quotes you piles with no engineering line in the price, that cost has not disappeared, it has just not been mentioned yet.

The ledger, and brick veneer

Richmond Hill puts it as plainly as anyone: the Building Code does not allow a deck to be attached to brick veneer. The load has to reach the foundation wall.

Brick veneer is a skin. It carries itself and nothing else. A ledger lagged into it will hold for a while and then take the brick with it when it goes. Where a house is brick clad, the ledger either goes through to the structure behind or the deck goes free-standing with its own posts along that side.

That second option is more common than people expect, and it is often the better build anyway. A free-standing deck moves as one thing, and the house moves as another, which is exactly what you want when the ground heaves.

What shallow footings actually do

This part is not in the Code. The Code tells you the depth and stops.

Water in soil expands when it freezes and pushes whatever is above it upward. A footing above the frost line gets lifted every winter and dropped every spring, never evenly, because one corner of a yard freezes deeper than another. Over a few seasons the posts go out of level, the deck develops a twist, gates stop latching and stairs pull away from their landing.

Where the deck is bolted to the house it is worse, because the house does not move and the deck does. That is the joint that opens up, and it opens up at the ledger, which is the one connection holding the deck to the building.

None of this shows in year one. It shows in year three, which is why it is so often somebody else’s deck by the time we see it.

What we build to

When a job crosses municipal lines, or the town publishes no figure, we build to a spec that clears all of them:

• 4 ft 0 in below finished grade, which covers both the 1200 mm and the 4 ft municipalities
• Bearing on undisturbed native soil, not backfill, confirmed when the hole is open
• Footings within 4 ft of the house taken down to the level of the existing foundation
• Minimum 10 in piers under 6x6 posts, larger where the soil says so
• No ledger into brick veneer, ever. Through to structure, or free-standing
• Free-standing decks over 600 mm braced against lateral movement

The depth is the cheap part of a deck. It is an hour with an auger and a bit more concrete, decided before anything visible gets built, and it is the one decision you cannot revisit later without taking the deck apart. Our deck repair work is very largely other people’s footings.

If you are still working out whether you need a permit at all, the Toronto deck permit guide covers that side for the city, and every municipality above lists its own threshold.

Sources: 2024 Ontario Building Code, Richmond Hill Residential Deck Checklist, Oakville deck sample drawings, Milton deck permit sample drawings, Clarington standard deck drawing, Oshawa deck construction guide, Toronto permit thresholds, Markham deck permits, Oakville decks, Burlington decks. Checked September 2026. Confirm the depth with your municipality before you dig, and treat your plans examiner as the final word.