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How deep do deck footings have to go in Niagara? Most people have heard "4 feet", and on clay soil that is roughly right, but the Ontario Building Code does not actually say 4 feet for every deck. It says two things. First, a footing has to go down to undisturbed soil (Article 9.12.2.1.). Second, its depth follows Table 9.12.2.2., which sets the depth by soil type. For a foundation with no heated space under it, which is what a deck is: on rock, there is no minimum depth. On coarse-grained soil with good drainage, there is no minimum depth either; with poor drainage, the footing goes below the depth of frost penetration. On silt, it goes below the depth of frost penetration. On clay, or soil that is not clearly defined, it is 1.2 m (about 4 feet), and not less than the depth of frost penetration. The Code does not publish a frost depth for Niagara; it says frost depth "shall be established on the basis of local experience" (Division B, 1.1.3.2.), which in practice means your building department. Port Colborne's own deck guide puts it simply: a minimum depth of 1.2 metres (3.94 feet) from grade, and Welland's sample deck drawings show a concrete pier 1200 mm below grade. So 1.2 m is the figure to plan around unless you know you are on rock or well-drained gravel. There is one real exception. Sentence 9.12.2.2.(7) says the depth rule does not apply to a deck that is all five of these: not more than one storey; not more than 55 m2 (about 592 sq ft) in area; no more than 600 mm (about 2 feet) from finished ground to the underside of the floor joists; not supporting a roof; and not attached to another structure, unless it can be shown that movement of the deck will not harm that structure. A low, freestanding ground-level deck can meet all five. A deck bolted to the house, a raised deck, or a deck with a roof over it cannot, and goes to full depth. These figures were checked against the current Ontario Amendments document (dated July 17, 2026), which leaves Sentence (7) and the table unchanged. In Niagara Falls, St. Catharines and Port Colborne the footing inspection happens before any concrete is poured, so the inspector sees the depth before it is covered.
What the footing itself has to be. It rests on undisturbed soil, rock, or compacted granular fill (Article 9.15.3.2.), and it is at least 100 mm thick, or thicker if it sticks out past the post or pier by more than that (9.15.3.8.). A post sits centred on its footing (9.17.2.1.). Wood posts are at least 140 mm by 140 mm square, or 184 mm round, and never narrower than the beam they carry, unless calculations show a smaller post is enough (9.17.4.1.). Where a wood post meets concrete that is in contact with the ground, it is separated from it by 0.05 mm polyethylene or Type S roll roofing (9.17.4.3.). Posts that are 600 mm or shorter, under a deck with nothing built on top of it, do not need the bracing against racking that taller posts need (9.17.2.2.(3)). A roof changes the footing, not just the depth: Port Colborne's deck guide says that under a covered deck "a sono tube is insufficient on its own" and calls for a footing of at least 0.4 m2 (4.3 sq ft) beneath it. Niagara Falls lists helical piles as an accepted footing type, inspected at the same pre-pour stage. Whatever you use, St. Catharines wants the pier size, depth and spacing shown on your permit drawings. For the inspection sequence around the footing, see /niagara-deck-inspection-guide/. For each town's permit process, see /niagara-deck-permit-guide/. Confirm the depth that applies to your lot with your building department before you dig.
It depends on the soil. Under Table 9.12.2.2. of the Ontario Building Code, a deck footing on clay or soil that is not clearly defined goes down 1.2 m (about 4 feet), and not less than the frost depth. On silt, or poorly drained coarse soil, it goes below the frost depth. On rock or well-drained coarse soil there is no minimum. Port Colborne's and Welland's deck documents both use 1.2 m.
Yes, if the deck meets all five conditions in Sentence 9.12.2.2.(7): one storey or less, 55 m2 or less, no more than 600 mm from the ground to the underside of the joists, no roof, and not attached to another structure (unless it can be shown the deck's movement will not harm it). Miss any one and the normal depth rule applies. You may still need a permit; see the permit guide.
An attached deck cannot use the 9.12.2.2.(7) exemption unless it can be shown that its movement will not affect the house. In practice it goes to full depth, which on clay means 1.2 m, the figure Port Colborne's and Welland's deck documents show.
Not on its own, according to Port Colborne's deck guide: when the piers carry a roof, "a sono tube is insufficient on its own", and a footing of at least 0.4 m2 (4.3 sq ft) is required under it. A roof also rules out the 9.12.2.2.(7) shallow-footing exemption.
At least 140 mm by 140 mm for a square wood post, or 184 mm for a round one, and no narrower than the beam it supports, unless calculations show a smaller post is adequate (Article 9.17.4.1.). Where the post sits on concrete that touches the ground, put 0.05 mm polyethylene or Type S roll roofing between them (9.17.4.3.).
Before the concrete is poured. Niagara Falls lists "Footing/excavation (helical pile) - prior to pouring of concrete" as the first deck inspection, and St. Catharines and Port Colborne require the same. Pour first and you may be digging it back out.
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