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What does the Ontario Building Code actually require of a deck? This page goes through the rules one at a time and points to our deeper guides where we have them. Every figure below was checked against the Ministry's 2024 Building Code Compendium and the current Ontario Amendments document, dated July 17, 2026, which leaves all of them unchanged. Permit first. The Building Code Act says no one may construct a building without a permit (s.8(1)), and it defines a building to include "a structure occupying an area greater than ten square metres" (s.1(1)). That is about 108 sq ft. The Code's list of structures exempt from the Act (Division C, 1.3.1.1.) names small sheds and tents but not decks. Each town also sets its own triggers, such as height above grade or attachment to the house; those are on /permit-guide/ and, town by town, on /niagara-deck-permit-guide/. Footings. A deck footing goes down to the depth Table 9.12.2.2. sets for your soil, which on clay is 1.2 m. Sentence 9.12.2.2.(7) exempts a low deck that meets five conditions: 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. The details are on /deck-footing-depth-guide/. Guards. Any side of a deck where the drop is more than 600 mm to the surface within 1.2 m needs a guard (9.8.8.1.). For a deck serving one home, the guard is at least 900 mm high where the deck is 1 800 mm or less above the ground, and 1 070 mm above that (9.8.8.3.). Openings must stop a 100 mm sphere (9.8.8.5.), and nothing between 140 mm and 900 mm up can make it easy to climb (9.8.8.6.). The full rule is on /deck-railing-height-guide/. Stairs. Exterior stairs serving a single home are "private stairs" under the Code (Table 9.8.4.1., Note 1). Each rise is between 125 mm and 200 mm. Each run is between 255 mm and 355 mm, and a rectangular tread is no shallower than its run and no more than 25 mm deeper (9.8.4.2.). The stair is at least 860 mm wide (9.8.2.1.). No single flight may rise more than 3.7 m (9.8.3.3.). A handrail is not required on exterior stairs with 3 risers or fewer; above that, one handrail is enough for a stair serving one home (9.8.7.1.), set between 865 mm and 1 070 mm high (9.8.7.4.).
What the frame has to carry. A deck serving one home is designed for the specified roof snow load or 1.9 kPa, whichever is greater (9.4.2.3.). Its beams, joists and decking may deflect no more than 1/240 of the span (Table 9.4.3.1.). Joist spans. Part 9 sends wood joists to Span Tables 9.23.4.2.-A to -G (9.23.4.2.), and those tables apply only to floors serving residential areas (9.23.4.1.). None of the tables is titled for decks or exterior use, so this page does not quote a span; your drawings or a designer settle joist size and spacing for your deck. Attaching to the house. Part 9 has no article on bolting a deck ledger to a house. What the Code does say is that an attached deck loses two exemptions unless it can be shown that its movement will not harm the house: the shallow-footing exemption in 9.12.2.2.(7) and the post-anchorage exemption in 9.23.6.2.(3). Posts on a low deck. Where the underside of the joists is 600 mm or less from the ground, the posts are anchored to the foundation against uplift and sideways movement, or the joists or beams are anchored to the ground (9.23.6.2.(2)). A deck of one storey, 55 m2 or less, with no roof and not attached, does not need that anchorage (9.23.6.2.(3)). Treated wood. Structural wood less than 150 mm above finished ground must be pressure-treated against decay (9.3.2.9.(3)). Wood in contact with the ground calls for Use Category 4.1 treatment; exterior wood above ground is Use Category 3.2 (9.3.2.9.(5)). For deck boards themselves, see /decking-material-guide/. For the inspection sequence, see /niagara-deck-inspection-guide/. Your building department has the final word on your lot.
A deck over 10 m2 (about 108 sq ft) is a "building" under the Building Code Act, and the Act requires a permit to construct one. The Code's own exemption list does not include decks. Towns add triggers of their own, like height or attachment; see /niagara-deck-permit-guide/ for Niagara Falls, St. Catharines and the rest.
Exterior stairs serving one home are private stairs under Table 9.8.4.1.: rise from 125 mm to 200 mm, run from 255 mm to 355 mm. A rectangular tread is at least as deep as the run and no more than 25 mm deeper. The stair is at least 860 mm wide.
Not if the exterior stair has 3 risers or fewer (9.8.7.1.(3)). With more than 3 risers, one handrail is enough for a stair serving a single home (9.8.7.1.(4)), and it sits between 865 mm and 1 070 mm high (9.8.7.4.).
A guard is required where the drop is more than 600 mm (about 2 feet) to the surface within 1.2 m of the edge (9.8.8.1.). It is at least 900 mm high up to 1 800 mm above ground and 1 070 mm above that. /deck-railing-height-guide/ has the detail.
For a deck serving a single home, the specified roof snow load or 1.9 kPa, whichever is greater (Article 9.4.2.3.). Beams, joists and decking may deflect no more than 1/240 of their span (Table 9.4.3.1.).
Not in Part 9; there is no article on deck ledger attachment. The Code instead removes the shallow-footing and post-anchorage exemptions from an attached deck unless its movement is shown not to harm the house. Ask your building department what it wants to see for the ledger on your drawings.
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