Deck stair tread depth limits in Ontario?

Under Ontario's 2024 Building Code, a private exterior stair tread has a minimum run of 255 mm and a maximum run of 355 mm. A rectangular tread must be no shallower than its run and no deeper than its run plus 25 mm.

Those numbers come from Ontario Table 9.8.4.1 and section 9.8.4.2(2), which apply to private stairs, including exterior stairs serving a single dwelling unit. The rise rules sit alongside them: minimum 125 mm, maximum 200 mm per step.

A few things shape how those numbers play out on your deck:

• The tread depth rule is tied to the run: a tread can be up to 25 mm deeper than the run, never shallower.
• The rise and run together determine how many steps you need and how far the stair lands from the deck. A shallower rise with a longer run means the stair travels further into the yard.
• Once there are more than three risers, one handrail is required on exterior stairs serving not more than one dwelling unit, and required handrails are 865 mm to 1 070 mm high.
• Guards are required alongside the stair where the drop beside the stair exceeds 600 mm within 1.2 m of the edge, and those guards must meet the no-climb rule between 140 mm and 900 mm above the walking surface.

If your deck needs a building permit, the permit drawings must show the stair rise and run, the guard and handrail heights, and the stair location on the framing plan. The City of Ottawa lists those as required elements on the elevation drawings.

Whether a permit is needed depends on the deck's height above grade, whether it is attached to the house, and whether it is the principal access to the building. A deck with a walking surface more than 600 mm (24 in.) above adjacent grade and attached to the house requires a permit regardless of size.

Your builder or a designer is the right first call, because they can work out the rise and run combination that fits your deck height, the available landing space and the permit drawings in one step, rather than designing the stair and the drawings separately.