What Is the Minimum Stair Width in Ontario?

The Ontario Building Code Part 9 sets minimum clear width for stairways serving dwelling units. For a stair serving a single dwelling unit, 860 mm is the common residential minimum unless the stair also acts as a required exit for multiple units or higher occupant loads.

Clear width is measured between walls, guards, or other fixed obstructions at the narrowest point along the run. Handrails may project into the required width within the limits specified in the code — measure the clear path, not the rough opening alone.

Exterior stairs that serve a dwelling unit generally follow the same Part 9 width rules as interior private stairs, but materials, landings, and weather exposure still apply. Stairs that form part of a required exit path may need to meet stricter exit dimensions.

Exit stairs and stairs serving more than one dwelling unit are sized for occupant load and exit capacity. Do not assume the 860 mm private-stair minimum applies to a shared duplex stair or a public exit.

Existing houses being renovated may have narrow stairs that were legal when built. Adding a secondary suite, changing use, or creating a required exit can trigger upgrades to current width, headroom, and enclosure rules — verify with your building department rather than assuming grandfathering.

New construction must meet current Part 9 dimensions throughout. Common design mistakes include measuring only at the top landing, ignoring handrail projections, and applying private-stair limits to exit stairs without checking classification.