Which code limits apply to a curved staircase in an Ottawa home?

A curved staircase inside a dwelling unit follows the same 2024 Ontario Building Code private-stair rules as any other interior stair, with one exception: if the curve is achieved through winders, specific winder rules apply, and if the design is a true spiral, a separate set of numbers takes over entirely.

Private stair dimensions that apply to a curved stair

For a curved stair built with winders, the Code (9.8.4.6.) permits winders inside a dwelling unit to turn not more than 90 degrees in total, with each tread spanning 30 to 45 degrees, and adjacent winders at the same angle as each other. The standard rise and run limits still govern: each rise is between 125 mm and 200 mm, each run between 255 mm (about 10 in.) and 355 mm, measured nosing to nosing. Risers and treads within a flight must be within 5 mm of adjacent ones and within 10 mm across the whole flight. Nosings need a rounded or bevelled edge of 6 mm to 14 mm.

Width must be at least 860 mm (about 34 in.) for at least one stair between each floor level. Headroom is at least 1 950 mm over the stair and landings. A flight rises no more than 3.7 m. Wood stringers need an effective depth of at least 90 mm and an overall depth of at least 235 mm, spaced not more than 900 mm apart, with treads at least 25 mm thick.

For a true spiral stair, the Code (9.8.4.7.) replaces the numbers above with its own set: clear width at least 660 mm, risers not more than 240 mm, treads at least 190 mm deep measured 300 mm from the centre line, and headroom at least 1 980 mm. A spiral stair also requires handrails on both sides, with the outer one at least 1 070 mm high. A spiral may serve as the only exit for not more than 3 persons.

Handrails on a curved or winder stair follow the standard rule: 865 mm to 1 070 mm high, measured from a line tangent to the nosings, continuously graspable from the bottom riser to the top. A stair less than 1 100 mm wide in a single dwelling unit needs a handrail on one side. Where the drop beside the stair exceeds 600 mm, a guard at least 900 mm (about 35 1/2 in.) high is required, with openings that stop a 100 mm (about 4 in.) sphere.

The geometry of a curved stair, particularly the winder angles and where each tapered tread's run is measured (9.8.4.3.: at a point 300 mm from the centre line of the handrail at the narrow end, with at least 150 mm at the narrow end), is something a carpenter works out against these numbers before cutting a single stringer. Ask the carpenter to show you where each tread's run is measured and how the winder angles were calculated, so you can see that every step falls within the Code ranges. A designer or the City of Ottawa (Building Code Services) can confirm whether the proposed layout meets the requirements before work begins.