The Ontario Building Code | Curved Stairs
3.4.6.9. Curved Stairs
(1) Except as permitted by Sentence (2), tapered treads shall not be used in an exit.
(2) A curved stair used as an exit shall have,
(a) a handrail on each side,
(b) treads with a minimum run of 240 mm exclusive of nosings,
(c) treads that conform to Article 3.4.6.8. where they are measured 230 mm away from the handrail at the narrow end of the tread, and
(d) an inside radius that is not less than twice the stair width.
In Plain Language
Tapered treads generally aren't allowed in an exit, but a curved stair used as an exit can use them if it meets specific handrail, minimum-run, and measured-tread requirements designed to keep the narrow end of each tread safely usable.
This is a plain-language summary of the 2012/2017 Ontario Building Code text above, for general understanding only and not a substitute for the Code. Always confirm current requirements against the 2024 edition (O.Reg. 163/24) or a qualified professional before construction.