3.4.6.8.Treads and Risers (See Note A-9.8.4.)

(1) Except as permitted for dwelling units and by Sentence 3.4.7.5.(1) for fire escapes, steps in flights shall have a run of not less than 280 mm and not more than 355 mm between successive steps. (2) Steps for stairs referred to in Sentence (1) shall have a rise between successive treads not less than 125 mm and not more than 180 mm. (3) Except as provided in Article 3.3.4.7. and except for fire escape stairs, stairs that are principally used for maintenance and service, and stairs that serve industrial occupancies other than storage garages, steps for stairs shall have no open risers. (4) Except in fire escape stairs and where an exterior stair adjoins a walkway as permitted in Sentence 3.4.6.3.(3), risers, measured as the vertical nosing-to-nosing distance, shall be of uniform height in any one flight, with a maximum tolerance of (a) 5 mm between adjacent treads or landings, and (b) 10 mm between the tallest and shortest risers in a flight. (5) Except in fire escape stairs, treads shall have a uniform run with a maximum tolerance of (a) 5 mm between adjacent treads, and (b) 10 mm between the deepest and shallowest treads in a flight. (6) Treads and risers shall not differ significantly in run and rise in successive flights in any stair system. (See Note A-3.4.6.8.(6)) (7) The slope of treads or landings shall not exceed 1 in 50. (8) Except as permitted by Sentence (10), the top of the nosing of stair treads shall have a rounded or bevelled edge extending not less than 6 mm and not more than 13 mm measured horizontally from the front of the nosing. (9) The front edge of stair treads in exits and public access to exits shall be at right angles to the direction of exit travel. (10) If resilient material is used to cover the nosing of a stair tread, the minimum rounded or beveled edge required by Sentence (8) is permitted to be reduced to 3 mm.