A carpenter is building a straight-run stair for a residential dwelling with a total vertical rise of 2740 mm. If the maximum allowable riser height is 200 mm and the minimum allowable tread depth is 235 mm, what is the most appropriate number of risers and corresponding tread depth to ensure compliance with the National Building Code of Canada (NBC) 2020?

Question: A carpenter is building a straight-run stair for a residential dwelling with a total vertical rise of 2740 mm. If the maximum allowable riser height is 200 mm and the minimum allowable tread depth is 235 mm, what is the most appropriate number of risers and corresponding tread depth to ensure compliance with the National Building Code of Canada (NBC) 2020?

Answer options:

• 14 risers at 195.7 mm, 235 mm tread depth ✅ 15 risers at 182.7 mm, 235 mm tread depth
• 13 risers at 210.8 mm, 235 mm tread depth
• 14 risers at 195.7 mm, 260 mm tread depth

Correct answer: 15 risers at 182.7 mm, 235 mm tread depth

Explanation: To find the number of risers, divide the total rise by the maximum riser height (2740 mm / 200 mm = 13.7). Round up to 14 risers or 15 risers to stay within the maximum. 15 risers (2740/15 = 182.7 mm) is a common choice and meets code, with the minimum tread depth of 235 mm also being compliant. 14 risers at 195.7 mm would also be acceptable, but 15 risers allows for a slightly shallower and often more comfortable rise.