What is the steepest residential stair allowed?
Friday, July 27, 2018
The International Residential Code sets the maximum riser (vertical part) of a stair at 7-3/4” and the minimum tread (horizontal part) at 10”. Put them together, and that makes the steepest stair allowed. However, when a tread is less than 11” it must have a nosing (extension past the face of the riser below) long enough so that the combined length of tread and nosing equals 11”. So a 10” tread would require a 1” nosing. See our blog post When is a nosing required on a stair tread? for more on this.
But the ratio of the riser height to tread depth is actually more important than their individual dimensions in achieving a stair that is both safe and comfortable to use. If you have ever walked down a garden path with stepping stones that were either too close or too far apart, then you are familiar with the kind of awkwardness that a stair with the wrong riser to tread ratio creates in the cadence of the person ascending or descending them. The only difference is that a stair with a bad ratio can be dangerous.
The building code does not specify a ratio, but one standard that is often used is that one tread plus two risers should equal 24 to 26 (T + 2R = 24 to 26). So, as the tread lengthens, the riser must get shorter to make a comfortable stepping pattern—and vice-versa. When that formula is applied to maximum riser and minimum tread allowable, the total is 25-1/2.
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