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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Here’s links to a collection of our other blog posts about STAIRS:

• What do home inspectors check when inspecting stairs?

• Is a landing always required at the top and bottom of stairs? 

• When is a railing required at stairs?

• What is the building code for the minimum height of stair steps (risers)? 

• What is the building code requirement for receptacle outlets at stairs and stair landings?  

• Are open stair risers acceptable?

• What is the steepest residential stair allowed?

• Why is a single step dangerous in a house?

• Do I need stairs at all exit doors from a mobile home? 

• The stairs feel too steep. What's the building code? 

• What is the longest stair run allowed? 

• What is the lighting requirement for stairs?

• A light is required over a stair after how many steps/risers? 

• When is safety glass required for windows at stairs and stair landings?

   Visit our STAIRS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.