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## 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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Here’s links to a collection of our other blog posts about 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?

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.

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