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I received a question about stair width from one of our clients and thought it made sense to clarify the typical stair design rules for houses. These are the rules that are in the 2006 International Residential Code.

Stairways have to be at least 36" clear width above the handrail height. Handrails can't project more than 4.5" into each side.

You need at least 80" of head height throughout the stair.

The maximum height of a riser is 7.75" and the treads have to be at least 10" deep. Treads also need to be a minimum of 4" tall.

If you have winders they can't be shallower than 6" and they have to be at least 10" deep when you're 12" into the winder.

The landings must be at least as deep as the stairs are wide and you can't go up more than 12' without a landing.

Handrails have to be mounted between 34" and 38" above the tread nosing and must run the full length of the stairs. The handrails have to be at least 1.5" off the wall and where they are circular they need to be 1.25" to 2" in diameter. If the handrail isn't circular it needs to have a perimeter between 4" and 6.25" with a maximum cross section of 2.25".

There are a number of other rules for particular situations but these are the general guidelines. There are more difficult stair and handrail rules for commercial and multifamily dwellings. I've also written a separate post for spiral stairs.