The International Residential Code (IRC), has specific instructions the way stairs can be built. But not every municipality adopts every part of every code, and some cities and states add in their own additional rules. Bottom line, check with your local building official before cutting those stringers or forming that stoop. Here’s a clear summary of what the IRC code says about stair rise, run and nosing.
The IRC code does not allow a distance greater than 7-¾ of an inch from one step/tread to the next.
The “run” is the measurement of the tread, which needs to be a minimum of 10 inches if the tread has and overhang on it (see image). This is a measurement from the nose of the tread to nose of the tread. You are not required to have an nosing/overhang on your stair treads. If your stairs don’t have a nosing you are required to have a tread that is at least 11 inches long.
From the top flight of stairs to the bottom of the stairs, each individual riser cannot exceed a difference of 3/8 of an inch.
The nosing on treads, believe it or not, has a code minimum and maximum of how far the nosing projects over the next stair. Nosings are required to overhang a minimum of ¾-in. to a maximum of 1-¼-in. Also the maximum nosing radius cannot exceed 9/16 of an inch.
All of these measurements include the finished flooring, so don’t forget to take those thicknesses into account when planning your stair dimensions.
Here are three reference numbers for the codes pertaining to rise, run, and nosing in a straight run of stairs: