Maximum Angle of the Stair Risers

The maximum angle a stairway riser can slope is 30 degrees.

To give you a general idea, a stairway riser with a 1 inch undercut on a 7 1/2 inch riser is about 10°. You would almost need to provide a 3 inch undercut to get 30°.

There is a good chance that most stair builders will never reach the maximum angle for stair risers to end up with a problem here.

Attention: Don't forget to check with your local building department to verify any building codes on this website. This information could be incorrect or outdated. Building codes are updated regularly and to some building departments have different interpretations for certain building codes.

one inch stair tread undercut 3d side view

1 inch is the most common length of an undercut measurement, just like the one inch overhang on most steps.

3d view of the degrees of a one inch under cut

The 1 inch undercut will provide you with about an 8 degree slope, with a 7 1/2 inch rise, which is well below the 30° maximum requirement from the International Building Code. However, if you look at the picture below, things start to look a little wacky.

3d view of stair riser with 30 degree angle for building code review

I left the 1 inch undercut on the first and last risers, but in the middle went ahead and drew a 30° angle and as you can see it provides us with almost a 16 inch wide step. There's a good chance your stairway won't ever need to worry about this particular building code, but as a stair builder, you should be familiar with it.