Stair Tread Minimum

minimum stair tread or step width to meet building code

The minimum stair tread is 11 inches, but there are some exceptions.

The minimum stair tread for private stairways is 10 inches, but be aware that it was 9 inches and this could confuse anyone who has been building stairs before they changed.

Reference: 2012 International Building Code - 1009.7 Page 254
Reference: 2012 International Residential Code - R311.7.5.2 Page 60

Again, this is where we could run into problems with architects, contractors and building department officials. Just exactly how do you interpret a private stairway. My interpretation of this particular building code is that a private stairway would be a stairway used in a single-family residence with less than 10 occupants.

The 2010 California Building Code Book has a 10 inch minimum tread for Residential 2 and Residential 3. I couldn't find anything on Residential 1 building types, suggesting a 9 inch minimum stair tread depth.

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 This information could be incorrect or outdated.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.

How Do We Measure The Stair Tread Width With A Nosing?

how to measure tread width with 90 degree angle or framing square

If you look at the picture above, the actual stair tread width is measured from the upper face of the stair tread nosing, to the lower face of the stair tread nosing.

You can use a framing square to check this also.

How Do We Measure The Stair Tread Width Without A Nosing?

international building code for stair step without nosing

If you don't have a nosing, then the stair tread can be measured from the face of the lower riser to the face of the upper riser as shown in the picture above. If you're having a difficult time figuring out the minimum stair tread depth, then feel free to send me an e-mail and I will see what I can do. Don't forget that some public libraries have updated building code books and you can always contact your local building department for more information.