Maximum Variation In Stair Risers
The maximum stair riser variation for a stairway is 3/8 of an inch.
Reference: 2012 International Building Code - 1009.7.4 page 254
Again, this one can be tricky to interpret, and I'm going to do my best. The maximum variation from the first riser to the last can't be more than 3/8 of an inch on any individual stair riser. Remember, these are finish stair riser measurements, not rough framing riser measurements.
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 This information could be incorrect or outdated.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.
You can't average out the riser measurements either. You take the smallest riser measurement and then try to find a largest riser in the stairway, it can't be more than 3/8 of and inch larger than the smallest one.
Three Step Stairway Example 1: The bottom riser is 7 inches, the second riser is 7 inches, the third riser is 7 inches but the fourth riser is 7 1/2 inches. There is more than 3/8 of an inch variation between the smallest and tallest riser and this particular stairway wouldn't pass most building inspections.
Three Step Stairway Example 2: The bottom riser is 7 inches, the second
riser is 7 - 1/8 inches, the third riser is 7 inches, the fourth riser
is 7 - 1/4. This stairway would be
acceptable by most building departments, because the smallest riser is 7
inches and the tallest is 7 1/4, giving it only 1/4 of an inch maximum
The reason why building departments allow for variations in your stairway risers are simple. It's hard to cut and install a perfect set of stairs without small riser variations. Large riser variations greater than 3/8 of an inch can create safety problems, but smaller ones can be tolerated. This building code gives stair builders a little room for errors made during construction, variations in building materials sizes and lumber expansion and shrinkage.