Section 1009.3.1 of the International Building Code states that:
Stair treads and risers shall be of uniform size and shape. The tolerance between the largest and smallest riser or between the largest and smallest tread shall not exceed 0.375 inch (9.5 mm) [translated 3/8”] in any flight of stairs.
- Nonuniform riser dimensions of aisle stairs complying with Section 1024.11.2.
- Consistently shaped winders, complying with Section 1009.8, differing from rectangular treads in the same stairway flight.
Where the bottom or top riser adjoins a sloping public way, walkway or driveway having an established grade and serving as a landing, the bottom or top riser is permitted to be reduced along the slope to less than 4 inches (102 mm) in height with the variation in height of the bottom or top riser not to exceed one unit vertical in 12 units horizontal (8-percent slope) of stairway width.
The above building and accessibility code is consistent with the current California Building Code Section 1009.7.4
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