Q. I’m building an exterior stairway from a deck to grade. Is there a building code requirement that the risers of an exterior stairway all be the same height?
A. Corresponding Editor Paul Fisette responds: Most building codes, including the Massachusetts Building Code, BOCA, and the Life Safety Code, require that the variation in the depth of adjacent treads, and in the height of adjacent risers, not exceed 3/16 inch. In any flight of stairs, there should be no more than 3/8 inch difference between the largest and smallest riser, and the same rule holds for treads. There are a couple of exceptions: 1) where the bottom riser adjoins a sloping public way, walk, or driveway that has an established grade and serves as a landing, and 2) on stairways serving as aisles in assembly seating. In those cases, the bottom riser may be wedge-shaped if necessary.
As far as I know, there is no reason to think that an inspector would treat exterior stairs differently from interior stairs with regard to dimensional uniformity.