What Are Standard Stair Tread Depths?

International Residential Code

• The International Code Council is the organization responsible for defining building codes used by many municipalities, states and even some federal organizations. The ones applied specifically to residential stairs are part of the International Residential Code. It defines minimum tread depths to be 10 inches, as measured horizontally across the stair step from one adjacent tread to the next. The depths can vary only by 0.375 inch across all steps in any flight of stairs. The risers, which are the vertical spaces between treads, can indirectly affect tread depth. They must be a maximum of 7.75 inches high. And measurements across all risers can vary only by 0.375 inch.

Spirals

• If the run of a flight of stairs takes up too much space, spiral staircases can connect floors in about half the space of standard inclined stairway. The IRC allows for these constructions as long as the width of the tread below the required handrail is at least 26 inches and the minimum headroom above each tread is 78 inches. Treads must have a minimum depth of 7.5 inches, as measured at 12 inches from the narrower edges. Treads cannot vary across the entire stair and cannot rise more than 9.5 inches.

Exceptions

• Bulkhead stairways connect an outside grade, such as a lawn or driveway, to a lower interior level, such as a basement. They are exempt from the tread depth standards only if they meet certain specifications. The lower level must have another exit method, the maximum height from the finished basement floor level to the connected grade must be 8 feet or less, and the opening from the upper level to the stairway must be covered by an enclosure such as double doors that hinge outwards. If the bulkhead stairway does not meet these criteria, it must follow the stair measurements and tread depths specified by the IRC for all stairs.

Americans With Disabilities Act

• The Americans With Disabilities Act defines building standards that enable the handicapped to use stairways. While these routes are obviously inaccessible to those in wheelchair, they must still be usable by those with balance, hearing or vision problems. The ADA standards apply primarily to commercial spaces, but may be useful to homeowners who either have handicaps themselves or want to sell their homes to the disabled. They specify minimum tread depths of 11 inches, with riser heights of between 4 and 7 inches. All measurements must be uniform throughout a flight of stairs.