QUESTION: On a recent project, we were informed that our concrete stairs did not meet the building code requirements with regard to the allowable tolerances on riser heights. After several re-measures by a third party, our work was finally accepted. Considering the cost of tearing out and replacing stairs and landings, I want to understand what’s required to ensure our work satisfies code requirements on future jobs. Also, how do you measure riser heights when the tread surface has been sloped for drainage?
ANSWER: Because stairs are one of the most common locations of missteps and falls, building codes including the International Building Code (IBC), International Residential Code (IRC), and National Fire Protection Association (NFPA) 101 Life Safety Code address stair geometry, dimensional uniformity, and slip resistance of surfaces. Detailed requirements are specified to ensure both safe and effective use of stairs during both emergency egress and day-to-day usage. Codes address riser heights, tread depths, stair landings, slopes, and slip resistance of surfaces.
For nonresidential buildings (IBC), stair riser heights shall be 4 inches minimum and 7 inches maximum as shown in the chart. For residential buildings (IRC), the maximum riser height is 73/4 inches. The IRC does not specify a minimum riser height. Also, risers can be vertical or sloped under the tread at a maximum angle of 30 degrees. Riser heights shall be measured vertically between the nosings of adjacent treads.
Stair risers must be uniform size and shape. The tolerance between the largest and smallest riser height shall not exceed 3/8 inch in any flight of stairs and 3/16 inch between adjacent risers.
For special conditions, building codes do allow uniformity exceptions for riser heights. For example, riser heights can be less than 4 inches when bottom or top risers adjoin an existing sloping public way, walkway, or driveway having an established grade and serving as a stair landing. However, the variation in height of the bottom or top riser shall not exceed a slope of 8% (1 inch per foot) of the stairway width and the nosing or leading edge of the tread of the nonuniform riser height must have a distinctive and non-slip marking stripe.
Tread depths and maximum slope
For nonresidential buildings, rectangular tread depths shall be 11 inches minimum, measured horizontally between the vertical planes of the foremost projection of adjacent treads and at a right angle to the tread’s nosing. For residential buildings, the minimum tread depth shall be 10 inches.
Identical to riser heights, tread depths must be uniform with an allowable tolerance of 3/8 inch between the largest and smallest tread depth in any flight of stairs and 3/16 inch between adjacent treads.
The radius of curvature at the nosing or leading edge of treads shall not exceed 9/16 inch. For outdoor stairways, treads must be slightly sloped so water will not accumulate on the stairs. Install a 1% slope (1/8 inch per foot) to ensure water runoff but do not exceed the maximum allowable slope of 2% (1/4 inch per foot) in any direction.
Tread slopes can be sloped either to the front, back, or perpendicular to the direction of travel. Building codes limit the tread slope to reduce the nonuniformity of the effective riser heights and to reduce the risk of occupants slipping on the sloped tread surfaces.
There shall be a landing at the top and bottom of stairs. Minimum landing widths shall be at least the stairway width and minimum depths shall be at least 48 inches for nonresidential and 36 inches for residential buildings. Landing surfaces shall not be sloped more than 2% (1/4 inch per foot) in any direction.
For sidewalks adjoining stair landings, the maximum running slope (parallel to the direction of travel) is 5% (1/2 inch per foot) and the maximum cross slope (perpendicular to the direction of travel) is 2% (1/4 inch per foot).
If the running slope of a sidewalk exceeds 5%, the sidewalk would be considered a ramp and code requirements for ramps would apply. Also, abrupt changes in elevation of surfaces shall not exceed 1/4 inch.
Building codes require all walking surfaces, including stair treads, landings, and sidewalks, to be uniformly slip-resistant along the natural path of travel. If walking surfaces are exposed to weather or moisture, do not install a steel-troweled finish as they can become extremely slippery when wet. Instead, install a broom finish or other nonslip surface.
Kim Basham, PhD, PE, FACI, is president of KB Engineering. He specializes in concrete construction, troubleshooting, nondestructive testing, forensics, and repair. E-mail firstname.lastname@example.org; visit www.kbengllc.com.