The radius of curvature at the leading edge of the tread shall be not greater than 9/16 inch (14.3 mm). Beveling of nosings shall not exceed 9/16 inch (14.3 mm). Risers shall be solid and vertical or sloped under the tread above from the underside of the nosing above at an angle not more than 30 degrees (0.52 rad) from the vertical. The leading edge (nosings) of treads shall project not more than 11/4 inches (32 mm) beyond the tread below and all projections of the leading edges shall be of uniform size, including the leading edge of the floor at the top of a flight. Perforated risers with openings not exceeding 1/4 inch (6.4 mm) shall be considered solid risers.
Exceptions:
- Solid risers are not required for stairways that are not serving as exits or that are not required to comply with Section 1007.3, provided that the opening between treads does not permit the passage of a sphere with a diameter of 4 inches (102 mm).
- Solid risers are not required for occupancies in Group I-3 or in Group F, H and S occupancies other than areas accessible to the public. There are no restrictions on the size of the opening in the riser.
- In Group R-3 occupancies, the following shall apply:
- The radius of curvature at the leading edge of the tread shall be no greater than 9/16 inch (14.3 mm).
- Solid risers are not required provided that the opening between treads does not permit the passage of a sphere with a diameter of 4 inches (102 mm). However, the opening between treads is not limited on stairways with a total rise of 30 inches (762 mm) or less.
- Solid risers are not required for spiral stairways constructed in accordance with Section 1009.9.
- Solid risers are not required for alternating tread devices constructed in accordance with Section 1009.10.