Open risers can create problems for individuals with disabilities….and for those of us who are in a hurry. To prevent a foot from being caught under the tread, a solid riser that is vertical or sloped is required in most conditions.
When are open risers permitted? In new construction, open risers will only be permitted on stairs that are neither a means of egress nor part of an accessible route.
The Texas Accessibility Standards (TAS) and the ADAAG do not have requirements for stairs unless the stair serves as the only access to a level. The section below is copied from the TAS (italics denote an addition or change from the ADAAG):
4.1.3(4) Interior and exterior stairs connecting levels that are not connected by an elevator, ramp, or other accessible means of vertical access shall comply with 4.9; however, stairs adjacent to areas of rescue assistance required by 4.1.3(9) shall comply with 126.96.36.199 whether or not the stairs connect levels also connected by an elevator.
The International Building Code 2006 (IBC) does not permit open risers except in Group I-3 occupancies or when the stair is not an accessible means of egress (IBC 2006, 1009.3.3). Because all spaces in new construction are required to have at least one accessible means of egress, and two when more than one means of egress is required, most stairs will require closed risers.
To sum it up, open risers can be used on monumental stairs that are not part of the egress and only if the second level is also served by a ramp or elevator.