Subsection 1009.10 - Alternating tread devicesAlternating tread devices are limited to an element of a means of egress in buildings of Groups F, H and S from a mezzanine not more than 250 square feet (23 m2) in area and which serves not more than five occupants; in buildings of Group I-3 from a guard tower, observation station or control room not more than 250 square feet (23 m2) in area and for access to unoccupied roofs.
1009.10.1Handrails of alternating tread devices.Handrails shall be provided on both sides of alternating tread devices and shall comply with Section 1012.1009.10.2 Treads of alternating tread devices.Alternating tread devices shall have a minimum projected tread of 5 inches (127 mm), a minimum tread depth of 81/2 inches (216 mm), a minimum tread width of 7 inches (178 mm) and a maximum riser height of 91/2 inches (241 mm). The projected tread depth shall be measured horizontally between the vertical planes of the foremost projections of adjacent treads. The riser height shall be measured vertically between the leading edges of adjacent treads. The combination of riser height and projected tread depth provided shall result in an alternating tread device angle that complies with Section 1002. The initial tread of the device shall begin at the same elevation as the platform, landing or floor surface. Exception:Alternating tread devices used as an element of a means of egress in buildings from a mezzanine area not more than 250 square feet (23 m2) in area which serves not more than five occupants shall have a minimum projected tread of 81/2 inches (216 mm) with a minimum tread depth of 101/2 inches (267 mm). The rise to the next alternating tread surface should not be more than 8 inches (203 mm).