188.8.131.52.1 Material and Type
(a) Step frames, treads, risers, and dynamic skirt panels, excluding the step's attachments or inserts, shall be metal, except that magnesium alloys shall not be used; or the materials, in their end-use configuration, shall have a flame spread index of 0 to 50 based on the tests conducted in accordance with the requirements of ASTM E84, UL 723, NFPA 255, or CAN/ULC-S102.2, whichever is applicable (see Part 9).
(b) Nonmetallic attachments and inserts (excluding wheels) shall be classified 94 HB or better in accordance with ANSI/UL 94.
(c) Step treads shall be horizontal and shall afford a secure foothold. The step supporting system shall be designed so that the back of the step cannot tip upward more than 6 mm (0.25 in.) at any point.