4.9.2 TREADS AND RISERS.
On any given flight of stairs, all steps shall have uniform riser heights and uniform tread widths. Stair treads shall be no less than 11 in (280 mm) wide, measured from riser to riser (see Fig. 18(a)). Open risers are not permitted on accessible routes.
Figure 18 Usable Tread Width and Examples of Acceptable Nosings
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