Winder treads shall have a tread depth of not less than 10 inches (254 mm) measured between the vertical planes of the foremost projection of adjacent treads at the intersections with the walkline. Winder treads shall have a tread depth of not less than 6 inches (152 mm) at any point within the clear width of the stair DRAWING 19. Within any flight of stairs, DRAWING 12 (p. 8) the largest winder tread
depth at the walkline shall not exceed the smallest winder tread by more than ⅜ inch (9.5 mm) DRAWING 20. Consistently shaped winders at the walkline shall be allowed within the same flight of stairs as rectangular treads and shall not be required to be within ⅜ inch (9.5 mm) of the rectangular tread depth DRAWING 20.
Exception: The tread depth at spiral stairways shall be in accordance with Section R311.7.10.1 DRAWING 54 (p. 19).