Alternating tread-type stairs have a series of steps between 50 and 70 degrees from horizontal, attached to a center support rail in an alternating manner so that a user of the stairs never has both feet at the same level at the same time. (See Figure D-11 of this section.)
(1) You must ensure that alternating tread-type stairs are designed, installed, used, and maintained in accordance with approved manufacturer's specifications, and have the following:
(a) Stair rails on all open sides;
(b) Handrails on both sides of enclosed stairs;
(c) Stair rails and handrails of such configuration as to provide an adequate handhold for a user grasping it to avoid a fall;
(d) A distance of 17 to 24 inches between handrails;
(e) A minimum width of 22 inches overall;
(f) A minimum tread depth of 8.5 inches;
(g) A minimum tread width of 7 inches;
(h) A maximum rise of 9.5 inches to the tread surface of the next alternating tread; and
(i) Open risers if the tread depth is less than 9.5 inches (24 cm).
(2) You must ensure that alternating tread-type stairs have not more than a 20-foot continuous rise. You must provide one or more intermediate stair platforms in accordance with WAC 296-24-74005 where more than a 20-foot rise is necessary to reach the top of a required stair.
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Angle | Type |
≤ 30° | Ramps |
30° - 50° | Standard Stairs |
50° - 70° | Ship Stairs |
50° - 70° | Alternating Tread-Type Stairs |
60° - 90° | Ladders |
Figure D-10 - Angles for Stairs, Ramps, and Ladders
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Figure D-11