Section 21.23. Wood steps

(a) Wood rungs, treads and cleats shall not be made from low density wood, but shall be thoroughly seasoned material free of all of the following defects:

(1) Shakes.

(2) Pitch pockets.

(3) Cross-breaks.

(4) Injurious checks.

(5) Decay.

(6) Compression failures.

(7) Compression wood.

(b) Cross grain in rungs and cleats of minimum dimensions shall not exceed a slope of one in 15. If the size of rungs and cleats is increased to afford at least 15% greater strength than the minimum specified in § 21.33 (relating to steps), the maximum allowable slope of cross grain may be increased to one in 12.

(c) In ladders having treads for steps, cross grain in treads shall not exceed that permitted for side rails as specified in § 21.21 (relating to wood side rails).

(d) Knots over 1/8 inch in diameter shall not appear in rungs.

(e) Knots shall not appear in the narrow faces of treads and cleats.

(f) Knots appearing in the wide faces of treads and cleats adjacent to the edges shall not exceed in diameter 1/12 the width of the face. The size of knots in the wide faces may increase proportionately from the size allowed adjacent to the edge of two times that size at the center line of the wide face.

(g) Wood treads shall be made from the species of woods according to the specifications of § 21.21.

(h) Wood rungs and wood cleats shall be made from white ash or approved equivalent. Species listed in § 21.61 (relating to group one woods) with the exception of Douglas fir and Southern yellow pine may be substituted for white ash in like sizes. Douglas fir and Southern yellow pine shall not be used for rungs or cleats.