34 Pa. Code § 21.35 – Single or straight ladders.

(a) Single ladders over 30 feet in length shall not be used.
(b) The width between side rails at the base shall in no case be less than 11 1/2 inches for ladders up to and including 10 feet in length. For longer ladders this width should be increased at least 1/4 inch for each additional foot of length.
(c) Rectangular wood side rails may have an end depth of not less than 2-1/4 inches and shall have center cross section to conform with the requirements of § 21.32 (relating to side rails). Rungs shall conform to the requirements of § 21.33 (relating to steps).
(d) Hooked tops which fit over shaftings, roofs and other similar places shall be employed on single ladders where working conditions warrant.

This section cited in 34 Pa. Code § 21.39 (relating to fire ladders).