34 Pa. Code § 21.3
This chapter applies to every establishment within this Commonwealth, specifies the kinds of materials suitable for ladders, and divides the species of wood into four classes according to their strength as related to minimum dimensional requirements of ladder parts. The quality of wood materials specified in this chapter shall be controlled by limiting the timber defects in ladders manufactured to the minimum dimensions. Where larger cross sections are employed, the timber requirements relating to the size of knots and slope of cross grain may be modified when and as the provisions of this chapter provide the basis of modification being that the strength of each ladder part shall be equivalent to or greater than a similar part manufactured to the minimum dimensional requirements.
34 Pa. Code § 21.3