Cal. Code Regs. tit. 8, § 7041 – (11-3) (11-5). Construction of Wooden Ladders for Underground Use.

(a) Wooden ladders shall be substantially constructed of sound lumber of strength equivalent to No. 1 Select Douglas Fir of the following dimensions:
1. (1) Side rails shall be not less than 2 inches by 4 inches nominal, in cross section.
2. (2) Ladder steps shall be clear, straight-grained, and absolutely free of knots.
3. (3) Ladder steps shall be not less than 1 inch by 4 inches nominal, in cross section.
(b) The distance between the tops of the steps of a ladder shall not exceed 14 inches and shall not vary more than 1 inch in any one ladderway.
(c) Ladders shall be constructed so there is at least 10 inches clear space between the side rails.
(d) Ladder steps shall be securely fastened to the side rails with nails or other equivalent fastenings, and shall be secured against pulling loose by one or more of the following methods:
1. (1) Mortised in the side rails so as to be flush with the surface, provided this is done without unduly weakening the side rails.
2. (2) Fastened to the surface of the side rails with filler pieces between the ends of the steps. Filler pieces shall be the same width as the side rail and the same thickness as the ladder steps.
3. (3) Fastened on the surface of the side rails with continuous wooden strips nailed over the ends of the steps for the full length of the ladder.
4. (4) Any combination of the above methods which in the opinion of the Division will provide equivalent protection.
(e) Nails and other metal fasteners used in ladders that are exposed to corrosive water or corrosive mine atmosphere shall be made of metal that is resistant to corrosion of the type to which they are exposed.