Cal. Code Regs. tit. 8, § 7043 – (11-3). Construction of Flexible Ladders for Underground Use.

(a) Flexible ladders shall have sides made of chain, connecting links, wire rope, or fiber rope.
(b) Each side of a flexible ladder shall have a tensile strength not less than that of 1-inch best Manila rope. If chair is used, the links shall be made of stock not less than three-eighths inch.
(c) There shall be at least 12 inches of clear space between the sides of a flexible ladder.
(d) Each step of a flexible ladder shall be made of stiff material and shall be strong enough to safely support a weight of 300 pounds at the center of the step. The distance between the tops of the steps of a ladder shall not exceed 14 inches.
(e) The steps shall be fastened to the sides in such manner that they will be held securely in place without damage to the sides of the ladder.