Cal. Health & Safety Code § 9635

(a) All individual crypt openings shall be sealed with a solid panel of precast concrete, not less than 1 1/2 inches thick, fiber reinforced cement board not less than 7/16 inch thick, or other incombustible material that meets all of the following requirements:
1. (1) A minimum density of 80 pounds per cubic foot.
2. (2) A minimum modulus of rupture of 270 pounds per square inch.
3. (3) A minimum compressive strength of 2500 pounds per square inch.
4. (4) A rating that conforms to Underwriters Laboratories fire hazard class 1.
(b) All panels shall be securely set in with a high quality, nonflammable, resilient, and nonhardening urethane, silicone base, or other appropriate sealant for permanent sealing after interment is made in the crypt. Seal panels shall be set independent of crypt fronts.