Cal. Health & Safety Code § 9633

Horizontal and vertical partitions separating crypts comprising companion crypts or a nest of crypts entered through a single crypt opening may be constructed of precast reinforced concrete; provided, the horizontal partitions are not less than one and one-half inches in thickness and the vertical nonbearing partitions are not less than one inch in thickness, and vertical partitions bearing any load are not less than three inches in thickness, and provided the crypt walls enclosing the nest of crypts are constructed as required in Section 9632. Crypts shall be vented at roof level of the structure, and vents shall continue to a gravel filled trench below the floor of the bottom crypt to provide adequate circulation of air. Nonstructural horizontal and vertical partitions separating columbarium niches may be constructed of precast reinforced concrete or other incombustible material.