N.Y.C. Admin. Code § 27-873: § 27-873 Underground chimney connectors.

Underground chimney connectors shall be covered with at least twelve inches of solid masonry or an equivalent insulation. Each cleanout opening shall be provided with double iron doors or covers, and the doors or covers shall be twelve inches apart with the intervening space filled with insulating material. The doors or covers shall be not less than 10 manufacturers standard gauge. No combustible flooring shall be permitted over such connectors.