Every hot-air furnace shall have at least one register without valve or louvres. A register box placed in the floor over a portable furnace shall have an open space around it of not less than four inches on all sides and be supported by an incombustible border. Hot-air registers placed in any woodwork or combustible floors shall be surrounded with borders of incombustible material, not less than two inches wide, securely set in place. The register boxes shall be of metal and be double and the distance between the two shall be not less than one inch, or they may be single, if covered with asbestos, not less than one-eighth inch in thickness and if all woodwork within two inches be covered with tin. Cold-air ducts for hot-air furnaces shall be made of incombustible material.