M1601.5 Under-Floor Plenums
The space shall be cleaned of loose combustible materials and scrap, and shall be tightly enclosed. The ground surface of the space shall be covered with a moisture barrier having a thickness of not less than 4 mils (0.1 mm). Plumbing waste cleanouts shall not be located within the space.
A duct shall extend from the furnace supply outlet to not less than 6 inches (152 mm) below the combustible framing. This duct shall comply with the provisions of Section M1601.1. A noncombustible receptacle shall be installed below any floor opening into the plenum in accordance with the following requirements:
- The receptacle shall be securely suspended from the floor members and shall be not more than 18 inches (457 mm) below the floor opening.
- The area of the receptacle shall extend 3 inches (76 mm) beyond the opening on all sides.
- The perimeter of the receptacle shall have a vertical lip not less than 1 inch (25 mm) in height at the open sides.