Noncombustible refers to the fuel the building contributes, not its resistance to spread of fire. Typically exposed metal floor and roof system and metal or masonry walls. Least stable in terms of collapse, when exposed to fire. Walls of masonry or other noncombustible walls with a 2 hour fire rating. Floors, roofs and interior partitions are of wood.  Interior columns, beams and girders are heavy timber (minimum 8x8). Floors and roofs are heavy planks (3x6 minimum).