Solid shelving is fixed in place, slatted, wire mesh or other type of shelves located within racks. The area of a solid shelf is defined by perimeter aisle or flue space on all four sides. Solid shelves having an area equal to or less than 20 ft2 (1. 9 m2) shall be defined as open racks. Shelves of wire mesh, slates, or other materials more than 50 percent open and where the flue spaces are maintained shall be defined as open racks.