Plywood for building construction has a span rating that governs how it is used to build subfloors and to sheath roofs. On floors, the plywood panels have tongue-and-groove edges that fit together. On a roof, the plywood carries less load and the tongue-and-groove edges are not required. The manufacturer-assigned span rating for roofs and floors specifies the maximum allowed distance between rafters for a roof and between joists for a floor.