ABSTRACT
In a wood structure, plywood is used primarily as roof and floor sheathing and wall sheathing. The span rating for floor and roof sheathing appears as two numbers separated by a sloped line. To design roof sheathing for gravity loads, there are several options to choose from. Two types of systems are used for floor sheathing. The first is a single-layer system, which is also referred to as a single-floor or combination subfloor-underlayment. The second type is a two-layer system in which the bottom layer, or subfloor, acts as the primary structural support. The floor and roof sheathing supports gravity loads and also acts as a horizontal diaphragm to support lateral wind and seismic loads. For a rigid diaphragm, a torsional moment is developed and must be considered in the design of shear walls. The chords and drag struts are the top plates in horizontal diaphragms.
