Evolution of the Use of Plywood for Structures

From 1940 to 1965, use of softwood plywood increased from 1 billion to 13 billion sq ft, the increase being mostly in building construction. Major construction uses are sheathing and subflooring. Contributing factors were low in-place costs ($0.10 per sq ft for roof sheathing) through labor savings plus acceptance of plywood as an engineering material. An important engineering property of plywood is its ability to act as a shear diaphragm in bracing a building against lateral wind or seismic loads. This was first recognized through wall racking tests, and later extended to horizontal diaphragms by the development of tested design methods. These design methods are also used in analysis of folded plate construction. Interesting uses for plywood have been developed as gusset plates to splice lumber into moment-resisting joints. Applications include trussed rafters and rigid gable frames. The connections are commonly made with nails, or glue or both. More recent developments include the acceptance, based on extensive testing, of plywood for one-hour fire-rated floor and wall assemblies, for heavy-timber roof construction, and for fire-retardant treated roof assemblies.