Plywood Plywood is manufactured from thin veneer sheets that have been peeled from logs and bonded together with resin in alternating cross-grained plies using a hot press.3 To maximize strength and stability, the resulting plywood panels always have an odd number of layers so that each panel is balanced around a central axis. Structural plywood has been manufactured since 19054; however, delamination resulting from exterior exposure was a common problem until the first synthetic “waterproof glue” was developed in 1934.5 • “Since layers can consist of a single ply or of two or more plies laminated such that their grain is parallel, a panel can contain an odd or even number of plies, but always an odd number of layers. ...To distinguish the number of plies (individual sheets of veneer in a panel) from the number of layers (number of times the grain orientation changes), panels are sometimes described as three-ply, three-layer or four-ply, three-layer.”6
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