Plywood

Plywood is a type of engineered board made from thin sheets of wood, called plies or wood veneers. The layers are glued together, each with its grain at right angles to adjacent layers for greater strength. There are usually an odd number of plies, as the symmetry makes the board less prone to warping, and the grain on the outside surfaces runs in the same direction. The plies are bonded under heat and pressure with strong adhesives, usually phenol formaldehyde resin, making plywood a type of composite material. Plywood is sometimes called the original engineered wood.

The adhesives used in plywood has become a point of concern, due to the off gassing of the formaldehyde. Both urea formaldehyde and phenol formaldehyde are carcinogenic, so their use is undesirable. Many manufacturers are turning to "Greener Products" as government regulations become stronger against the use of these adhesives.

A common reason for using plywood instead of plain wood is its resistance to cracking, shrinkage, twisting/warping, and its general high degree of strength. In addition, plywood can be manufactured in sheets far wider than the trees from which it was made. It has replaced many dimensional lumbers on construction applications for these reasons.