Plywood is a building material that is useful in many different types of construction due to its relatively low cost and its flexibility, strength and resistance to warping and cracking. Plywood is a cheap and durable building supply that is used both for exterior and interior building purposes as well as for building furniture and cabinets.
Plywood can be made from several different types of wood, including hardwoods and softwoods. Common hardwood trees used for plywood production include ash, maple, mahogany, oak and teak, while softwoods include Douglas fir, pine, spruce and redwood. The type of glue used to hold the veneer sheets together differs based on the intended application. Sheets made of softwoods for exterior applications are typically bonded with phenol-formaldehyde resin due to its strength and moisture resistance. Interior softwood applications may also use a blood protein or soy protein as an adhesive. Hardwood sheets for interior applications, such as furniture, are usually bonded with a urea-formaldehyde resin.
How It's Made
Raw timber is softened with steam and is then mounted on a lathe, where a large blade peels off thin layers of wood in a contiguous sheet of veneer. The veneer sheet is dried and then stacked with other veneer sheets on top of one another so the grain of each concurrent sheet is facing the opposite direction of the previous sheet. The sheets are glued together and the stack of veneer sheets is heated and pressed, forming a strong, sturdy sheet of plywood.
Other Construction Methods
Oriented strand board (OSB) is less expensive -- at half the cost -- than traditional plywood sheets and uses wood chips rather than thin sheets of wood veneer. As with the veneer, these chips are glued, heated and pressed together. Composite plywood is used for applications that require thick sheets of wood, such as flooring or exterior walls, and often has a particle-board core with thicker veneer pieces layered on either side of the particle board.
Construction and industrial plywood are either rated for interior or exterior applications based on the veneer quality, with grades ranging from N, A, B, C or D. N-grade plywood sheets are of the best quality and do not have any defects, while D-grade sheets may have knots and splits throughout the front and back of the sheet. Plywood sheets with a combination of grades, such as C-D, have a C-grade veneer on the front and a D-grade veneer on the back. Hardwood and decorative plywood sheets are intended for aesthetic purposes and are graded based on moisture resistance.