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Plywood, a product made by gluing a number of thin layers of wood together, is an inexpensive and convenient material often used as a base for other materials. The two most common areas in construction projects where plywood decking is used are roof decks and sub-floor decks. Plywood decking is fastened to roof trusses or floor joists to provide a substrate for other materials like roofing shingles, finished flooring etc.
There are many types of plywood decking. Softwood plywood consists of between three and five layers and is sold in sheets measuring 4 by 8 feet (1.2 by 2.4 m). This type of plywood decking is commonly used for roofing applications at a thickness of 5/8 inch (15.87 mm).
Plywood decking used as a sub-floor typically has a thickness of 3/4 inch (19.05 mm). It is also a common practice to use tongue and groove plywood, a type that joins at its seams, for flooring applications. It reduces movement between the two pieces, providing a stable surface to which finish flooring can be attached.
Several benefits were provided by the invention of plywood decking and sheathing. These are increased construction speed; a uniform product that is readily available worldwide; and a strong, durable material that is suitable for many applications. The manufacturing of plywood drastically reduced the amount of waste created and provided a more efficient method versus cutting sheathing and decking boards out of logs.
Plywood is manufactured by gluing a set number of layers, or piles, of wood veneers together with special glues. The number of piles will vary to accommodate different strength requirements. The weather resistance properties of the glues used to bond the piles together determines whether the plywood is exterior or interior sheathing.
Marine plywood is treated with a special additive that makes it resistant to rotting in areas of high moisture. This property makes the plywood ideal for use as floor decking in boats and other types of watercraft. Pressure treated plywood is similar to marine plywood as it is specially treated to resist rot and water damage. The plywood is allowed to soak under high pressure in a mixture of chemicals, which gives it weather, rotting and insect resisting properties.
Plywood is graded according to the amount of defects present in the material. Common defects include knots, cracks and rough surfaces created during its manufacture. Plywood is also produced with a finish grade side or sides. Different countries have their own grading scales with the two most common being British Standard (BS) or the American Standard (ASTM). This is beneficial to consumers to aid them in selecting the right grade of plywood for their project.