Plywood is made of three or more thin layers of wood bonded together with an adhesive. Each layer of wood, or ply, is usually oriented with its grain running at right angles to the adjacent layer in order to reduce the shrinkage and improve the strength of the finished piece. Most plywood is pressed into large, flat sheets used in building construction. Other plywood pieces may be formed into simple or compound curves for use in furniture, boats, and aircraft.
The use of thin layers of wood as a means of construction dates to approximately 1500 B.C. when Egyptian craftsmen bonded thin pieces of dark ebony wood to the exterior of a cedar casket found in the tomb of King Tut-Ankh-Amon. This technique was later used by the Greeks and Romans to produce fine furniture and other decorative objects. In the 1600s, the art of decorating furniture with thin pieces of wood became known as veneering, and the pieces themselves became known as veneers.
Until the late 1700s, the pieces of veneer were cut entirely by hand. In 1797, Englishman Sir Samuel Bentham applied for patents covering several machines to produce veneers. In his patent applications, he described the concept of laminating several layers of veneer with glue to form a thicker piece—the first description of what we now call plywood.
Despite this development, it took almost another hundred years before laminated veneers found any commercial uses outside of the furniture industry. In about 1890, laminated woods were first used to build doors. As the demand grew, several companies began producing sheets of multiple-ply laminated wood, not only for doors, but also for use in railroad cars, buses, and airplanes. Despite this increased usage, the concept of using "pasted woods," as some craftsmen sarcastically called them, generated a negative image for the product. To counter this image, the laminated wood manufacturers met and finally settled on the term "plywood" to describe the new material.
In 1928, the first standard-sized 4 ft by 8 ft (1.2 m by 2.4 m) plywood sheets were introduced in the United States for use as a general building material. In the following decades, improved adhesives and new methods of production allowed the plywood to be used for a wide variety of applications. Today, plywood has replaced cut lumber for many construction purposes, and plywood manufacturing has become a multi-billion dollar, worldwide industry.
The outer layers of plywood are known respectively as the face and the back. The face is the surface that is to be used or seen, while the back remains unused or hidden. The center layer is known as the core. In plywoods with five or more plies, the inter-mediate layers are known as the crossbands.
Plywood may be made from hardwoods, softwoods, or a combination of the two. Some common hardwoods include ash, maple, mahogany, oak, and teak. The most common softwood used to make plywood in the United States is Douglas fir, although several varieties of pine, cedar, spruce, and redwood are also used.
Composite plywood has a core made of particleboard or solid lumber pieces joined edge to edge. It is finished with a plywood veneer face and back. Composite plywood is used where very thick sheets are needed.
The type of adhesive used to bond the layers of wood together depends on the specific application for the finished plywood. Softwood plywood sheets designed for installation on the exterior of a structure usually use a phenol-formaldehyde resin as an adhesive because of its excellent strength and resistance to moisture. Softwood plywood sheets designed for installation on the interior of a structure may use a blood protein or a soybean protein adhesive, although most softwood interior sheets are now made with the same phenol-formaldehyde resin used for exterior sheets. Hardwood plywood used for interior applications and in the construction of furniture usually is made with urea-formaldehyde resin.
Some applications require plywood sheets that have a thin layer of plastic, metal, or resin-impregnated paper or fabric bonded to either the face or back (or both) to give the outer surface additional resistance to moisture and abrasion or to improve its paint-holding properties. Such plywood is called overlaid plywood and is commonly used in the construction, transportation, and agricultural industries.
Other plywood sheets may be coated with a liquid stain to give the surfaces a finished appearance or may be treated with various chemicals to improve the plywood's flame resistance or resistance to decay.
Plywood Classification and Grading
There are two broad classes of plywood, each with its own grading system.
One class is known as construction and industrial. Plywoods in this class are used primarily for their strength and are rated by their exposure capability and the grade of veneer used on the face and back. Exposure capability may be interior or exterior, depending on the type of glue. Veneer grades may be N, A, B, C, or D. N grade has very few surface defects, while D grade may have numerous knots and splits. For example, plywood used for subflooring in a house is rated "Interior C-D". This means it has a C face with a D back, and the glue is suitable for use in protected locations. The inner plies of all construction and industrial plywood are made from grade C or D veneer, no matter what the rating.
The other class of plywood is known as hardwood and decorative. Plywoods in this class are used primarily for their appearance and are graded in descending order of resistance to moisture as Technical (Exterior), Type I (Exterior), Type II (Interior), and Type III (Interior). Their face veneers are virtually free of defects.
Plywood sheets range in thickness from. 06 in (1.6 mm) to 3.0 in (76 mm). The most common thicknesses are in the 0.25 in (6.4 mm) to 0.75 in (19.0 mm) range. Although the core, the crossbands, and the face and back of a sheet of plywood may be made of different thickness veneers, the thickness of each must balance around the center. For example, the face and back must be of equal thickness. Likewise, the top and bottom crossbands must be equal.
The most common size for plywood sheets used in building construction is 4 ft (1.2 m) wide by 8 ft (2.4 m) long. Other common widths are 3 ft (0.9 m) and 5 ft (1.5 m). Lengths vary from 8 ft (2.4 m) to 12 ft (3.6 m) in 1 ft (0.3 m) increments. Special applications like boat building may require larger sheets.