Plywood is incredibly useful, with practical applications that include interior, structural, and exterior projects. It can be used to complete jobs that range from formwork all the way to paneling. But just what is plywood, and how is it made? In this post, we will answer these questions and learn more about why plywood is such a versatile material for outdoor and indoor construction projects alike.
What Is Plywood?
Plywood is a material (wood) manufactured from thin layers or “plies” of wood veneer that are glued together with adjacent layers having their wood grain rotated up to 90 degrees to one another. In the most basic of terms, plywood is made by combining wood veneers together in order to create a flat sheet.
Process
Plywood pieces are formed differently depending on their intended use. For example, if the plywood is to be used for building purposes, it is pressed into flat large sheets. If it is intended to be used in aircraft or boat construction or to build furniture, it is formed into curves. Every layer of ply has its grain running in right angles. This helps to keep the layer strong and durable while also limiting any chance of it shrinking. The plywood itself is constructed of a minimum of three layers of wood that are held together using an adhesive.
The two outside areas of the plywood are called the back and the face. The back is typically the part that is hidden from view while the face is the one that can be viewed. The layer in the middle is referred to as the core and if there are five or more plies used, the additional internal layers are considered crossbands.
Materials
Determining how plywood is made depends a great deal on the materials used. Plywood can be made from softwoods or hardwoods. It can also be constructed of a mix of the two. Some common softwoods used to create plywood include cedar, pine, redwood and spruce but the most commonly used is Douglas fir. For hardwoods, plywood is commonly created using oak, mahogany, teek, maple or ash. If using composite plywood, the core will be made of either solid lumber pieces or particle board. Composite plywood is typically used when the project calls for very thick sheets.
There are various types of adhesive that can be used to combine the layers of wood to create a sheet of plywood. The type used depends on the usage for the plywood. For example, a phenol-formaldehyde resin will be used if the sheets are to be used for a structure’s exterior. That is because the adhesive is very strong and will resist any damage caused by moisture, an important feature for any outdoor project.
If the plywood is to be used for the interior of a structure, the adhesive used is usually either made from a soybean protein or a blood protein. However, many of these interior plywood sheets will now often use the same phenol-formaldehyde resin that is used for exterior sheets. Finally, if the plywood is to be used for building furniture, the adhesive is usually made with a urea-formaldehyde resin.
There are other materials that can also be used in the construction of plywood. For example, some projects require that a layer of metal, plastic, paper or fabric be bonded to the back or face, or sometimes both, of the plywood. This helps to make the sheet even more resistant to moisture. That plywood is referred to as overlaid plywood and is most often used in the transportation, agricultural and construction industries.
Plywood can also have a layer of liquid stain used to give it a more polished look, which is used for more decorative projects. Finally, plywood can be treated with a variety of different chemicals to promote different properties. For example, it can be treated to be more resistant to flames or decay.
Classification
In general, there are two classifications of plywood, both of which have their own grading system. The two classes are construction/industrial and hardwood/decorative. Those plywoods that are included in the construction/industrial classification are used for their strength. They are rated depending on the grade of veneer that is used on their back and face as well as their exposure capability. That exposure capability can be either exterior or interior and usually depends on the type of adhesive used. The grades of veneer in this classification can be D, C, B, A, or N. The D grade is the lowest with multiple defects while the N grade has very few defects. As a rule of thumb, plywood that is used for subflooring usually has a rating of C to D.
The second classification of plywood, hardwood/decorative, is used for its overall attractive appearance. The grading in this class is dependent upon the plywood’s ability to resist moisture. The face of this plywood is almost always free from defects, at least by the naked eye, given that it is used for its decorative purposes. The gradings are Type III, Type II, Tile I, and Technical.
Sizes
Plywood can range quite substantially in size. The thickness can be anywhere from 0.6 inches all the way to 3 inches, with the most commonly used being somewhere in the range of 0.25 to 0.75 inches thick. The back and face of any veneer must have the same thickness and the top and bottom crossbands must also be of the same size.
Given the universal properties of plywood, it can be used in virtually any building or decorative project. If you need more information or are still wondering how is plywood made, reach out to Curtis Lumber & Plywood for more information.
