Plywood is widely used in the construction industry, especially in the building of new homes. It is used for subfloors, as an underlayment for exterior walls to cover the framing, and for roofs. Plywood is graded based on its ability to withstand moisture, and there are two main types. These types of plywood are interior grade and exterior grade, and must only be used for its intended purpose to be in compliance with building codes when used for new home construction.
The creation of plywood in the United States can be dated as far back as the mid-1800s. John K. Mayo developed and patented the idea to glue together thin strips, or veneers, of wood to form what is known as sheets of plywood, but it wasn't until the early 1900s that plywood was recognised as a valuable product that could be used in the woodworking industry. The recognition of the value of plywood created an industry that is still in demand today.
Plywood is constructed out of thin sheets of soft or hard woods that are glued together, heated to over 93.3 degrees C, and machine pressed. Poplar is the most common wood used to create the sheets of plywood, and the average size of each sheet is 4 by 8 feet. The advantages of using plywood over regular wood is that plywood is stronger than regular wood, and it is less likely to crack, warp or shrink when exposed to weather extremes.
Defining Exterior Grade
Exterior sheets of plywood are graded with the letters A through D for each side, and the letter X that represents it has been made for exterior use. Plywood has a front and a back, and the front or face of the plywood will always be a better grade than the back. The letter A signifies that the plywood has no knots or blemishes, is sanded smooth on the face, and is suited for fine finishing. The letter D is the lowest quality of plywood, and is used most often for construction purposes. There is no need for construction grade plywood to be blemish free or finely sanded.
Exterior plywood is designed to be more resistant to moisture than interior plywood. The glue that is used to bond the layers of plywood together also determines its ability to be exposed to moisture. Plywood that is classified as exterior can be used for construction applications that leave the plywood exposed to weather elements. Plywood that is classified as having an exposure 1 level can withstand exposure during construction, but must ultimately be covered.`One other classification for exposure is known as marine. This type of plywood is often used for the construction of boat hulls, and must be constructed out of the highest quality of plywood.
Each sheet of exterior plywood is marked with the letter grade and the exposure rating. Plywood is always made with an uneven amount of veneers, which gives it its strength. The weakest of plywood sheets is known as 3-ply, and the strongest is known as 9-ply. The ply rating means how many layers of veneers a sheet of plywood has. All of these factors are marked on each sheet of exterior plywood, and will help you to purchase the correct type that is needed for each job.
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- APA: The Engineered Wood Association: Plywood Glue Bond-Durability Classification
- American Plywood Association: Softwood Plywood Grading and Classifications