Plywood Grades: When buying plywood at your local supplier, you may notice that there are a number of different grades. The normal grading system uses the letters A, B, C & D, where A is the best quality, with virtually no blemishes and very well sanded. Grade D typically contains up to the maximum number of blemishes allowed. The letter grades typically come in pairs, where one letter refers to the "better" side, called the face, and the other letter to the back side, opposite the face. As such, a sheet of A-C plywood will be very well finished on the face with a relatively unfinished back. Conversely, construction grade plywood would be C-D (commonly referred to as CDX plywood), which is great for structural use but not suited to be finish material. Plywood Bonding Types: In addition to the plywood grades, there are four common plywood bonding types. The difference is in the glues that are used to bind the plies, or layers of the plywood.
Interior Plywood: Plywoods for interior use only are made from various hardwood and softwood species, and can be used only in interior applications such as wall sheathing, furniture (where exposure to moisture is limited), cabinetry and the like. Interior plywood is available in most grades, as well as a number of hardwood species such as birch, oak and cherry. Exterior Plywood: The most common type of plywood, readily available at home centers. The glues used in exterior plywoods are much more resistant to moisture than interior plywoods. Once again, nearly all grades are available, with A-C, B-C and CDX the most common. Numerous hardwood species are also available in exterior varieties. Marine Plywood: When moisture resistance is a priority, look into marine plywood. This type uses the best adhesives and is manufactured to the highest standards. It also is most commonly graded as A-A, with two top grade faces, but is limited in the hardwood choices that are practical for use in marine settings. Structural Plywood: When the appearance of the face is of lesser concern than the strength and stability of the material, structural plywood will typically be the choice. The resins used to adhere the plies are designed for extra strength to avoid separating of the layers. Structural plywood is seldom found in a grade higher than C-D. It is commonly used in concrete forms on construction sites. Sizes of Plywood As with hardwood and softwood sizes, plywood sizing can be somewhat confusing. Plywood sheets usually sold as four-feet wide, but are occasionally found in two and five-foot widths. Similarly, a plywood sheet's length is commonly eight-feet, but some suppliers carry four and twelve-foot lengths. Metric sizes are also available. While the length and width of a sheet is pretty straight forward, the thickness dimensions are anything but. The most common sizes of plywood sold in the United States are 3/4", 1/2" and 1/4". However, that's not exactly correct. A 3/4" sheet of plywood is really 23/32" (and is now typically labeled as such). Similarly, 1/2" should be correctly labeled as 15/32" and 1/4" labeled as 7/32". However, everyone still refers to them as 3/4", 1/2" and 1/4". Although it doesn't seem like much, this 1/32" differential between the actual size and the common size can become an issue when working with plywood. For instance, if a woodworker is building a bookshelf where a 3/4" plywood shelf is placed into a dado cut into the shelf standards, there will be a very noticeable 1/32" gap, and the shelf will feel a bit sloppy in the dado. To combat this, the dado should be cut at 23/32" to ensure a proper fit.
