When buying plywood from your local Home Depot, Rona, Lowes or other wood supplier, you might have noticed that all the plywood is "graded". The most common plywood grading scheme is from A to D, with A being the highest quality with zero blemishes and great sanding, and D being the worst with the greatest number of blemishes (allowed).
Grading also typically comes in pairs where each grade addresses a different side or “face” of the stock piece, ie one letter will address the quality issues of the front face and the second, the side opposite to the face. So for instance, an A-C plywood sheet would be highly finished on the front face with a relatively poorer finish on the back. Similarly, construction grade C-D (referred to as CDX) plywood, is great for structural use but not for projects requiring a high quality finish.
Bonding Types
Along with the plywood grading system, plywood comes in different bonding where each type is differentiated by the glue used to bind the layers (aka plies) of plywood. We’ll cover each in turn.
Interior Plywood
This type is made for interior use only, from hardwood and softwood species and is generally used in places where exposure to moisture is minimal, e.g. furniture, wall sheathing, cabinetry, etc. Interior plywood is available in most grades and comes in a variety of hardwood species like birch, oak and cherry.
Exterior Plywood
By far, much more sturdier and moisture-resistant than interior plywood, this type can be used outdoors and is easily available from local suppliers. Like its interior counterpart, it also comes in various grades—A-C, B-C and CDX are widely available—and hardwood species.
Marine Plywood
If you’re really looking for highly moisture-resistant plywood, look no further than Marine Plywood, which is both manufactured in top quality and uses the highest adhesives. And though commonly graded A-A for two highly placed faces, hardwood choices for exterior use (where the type would be most useful) are limited.
Structural Plywood
If you’re looking for beauty over brawn, this type is ideal although it is rarely found in a grade higher than C-D and is atypically used in construction sites (as concrete forms). Special resins are used to adhere the layers together and they are designed in such a way that the plies are less likely to separate.
Plywood Sizing
Just like hardwood and softwood sizes, plywood sizes can be just as confusing if not more. Although sheets are usually sold as 4’ wide, they may sometimes be found in 2’ and 5’ widths. Similarly, just as a typical plywood sheet’s length is 8’ they can also be found in 4’ and 12’ sizes as well as metric sizes. The variety can easily confuse the best of us.
And that’s just the beginning; the variation of sizes above will be a walk in the park compared to the thickness dimensions. Common sizes on the US market are ¼”, ½” and ¾”. That said, a ¼” plywood sheet is really 7/32”; ½” a 15/32” and ¾”, a 23/32”.
And though the 1/32” doesn’t seem like much, it can make all the difference when working with plywood. Consider this: a wood craftsman is constructing a bookshelf where a ¾” shelf is inserted into a dado cut into the shelf standards; the 1/32” gap will not only be noticeable, the dado will feel sloppy and unprofessionally handled. To counteract such a situation, the dado will need to be cut at 23/32”, ensuring a snug fit.