Unlike hardwood grading, plywood grading falls short of indicating the actual quality of a sheet of plywood. In an attempt to find a simple way to help determine the quality of a sheet of plywood, many would like to reduce the entire issue to the number of plies. While we can empathize with the desire for an easy way to evaluate a sheet of plywood, it’s simply not possible. Why not? We’ll attempt to explain.
What Number of Plies Does & Doesn’t Mean
The reason counting plies fails to account for quality is the same reason plywood grading misses the mark: it only tells part of the story. A major factor in plywood quality has to do with how it’s constructed; and that isn’t just about the number of plies. Plywood core is important, too; and neither is accounted for in the grading scheme. More plies doesn’t mean everything, but it does mean something: the greater the number of plies, the greater the amount of glue. And that increase creates a product that’s more synthetic, rather than an organic, hydroscopic one. It might seem to follow that the greater the amount of glue, the greater the stability of the plywood. That line of reasoning may seem logical, but it isn’t always accurate.
The Nitty Gritty About Glue
A huge factor in plywood quality is tied to glue, to be sure; but it has to do with more than the amount of pieces glued: it has to do with the quality and amount of glue used. If poor quality glue is used, more plies mean more crappy glue, so the result isn’t higher quality plywood. As important as glue quality may be, glue application is important, too. Often, foreign mills use extenders to essentially water down the glue; automated mills allow for thinner glue lines, further extending the volume of glue. While automation saves money, it can also be quite costly: fully automated plants can have a glue vat run dry, leading to a lack of glue between plies and ending up causing delamination.
Why the Bottom Line is Still the Bottom Line
In the end, the whole issue is tied to expense. In fact, about 30% of your plywood’s cost is actually for the glue. Even though the price of wood can drastically change and is dependent on many variables, including grade and species, the cost of glue is far more stagnant. As a result, glue is the very thing over which mills have the greatest amount of control. If they want to increase profit margins, they know they can change up the glue they use or how it is applied. If they can figure out a way to extend the same volume of glue to make 10 additional panels, a cost savings for them will result. But as glue and other elements of the manufacturing process are manipulated to lower cost, quality takes a huge hit.