When it comes to plywood, a lot of people are concerned about how many plies, or layers, are involved. And that is significant. But some other things matter even more. Two panels can appear identical — same species, same thickness. But if one is comprised of 6 plies and the other has 12 plies, is the latter necessarily the better choice? Maybe. But just like bigger isn’t always better, more isn’t always better either.
Why More Plies May Be Better
Theoretically, more plies is better, but probably not because of the reason you’re thinking: It’s not because the thinner the veneer, the less the movement. Ready for the real reason more might be better? More plies mean more glue. More glue means less organic, hydroscopic movement, allowing the panel to function more like a synthetic product. The more glue that’s used, the more stable the plywood product will be. Essentially, the more glue that’s used, the less the wood will move.
Why More Plies Might Not Be Better
Think about it: Why might more plies not be better, after all? I’ll give you a hint: It’s the same reason that more plies may be better. If you guessed it has to do with the glue, you’re right! More glue may be a good thing, if that glue is quality stuff. But if poor quality glue is used, the more glue, the lower the quality of the plywood product. Both the glue itself and the application process of that glue come into play, when it comes to determining the quality of glue.
How To Determine Glue Quality
Really, only a small fraction of the glue-quality issues have to do with the actual chemical makeup of the glue. Foreign mills often do use extenders to essentially water down the glue and increase the amount of coverage they can get out of a given amount; however, technology does allow thinner glue lines to reasonably extend a given amount of glue.
The automation currently used in the plywood-construction industry has led to decreased costs, but even fully automated plants have to ensure that the glue vat doesn’t become empty while the line is being processed. If that situation occurs, panels can delaminate easily, revealing missing glue between plies. If only heat, pressure, and a little glue residue are holding your plies together, it doesn’t matter how many plies or how high quality they are — or how good the glue is, for that matter!
Why Plywood Pricing Helps
You can’t go evaluate the gluing process of every plywood manufacturer out there; we get that. But you can evaluate plywood based on pricing; in fact, for plywood, pricing is more of a determinate of quality than grading is. And glue is a huge part of the expense. It usually adds up to about 1/3 the cost of a sheet of plywood. Unlike the lumber itself, the glue price is fairly constant. When the lumber prices fluctuate, plywood mills can keep their prices constant by skimping on the glue. If you don’t want more cheaply made plywood, then you can’t expect bargain-basement pricing.
Read the Series
• Lumber Math: Fractions You Need To Know
• Lumber Math: Understanding the Board Thickness Dilemma
• Lumber Math: Possible Solutions for Board Thickness Dilemma
• Lumber Math: Plywood Ply Number Isn’t Always Important
• Lumber Math: Shorter Boards Can Be Better & Save You Money
• Lumber Math: Sometimes Less Is Really More