by Steven D. Johnson
Racine, Wisconsin

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Plywood Plies & Plywood Lies

Plywood with an odd number of plies starts with a central core ply with the grain oriented according to the total number of plies. In the case of this 7-ply 3/4" plywood, the central core ply grain ran across the width of the 4' X 8' sheet (see the labeled photo in the previous section). On either side of that central core are two plies running perpendicular, or lengthwise, down the 8' dimension of the plywood. Then two more cores run crosswise, and a final pair of appearance (or veneer) plies sandwich the entire assembly, and the grain of those plies, of course, runs lengthwise. A total of four plies running parallel to the 8' length of the sheet, three plies running across the sheet --- so the sheet will demonstrate its greatest deflection resistance lengthwise, with the face grain, right? Wrong!

As a point of fact, this plywood may actually be stronger across its face grain direction than with its face grain direction. And the reason has to do with the thickness of the face plies.

Perhaps in the "good old days" the outermost veneer appearance ply was as thick, or almost as thick, as the inner plies, but not today. The two veneer appearance plies on this plywood are little thicker than a cheap sheet of copy paper.

Figure 7 - The 5 core plies separated

I decided to deconstruct a sample of this plywood and get a good look at the core plies. After submersing the plywood in water and softening the adhesive, I was able to use a broad chisel and slice apart the core plies. As a side note, the new soy-based urea formaldehyde-free adhesives must be outstanding. It took eight days of soaking before the edges of the core plies began to separate enough to get a chisel in, and it was still a fair bit of work to get the core plies apart.

This 3/4" plywood is stated to be "nominally 23/32." Converting 23/32" to millimeters, this plywood should measure ~19.05mm. Using my digital caliper, the actual thickness of the plywood was 18.05mm (about 45/64"). I removed and measured several pieces of the face veneer, and found them to be a fairly consistent 0.50mm. The five core layers each measured ~3.41mm in thickness. Therefore, if we simply add the thicknesses of the plies running in each direction, we find that a combined 10.23mm of plies run across the width of the plywood sheet and 7.82mm of plies, including the face veneer plies, run lengthwise. From this we could deduce that the strongest flex resistance in a sheet of this plywood would be in what appears to be a cross-grain direction, or across the width of the sheet.

Admittedly, when I started looking into this, I was shocked. For much of my life I have assumed that plywood was "automatically" strongest in flex resistance along its length, not across its width. But dissecting the plies and taking measurements would indicate my assumptions (and correlating work habits) were incorrect.

Still, I have visited the "Show-Me" state enough times that I need empirical proof, not just theoretical evidence. So I cut two identically sized pieces of this 3/4" plywood, one with the face veneer grain running lengthwise, and one with the face grain running crosswise. I set up a little jig and added weight. Each of these stacks of landscape pavers weighs 24 pounds, and as you can clearly see from the photo, this plywood is stronger in what would appear to be the "cross grain" direction.

Figure 8 - The plywood "shelf" on the left is oriented with its face grain running length- wise and it is sagging demonstrably more under the same weight

Immediately, the deflection of the plywood was 1/8" for the plywood whose face grain was oriented lengthwise, and just 1/16" where the face grain was oriented widthwise. After an hour, the difference had increased. The plywood with the grain running the length of the board had deflected an addition 1/32" and the other board had held at the same 1/16" deflection. Twelve hours later, the long-grain (in appearance) board had deflected a total of 11/64" and the cross-grain board had still bowed only 1/16". At 24 hours I had to dismantle the experiment because I needed those pavers (that project is a "whole 'nother story!"). I suspect the sagging would have continued a bit more, and the difference may have become more pronounced.

It is important to note that the central core orientation will be dependent upon the total number of plies. For example, the 1/2" cabinet grade plywood I purchased at the same time has a total of 5 plies. The two outermost, super-thin veneer plies are oriented such that the grain runs the length of the sheet, which means there are two thicker core plies running crosswise, and the center core runs parallel to the face veneer grain. The two thicker core plies running cross grain add up to more cumulative thickness than the single central core ply and the two whisper-thin face plies. This 1/2" plywood is also going to demonstrate more flex resistance in an apparent cross-grain direction than in the more conventional "with-the-grain" orientation.

I suspect about 99.9% of all plywood bookcases that have ever been built have shelves oriented so the face grain of the plywood runs the length of the shelf, across the bookcase. And while this is cosmetically correct (or at least customary), some plywood with whisper-thin outer plies may actually be stronger with the face grain oriented the other direction. If you (or your project) can live with face grain that runs in a "non-customary" direction, it may pay to look closely at the next plywood you buy and figure out the strongest orientation before starting.

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