Plywood vs Lumber

[dusty]

What is meant when one says that plywood is "more stable" than lumber?

Is it a true statement? Are there any qualifiers to make it a true statement?

I've encountered a lot of warped plywood lately is why I'm asking.

[fixit]

I've always thought of plywood being more "stable" than lumber in terms of which one will change dimension in length or width the most with a change in humidity. Plywood doesn't change as much as lumber because of the fact that the layers of veneer are glued together at 90 degree angles.

With lumber there is only one "layer" which will expand across its width more than its length.

Warping is a different process. It comes from one side of the material being in compression while the opposite side is in tension. Given enough time (and force acting on it) plywood AND lumber will warp and have a concave side facing the direction from where the force was applied.

[dickg1]

Dusty, I too have been affected by warped plywood. The 4X8 sheets are fine until I start cutting them into workable panels. Then the panels warp. I have been getting them at a popular home center and suspect they are coming from somewhere in Asia and are being manufactured in a high humidity climate. Since the large panels appear to be OK and are stored flat, it seems they warp from internal tension which is released when cut.

Dickg1

[rkh2]

Just my two cents worth. I would be real careful when buying plywood at the big box stores. I work at one and a few weeks ago we had a "special buy" on some birch plywood which was priced too good to be true. I was tempted until I noticed that it was manufactured in China. Sorry, but I am sticking to plywood I know is manufactured here in the USA. Sometimes it pays to spend a little more for quality.

[Ed in Tampa]

What is meant when one says that plywood is "more stable" than lumber?

Is it a true statement? Are there any qualifiers to make it a true statement?

I've encountered a lot of warped plywood lately is why I'm asking.

Dusty
Plywood generally is more stable, however like everything else it is evolving. At one time plywood was made with multiple sheets of wood with each layer having it's grain perpendicular to the preceeding sheet. These glueups were usually 7+ sheets of ply and with little or no voids. Then manufactures found they could allow voids, then they cut down on the number of plies, and eventually they got to what is commonly called ply today. Basically Junk!

To make matters worst they began importing plywood from Asian countries and quality dropped even further. Then the super stores came on the scene and along with that storage problems. Many times ply is stored out doors under a few sheets of poly half heartedly covering the pile. When needed they get a fork lift bring the pile in and the unaware buyer pulls some sheets from the pile takes it home and watches it curl up.

However there is still real ply out there. Most small lumber yards, cabinet supply stores, and speciality wood stores carry a wide variety. Look in your phone book and start calling. Most of these places will sell to home hobbiest if you approach them right. Call a few local cabinet shops many will be glad to order a few extra sheets for you or give you the name and number where you can order yourself.

I use Sandply bought a HD which is easy to distinguish by the number of plies usually 9-13. I also use Marine ply, baltic ply (which is usually in 5x5 sheets) solid core ply which has MDF as a center, cabinet ply which is usually 10-13+ plies. I have never had any of these warp. Also I can't remember having a sheet of veneered (birch, oak, walnut, maple, etc) ply warp.

I know for a fact if I buy simple ply from the Home Store (HD, Lowes and some lumber yards) it will probably warp and curl.
Ed

[a1gutterman]

There is a reason that plywood is stored, laying flat, at the lumber store. Standing them on edge will lead to warpage before and after cutting to smaller size panels.

[Nick]

From "Wood and Woodworking Materials" by Nick Engler:

"Stability
"Plywood is much more stable across its width than solid wood. However, it is not completely stable. It expands and contracts across its thichness every bit as much as solid wood, and there is a small amount of movement along its length and width. How much movement? The amount depends on the type of plywood and the materials in it's plies. Generally, it's slightly more than the longitudinal movement of the wood from which its made, and ranges between 0.1 and 0.2 per cent. A 48-inch-wide piece of plywood may expand and contract 1/16 inch over the course of a year."

The warping and twisting you have mentioned has little to do with stability and more to do with balance. Laminated assemblies tend to warp and twist unless the stresses caused by wood movement are properly balance. The best way to balance plywood is to glue two outside plies to a middle ply, making three plies in all. The grain direction of each ply must be precisely perpendicular to the adjacent plies. For thicker plywood, additional plies are added in pairs to keep the assembly balanced -- this is why plywood always has an odd number of plies.

Other factors affect balance. The plywood remains flatter and more stable when all the plies are the same thickness (which is why Baltic Birch is so much more stable than many other plywoods). And grain direction is critical. If the grain direction of one face (outside) ply is cocked at a slight angle to the other, the plywood will twist, cup, or warp.

The quality of wood also effects the balance. Not only do knots and defects cause uneven stresses, certain species are easier to blance than others. The best core woods are relatively stable and lightweight. It also helps if the density is consistent. Poplar, for instance, make a good core wood. It expands and contracts less than most hardwoods, has a specific gravity of only 0.42, and there is little difference in density between springwood and summerwood.

With all good wishes,

[dusty]

There has been a lot of valuable information provided in this thread, at least it's valuable to me. But now that I have all that information, when I go to the lumber yard to purchase plywood - I am still ignorant. Unless I go in and buy baltic birch, I still can't look at a sheet of plywood and tell whether it is good or not. Yes, I can tell if it has a lot of surface knots and voids but that's where it ends. Plywood is plywood is plywood.

The guys in the lumber yards - I sure do hope they are patient.

Thank you all for the info provided.

[charlese]

If you want to make sure your plywood was made in the U.S.A. - check the bottom side for a grade stamp and mill location. This stamp will also tell you if is from Eastern or Western wood.

In the past I have had quite a bit more trouble (twisting/warping) with Southern pine ply as opposed to Douglas fir ply with a cottonwood center.

[james.miller]

I ran across an interesting plywood when I was stationed in North Carolina in the 70s. It was 3/4" thick, very heavy and 6 ply. The two center ply's ran in the same direction, the next two ply's were at right angles to them with the outer two ply's running in the same direction as the center ply's. All 6 ply's were the same thickness and appeared to be made out of the same type of wood.