Dumb plywood question...

[Brian Penning]

If the layers are cross-grain to each other why doesn't it stay flat?

[Joe Angrisani]

Because wood is not perfectly consistent and wood moves. Movement in one direction is overpowering movement (or a fault) in the other direction.

[Joe Angrisani]

There are no dumb questions.

[glenn bradley]

It used to. We don't make it very good anymore ;-) Seriously, stored correctly, quality sheetgoods remain flat. BORG plywood setting on end on your damp garage floor . . . . not so much.

[Julian Tracy]

This isn't one of them, but there are definitely dumb questions!

JT

[Jay Jeffery]

In order to not warp due to changes in moisture and temperature, an unrestrained laminated structure needs to be both balanced and symmetric. Balanced meaning that for every layer of a given degree there is a layer of the opposite degree, ex a laminate with layers at 90 deg, 0 deg, 30 deg and -30 deg would be balanced. Symmetric means just what it implies, that all the layers are on one half are repeated in reverse order in the other half, ex a laminate with layers at 90 deg, 45 deg, 0 deg, 45 deg, and 90 deg. Balanced and symmetric example would be 90/-45/45/0/0/45/-45/90.

If plywood were made with layers that had consistent grain patterns that were carefully aligned, and layers were matched to have similar properties, they could make very warp resistant. But of course plywood wouldn't be very economical to manufacture that way.

The more layers in a laminate, the less the balance will matter. So if warping is a concern, get that really thick Baltic birch that has lots of layers. The void free should also help.

Naturally, if moisture absorption is not consistent across the thickness, it can still warp. So keeping air on both sides, or keeping both sides blocked from moisture will make a big difference. Even completely isotropic wood materials can warp if you leave them on the concrete.

For example, if you paint the top of something, painting the bottom too can help keep things flat.

[Tony Bilello]

Poor quality while being manufactured, poor handling after manufacturing and poor storage by the wholesaler and retailer.
Some of the lower quality plywood is subject to tropical temperatures and high humidity while being stored and directly after being manufactured. This takes it's toll in the inferior inner plys.
Quality lumber and plywood dealers do not leave their plywood on open trucks during rain and snow and I have never gotten warped plywood from a quality dealer. Also, quality plywood manufacturers use a better quality limber for the inner plies. Most quality plywood should arrive to you with no dents and dings. Usually if it has any at all, the retailer puts it in another area and sells it as shop grade and not cabinet grade.
At the BORG's, everyone handles the plywood and shoves it around. At a quality hardwood lumber yard, you can usually select your own lumber but you can only look at the plywood and not touch.

['Jacques Malan']

Poor quality while being manufactured, poor handling after manufacturing and poor storage by the wholesaler and retailer.
Some of the lower quality plywood is subject to tropical temperatures and high humidity while being stored and directly after being manufactured. This takes it's toll in the inferior inner plys.


I received 12 sheets pine plywood from the dealer once that were so warped I couldn't get a flat 12"x12" piece out of it. I only find that out after cutting all of it. I put a piece of it in the press for a week and that didn't help at all. From that day on I am as careful about plywood as I could be and will never accept bent plywood again.

[Tony Bilello]

.......... I am as careful about plywood as I could be and will never accept bent plywood again.

Amen, brother.
There are many techniques I have heard about on how to re-flaten plywood or even boards. None have ever worked for me so I dont even bother. Buy the good stuff and you will have much less problems.

[Alan Schwabacher]

The question has been answered, but let me try to make it easier to visualize. It is true that moisture causes wood to expand across the grain, and loss of moisture causes it to shrink, while along the grain it is pretty close to constant dimension. But it can bend. If you take a straight piece of wood and attach a cord to each end pulling strongly, you can bow the wood sideways even though you are pulling along the grain. This is useful for archery.

Now consider a simple sheet of plywood with only three plies of alternating direction. There is straight grain in each direction, so the overall size of the panel should remain constant. But with change in moisture, the wood is straining to expand or contract, so the plies on each side of the center can be pulling quite strongly. The only reason it does not bend is that the pulls are balanced so the pull on one side cancels out the pull on the other side. So plywood remains flat not because of lack of stress, but because of balanced stress. As mentioned above, in lower quality plywood these stresses may not be balanced, because of wood of varying thickness, density, grain direction or continuity.

With more plies than three, there is more chance that the stresses will come closer to balance, but even putting one extra ply on a face can make the panel unbalanced enough to cause some bending.

Trees that have grown with their trunks at a slant, or branches rather than trunks, have wood with built in stresses from growing while resisting that sideways force. This "reaction wood" can make amazing contortions when you cut it.

[Mike Schuch]

From the look of the Borg plywood I have seen I figured it comes in a huge roll from China and they unroll it and cut it into 8' lengths when it get her.

Does anyone else's local Borg not even store the plywood on a flat surface? At mine the plywood is supported by three beams on there huge racks and the center beam is always 3" higher than the two outside beams. It is like they are trying to impart a curve to the plywood? Is that because if it is flat lengthwise it will start curling width wise? Miserable stuff to work with for sure!

[Dave Lehnert]

My local hardwood dealer said that import plywood most often had the core wood stored outside in the rain. They then apply the outer vernier that is dry and stored indoors.

[Phil Thien]

My local hardwood dealer said that import plywood most often had the core wood stored outside in the rain. They then apply the outer vernier that is dry and stored indoors.

While that may be an extreme example, I do think the problem w/ import plywood is that the moisture content of the plys is inconsistent during manufacturing.

[Jay Jeffery]

From the look of the Borg plywood I have seen I figured it comes in a huge roll from China and they unroll it and cut it into 8' lengths when it get her.

Does anyone else's local Borg not even store the plywood on a flat surface? At mine the plywood is supported by three beams on there huge racks and the center beam is always 3" higher than the two outside beams. It is like they are trying to impart a curve to the plywood? Is that because if it is flat lengthwise it will start curling width wise? Miserable stuff to work with for sure!

My guess is that it is safer and easier to get the forklift under the stacks that way. The big boxes really cater to the construction trade, and who cares if your roofing plywood is a little cupped. You will be nailing it to warped rafters anyways.

[David Hostetler]

While that may be an extreme example, I do think the problem w/ import plywood is that the moisture content of the plys is inconsistent during manufacturing.

I don't think it's so extreme of an example... Not sure if it is during manufacturing, shipping, or storage prior to retail, but I have gotten plywood from the BORG that nearly dripped...

[Phil Rose]

BORG plywood setting on end on your damp garage floor . . . . not so much.

Fortunately the local BORGs pre-dampen the plywood before I even get there ... not if I only wanted to build a conical house I'd probably have a shot at finding plywood there ... lol