Why Does Plywood Warp?

An extended technical and practical discussion of dimensional instability in plywood panels. January 20, 2010

Hi, I’m curious to know why plywood warps worse than anything else and what are ways of storing it to prevent warping?

Forum Responses
(Cabinetmaking Forum)
From contributor L:
I'm not an expert on this but I do know that after we installed an automatic moisturizer in our shop to keep the humidity around 35%, most of our warping problem was solved. I am amazed at the difference it makes.

From contributor D:
What type of ply are you having problems with? I know that Chinese birch will warp like crazy, it usually comes pre-warped. I have not had any major problems with warpage when using quality plywood, but even the most expensive ply can have a slight bow to it. As far as storage, if you have enough room in your shop, you should store your plywood laying flat.

From Gene Wengert, forum technical advisor:
Twist is a common type of warp with plywood. The reason is, one, that the moisture content changed after manufacturing. Plywood is almost always a low MC product, as the heat used in drying and gluing assures that the plywood is not wet, but rather is fairly dry.

Next, instead of cutting parallel to the bark, which minimizes warp even in sawing lumber, veneer is cut parallel to the pith (log center). This cutting pattern means that there is slope of grain (SOG) which in turn means that the wood is moving in all three directions, instead of two. Twist is the common end result. That is, one corner is out of plane with the other three. Construction plywood with warp is usually not an issue as it can be nailed or screwed flat when used for sheathing, etc. which is what it is designed for. When construction plywood is used for other uses, the warp is serious. One way to reduce the problem is to use a composite core (particleboard, MDF). Another is to use a higher grade of plywood, perhaps AB.

In any case, any attempts to reduce moisture changes (storage at 35% RH is well worth the effort, as mentioned in a previous posting) will minimize the warp issues. As an additional note, when using plywood, it is prudent to coat both faces with the same coating system to avoid moisture differences between the faces and warping.

From contributor R:
That brings to mind another question, why do they then sell prefinished plywood with one side unfinished? I was so glad to see it because it made me feel so much better about the few hundred or so jobs that I never sealed the backs or the ends of the cabinets on.

From Gene Wengert, forum technical advisor:
Isn't most prefinished plywood very thin? If so, it is easy to flatten. I think the issue arises with thicker plywood.

From contributor A:
Gene, prefinished plywood is sold 1/4", 1/2" and 3/4". We can get various grades of maple as well as red oak and cherry, prefinished with UV cure urethane 1 side or 2 side. We use Columbia. The 1 sided product is bowed like the big times. We used to buy it for painted end panels. However, it is so bowed that we gave up and just sand the finish on end panels before priming. As Gene mentioned the composite panels are typically better. We had good luck with Multicore (stranded core with veneers). Perhaps we are expecting a product to be flat, that by its nature should not be flat. Just a thought.

From Gene Wengert, forum technical advisor:
Just to make sure that no one missed it, plywood, when first made, is flat. It is only with MC changes that it warps. This change can occur in storage as well as after it is in a product, so warp can occur "before" or "after."

Note that as a consultant for 35+ years, people always contacted me with their problems. I do not recall anyone that asked me to visit their operation or sent me samples for analysis unless there was a defect or problem. In fact, the UPS man once asked me what I did with all the table tops - he thought that they were good ones and so I had a hundred tables in my house!

From contributor B:
Just something to throw out there for you - we used Multi-Core a lot and found it to be a very stable product, but it is sometimes hard to get. We are trying something new called Performance Core, and while it is a little too early to tell yet, so far it looks like it is going to be as good as if not better than the Multi-Core.

From Gene Wengert, forum technical advisor:
It would be common to find that plywood and other composite products are around 6% MC after being manufactured. In an unheated shed or outside storage protected from rain, the conditions are 12% EMC in most of the USA. Plywood stored in such a condition will indeed change MC and warp can be expected. It is much better to store such products at around 35% RH (7% EMC) so that moisture changes are minimized. Where does your supplier store plywood? Where do you store plywood?

From contributor R:
Gene are you telling me that if I stick my plywood on stickers in the paint room and it gets up to 120 degrees and I run my de-humidifier that it will go back to flat as when it came out of the hot dry press? I hope that’s what you’re telling me. I know the moist ground on the down side and the hot sun on the other and you can bow it the bow it again and again untill you remember at just the right time and fasten it off before it changes again, but it does not work on a small % of sheets, why is that ? some are amuened to my straitening method, why is that?

From Gene Wengert, forum technical advisor:
Indeed, it will straighten, but when it warps, there is also some plastic movement (also called set) that will not be easily recovered, so it may not end up perfectly flat. I await hearing the results.

From contributor A:
Gene, obviously if the plywood can't handle minor changes in EMC then I would not describe it as a flat stable product. Metals and plastics stay reasonable flat with changes in temp. Good wood stays flat (obviously gets bigger) with rising EMC. I believe the sheets are too big to stay flat. It would be interesting to see how flat the sheets are in the warehouse a day after they are manufactured. You could also say that a piece of wood is flat after it is face jointed; however, take a piece of poplar(with 5/16" growth rings) a few minutes and it won't be flat much longer.

From contributor R:
I'm not looking for perfect, just flat enough that my 1/4'' of adjustment is enough to allow my side mount euro drawer slides to work.

From contributor J:

Part of the problem is that the plywood mills can't get old growth logs anymore to make plywood from. The trees they use are smaller in diameter 2d and 3d growth plantation tree. States and Murphy plywood are just down the road, and my hardwood salesman is well informed about plywood. Bigger (trees that is) is better when it comes to wood for plywood.

I just got some decent birch plywood (made in Oregon), and it's so flat and nice I don't want to cut it up, because the next time it would be doing the big bow or crown. Chinese plywood is the worst; I have a 12" x 96" rip of 3/4" multi ply (poplar core) that has a crown in the middle that is 6" above the plane of the plywood. I'm saving it for no reason other than if I want a long shelf to store bricks on it would work perfectly. I think the Chinese are using trees that are 3" inch diameter and where watered by sludge from a Chemical or noodle factory.

From contributor D:
Contributor J hit on an important aspect. The core itself can vary in different and the same products. I'm in Oregon and have been to one of the Columbia plywood mills as well as seen the local small mills when they were open. I have seen units of plywood literally hot off the press get loaded out on a truck to a lumber yard. Have you ever seen the one sticker trick under the middle of the whole unit? Fork lift drivers are capable of this unit warping event. I saw them press a unit sand it and trim it and stack them cover them and band them ,at that point they are dead flat. Plywood is either flat when you get it or not, and it stays that way until you attach it to something.

From contributor S:
I am no plywood expert, but I know that storage and moisture content are definitely not the only factors on warpage. I think the posts that mention how it is manufactured is a big thing. I can cut a couple of lifts at a time and I will come across 80% of the sheets that are nice and flat, and the other 20% have varying degrees of warpage. I find the veneer cores are always the worst, no matter where they are manufactured.

From Gene Wengert, forum technical advisor:
I believe that warpage is 99% of the time caused by MC change in storage or use. However, not every panel is prone to warping when the MC changes. The worst ones will be the ones that have juvenile wood (the first 15 or so annual rings from the center of the log) in them, as well as crooked logs. So when this knotty juvenile wood is used, warp is likely compared to veneer cut further from the center (meaning large logs). As mentioned, lumber cut from cores is prone to warp as well. All this (veneer and lumber warpage) is because juvenile wood likes to shrink along the grain.

From contributor F:
The knowledge and opinions are good here, but the bottom line in my pond - quality is out the window, gone, forget it, good luck, etc, etc. The new rule as far as I can tell is basic B2 at around 60 plus a panel rates below a shop panel at 20 or less years ago. I see ripples in the veneer and core/veneer failures during finish stages ranging from veneer raising to core and veneer popping up from the slightest amount of solvent in sealer and top coat stages. The thought of checking each panel during delivery is something I never considered until the last 2-3 years.

Talking with the local suppliers about this has been an education. They admit to the low quality of the product and can only offer a suggestion to move up to A2, which is getting to be rare in the supply chain and insane price wise. 100-120 or more and hope the veneer and core will hold up during sizing and finish?

At one point I asked “So, three times the price for half the quality, right?” and the answer – “Yep, that's the way it is right now”. I appreciate the honesty from the rep, but man it's brutal for a small shop trying to make the bills. I'm beginning to wonder if a panel with a stable core and thick enough veneer is available at any price. Are we being moved into a particle core with whatever fancy new name the suppliers want to use and the thinnest veneer possible? Is ply too expensive to produce in any quality? Sure feels like it. Perhaps you guys up north are getting better luck, but the old south is wet on the ground and moist in the core ;)