Dealing with Bowed or Warped Plywood

Warped, bowed, cupped, twisted — these days, plywood seem to lie every which way but flat. Here's a long and somewhat technical discussion of what makes plywood deform, and what can be done about it. December 28, 2005

I have purchased a unit and a half of 3/4" A-1 maple plywood (5 ply + 2 veneer faces, made in Canada). I have gone through the pile and I noticed some of the plywood is not flat along the 8' length. It’s usually about a 1/2" curve. (If the board is vertical and a string is put on either end [8' way] there is a 1/2" space in the center). Has anyone had any success in taking the curve out by laying the ply down supported only by the ends on 2 x 4's with the curve facing up and letting gravity do it's thing? Or am I just wasting my time?

Forum Responses
(Cabinetmaking Forum)
From contributor S:
I wouldn’t waste your time. Call the sales rep where you bought it and have it replaced.

From contributor F:
The half inch hollow/bow sounds pretty typical to me. The way I deal with it depends on the season or the humidity in my shop. If I think it is dry in the shop, I will lay the plywood convex side up so that it will start to dry and begin going hollow on that face. If I think the air in the shop is moist, I will lay it hollow or concave side up for a while so that the hollow or dry side will take on moisture, begin to swell and then move toward being straight.

As to the question as a whole, yes, I think you can definitely influence the shape of plywood by the way it is stored and how it is supported, but it is a very slow process and will only be practical if you have the time to store the material for months before you use it.

For me, I see the bow when I get the sheet into the shop and I rip the longest parts from the straightest plywood and I stack the cutout parts on a very flat/straight surface. I monitor the top piece on the stack/stacks and flip it face for face as it changes shape until I put in into whatever assembly it is part of.

From contributor J:

I second Contributor F's response on this one. If you need flat these days you’re stuck with MDF or similar product. I have seen 3/4" doors on older (50 plus years) cabinets that were dead flat but for some reason the stuff they make today doesn't like to stay flat.

From the original questioner:
What I have noticed is that the top sheet is the real mover. Look at it one day and it is bowed one way, flip it over and the next day it is bowed the other way. So it does sound like I can make the curve go away. When it is flat (enough) seal it on both sides and it should stay that way. As I see it, wood is a natural product and it will respond to temp and humidity. I have yet to receive the perfect board, flat and true in all respects - you need to make it that way, so I figure the same is for plywood. It should be more stable, but it seems to succumb to the ravages of nature like everything else.

Even if I called up my distributor I don't think they would let me trade in individual pieces for straight ones, they need to be defective and they don't consider a slight bow a defect because that is the way all of their stuff is. And it is not like all the plywood has the same bow, as I pull them off some are straight, some have bows and some are worse then others. You just need to pick out the ones that are straight where you have to have straight and let the bowed ones go where it doesn't have to be straight. I know that when I make shelves I like to have a slight upward bow in them so they will settle straight.

From contributor V:
You might want to try covering the pile with cardboard. That will prevent moisture from getting to it.

From contributor E:
1/2 inch sounds average for good material these days. I feel lucky with any grade if that's as bad as it gets. Even if it is flat, a rip down the center will release tension in the panel and get worse sometimes. I stack on a heavy pallet rack and hope for the best.

From the original questioner:
I have to admit I don't have a problem with the thickness of this batch. I put two sheets together and they were shy 1/64" of 1 1/2". When I was getting the C-3 grade they were almost 11/16" thick, maybe a red hair thicker.

From contributor P:
I may be wrong, but it seems to me that the bowing is due to moisture migration through the face veneer, causing the first cross ply to expand or contract. The ever-thinner face veneers we are getting are probably the cause of this - older ply, with thicker faces, would be more resistant to this. If this is actually what is happening, then exposing both faces to the same humidity conditions should straighten them out, unless the cross-band was laid up out of balance.

From contributor Y:
I had a bad experience with a bunch of teak plywood a while ago. It was 13 ply and it actually looked pretty good when the sheets were whole, but when I got 16 sheets cut to size and dadoed, they twisted like a bag of potato chips. I couldn't even get them pulled into shape with pipe clamps. I called the place I had them made at and they replaced all of them with MDF core and refunded the difference in price between the ply and MDF. It seems to me that maybe the cores are not dried enough when they lay them up and that is what is causing all of the movement.

From contributor G:
It's pretty straightforward in my shop - MDF core on most veneered panels except where weight or linear strength are a major issue. I probably use 90% MDF core sheet stock, 10% V.C. Does anyone know of a source to buy 3/4" veneer with an ultralite MDF core? Is there any such thing?

From contributor Y:
You could have it made for you if you have a presser in your area. In Minnesota there is a plant called Buffalo Veneer and plywood that will put veneer on any substrate you want for the most part.

From contributor O:
Some people mentioned MDF core as being flatter. I also started using MDF about 10 years ago when my plywoods started to get unpredictable. But now in the past year or so I have been getting some MDF that is behaving like plywood and has been cupping, not just bowing. What is going on? MDF has always been a reliable product which I could count on to remain dead flat from raw sheet to finished product.

From contributor K:
I'm with Contributor P. It may be a moisture problem. I do similar to what Contributor V, and put a sheet of cardboard on the top sheet of a stack. Even if I have a couple of sheets lying out, I still cover the top. But be careful, the cardboard will curl, too.

Contributor P mentioned that to keep a panel flat, there must be the same amount of moisture on both sides so it says in equilibrium. If there is a bow/cup, introduce more moisture to that side - it has dried out/shriveled up, or take more moisture out of the other side - make it as dry as the other side. If it is humid, leave the curl up and let it gain moisture. If it is dry, curl down and let it flatten. When it is flat, cover it. If this doesn't work, the it is probably related to the core and how it was glued.

Have you started those frameless yet? If so, just be consistent in the way you orient the panels. Make it so that the sides curve into the box. When you join them to the next box, you can screw them together and get a straight edge. Put the shelves so that they curve is up, and weight will take care of the rest.

From Gene Wengert, forum technical advisor:
It would be very helpful if we knew the relative humidity in the area. This is then converted to an equivalent moisture content. This MC value is called the EMC of the air. The EMC must be within 2% of the moisture content of the wood. The warp being mentioned here is 100% related to an imbalance between EMC and MC.

From the original questioner:
The moisture in the air at my shop varies widely from day to day. I have a hygrometer in the shop along with my thermometer. The moisture usually varies from 45% to 85% depending on the weather. The real reason this question came about is because the sheets in the middle of the pile were not all flat or all bent. It varied from sheet to sheet. I would imagine if the moisture was the suspect, as I got deep into the pile the sheets would be straighter because they were not subject to the moisture in the air. I would dig into the pile looking for sheets that had the same #1 side, as it varied more than the A side. When I found one I would stand it on its side to see if it was straight enough for the cabinet I was using it for. To my surprise some of the sheets were flat and others (next in line) had a curve.

So I am thinking that it has less to do with the current moisture in my shop as to the way they were made/pressed/stored before they got to me. I tend to think it has more to do with the way they are processed (to quickly) then the moisture content of my surroundings, which in my thoughts should be considered a defect in manufacture, although I'm sure that a 1/2" curve in a piece of their plywood is within the manufactures specifications.

From contributor R:
I think we all contend with a certain amount of warped plywood sheets due to humidity inconsistencies. However, last year I kept getting 3/4" maple both unfinished and pre-finished that would warp 1" over 4". I just kept returning it to my distributor but the problem was not resolved until the distributor stopped buying from one particular mill.

From contributor U:
I have had good luck with classic core plywood (also called x-band, combo-core and probably a few other names.) It has two 1/8" plys of MDF directly below the veneer layer and then 3 standard veneer plys in the center. This product is available in all popular species of wood. It stays much flatter and is much lighter than solid MDF core.

From Gene Wengert, forum technical advisor:
Each sheet of veneer in a piece of plywood has a slope of grain (SOG). (This slope is the direction of the fibers. A split will follow the grain.) When the moisture content changes and there is slope of grain, the piece of plywood will warp, often twisting. If the moisture never changed, the piece would never warp (unless made in a warped shape initially). Certainly, some veneer mills have more SOG than other mills.

Assuming that the SOG is close to zero, a sheet of plywood (or any wood, for that matter), if exposed to one humidity on one face and to another humidity on the other face (that is, the moisture in the wood is different from face to face), moisture changes will cause size changes and warp can result, usually crowning, up or down. (As a rule of thumb, a 4% MC change results in a 1% size change across the grain. Further, wood does not change size along the grain.)

If, when manufacturing plywood with a decorative veneer face, the MC of the decorative veneer is not matched to the rest of the plywood, then again, as the moisture evens out, there will be shrinkage (or attempted shrinkage) that will cause warp. In this case, it would be common to see all the pieces warp in the same direction (such as toward the decorative veneer) rather than randomly.

A key point: Wood does not change its size or shape unless its moisture changes. The only exception is warp that occurs immediately when machining or cutting the piece of wood, or size changes that occur over many years when a piece of wood is under load (such as a loaded bookshelf).

From Gene Wengert, forum technical advisor:
Most decorative plywood is made at 6% MC to 8% MC. Part of the reason is that homes and offices in the wintertime in most of the USA have about 30% RH which will result in 6% MC in wood products.

Your shop is about 8% MC at the driest (45% RH) and very wet (17% MC at 85% RH). This means that any plywood brought into your shop will quickly gain moisture and swell, creating warp. It is also possible that some moisture gain could occur while the plywood is sorted in a retail or wholesale outlet.

From Gene Wengert, forum technical advisor:
Incidentally, at 85% RH, you would expect to see some rusting of tools and some mildew too, as this is a very high RH. A spot that is just a few degrees cooler would have condensation. I do wonder if your RH instrument is accurate.

From the original questioner:
The 85% reading only happens when it is raining outside. The gauge is electronic and responds very fast. On average I think it reads about 50-60%, in winter it is around 40%. I have no machine rusting or mold forming; I have however had moisture form on the floor where there is epoxy paint in extreme heat and humidity. I have plastic beneath all my wood storage to prevent any excess moisture transference.

From Gene Wengert, forum technical advisor:
I would suggest that you invest in a dehumidifier that you can run in the summer (evenings and nights) to keep the average RH a bit lower. If you use heat in the wintertime, unless you have a humidifier, the 40% RH reading sounds a bit too high naturally. Most shops heated in the wintertime will run 30% RH or lower. Radio Shack sells a $25 electronic sensor for RH that works well.

Regarding the moisture on the floor; it is a good idea when building a shop to insulate the floor by insulating the foundation. So-called "blue board" is put vertically to a depth of about 36" around the outside of the foundation. In wetter locations, there may be some plastic put down before the floor is poured to keep soil moisture out. Unfortunately, many shops today are in buildings that were not originally designed as a shop, etc.