Q. We have some cabinet doors that were flat but now, after installation, have warped a slight bit, enough to make them a reject in the customer’s eyes, and rightly so. Two questions: What caused this and what can we do about it to straighten the doors?
A: Moisture. Wood only changes size or shape in use because of a moisture change. (Of course, we could have a large outside force that would bend the wood too, but that is not a reasonable or likely event for cabinet doors.) So, we know that there was a moisture change between the time you manufactured the doors until they were put into use and the warp was noted. Now, the moisture in wood changes ONLY because the moisture content (MC) of the wood is not equal to (or in equilibrium with) the moisture in the air. Here is a brief summary of the relationship between MC of wood and the RH (relative humidity) of the air. Note that temperature is not a factor…it is only humidity. For convenience, we sometimes define the air’s moisture level as EMC, equilibrium moisture content of wood. If the air is at 7 percent EMC, the wood will be trying to achieve 7 percent MC.
RH       EMC
 0 percent       0 percent
30        6
50        9
65        12
In the wintertime, the interior conditions in most homes and offices are around 6 percent EMC. In the summertime, 9 percent EMC. Further, the outside conditions are 12 percent EMC, summer and winter, in much of North America.
Warp. So, with a MC change comes a size change or change in shape. Usually, we can tolerate a change of + or - 1 percent MC without any problems. It is with larger changes that we begin to see problems. Further, an abrupt or rapid change in conditions (going from your humid shop to a dry customer’s office for example in just a few days) will cause more problems that a slow change taking many months (such as the change from summer to winter). Hence, it is critical to have the wood, at the time of manufacturing, at a moisture level that the product will see when it is installed in the home or office. In most cases, this means you need to be certain that your wood supply is kiln dried to 6.5 to 7.2 percent MC, and that your storage area for wood and components is kept at about 35 percent RH (6.8 percent EMC).


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Gene Wengert, aka The Wood Doctor, troubleshoots wood related problems, and explores lumber and veneer qualities and performance, species by species, in Wood Explorer, inside FDMC's Knowledge Center.

The assumption often is that the rails or stiles are causing the warp. However, it can be the inserted panel. In fact, the inserted panel can be cut out of the door and then you will see if it was the panel or the frame that caused the warp. Of course, this is a destructive test, with no chance for repair.
So, why does wood warp? There are many reasons, but from a practical point of view, almost all warp results because
1) The grain of the wood within a piece is not parallel to the sides; that is, there is slope of grain. This slope can be due to the grain angle in the tree or due to the techniques used when sawing the log into lumber. If the MC changes in a piece of wood with slope of grain, then the piece will warp, especially twisting type of warp. When perfectly straight grain is required for expensive items, such as a pool cue, the wood is first split (which will follow the grain) and then sawn parallel to the pith. This is obviously too expensive for most lumber, so we need to work on minimizing the MC changes instead of grain selection.
2) The EMC on one side is different than on the other side, creating a MC gradient and therefore a shrinkage gradient even when the grain is straight. Note that this difference can be due to two different finishes on the two faces—one finish lets moisture through quickly while the other retards vapor flow fairly well. (Note that no finish seals the wood even close to perfect, so in reality if the outside RH changes, the wood MC will change slowly (which is better) or quickly, depending on the finish properties.)
3) A less common reason for cupping type of warp (warp across the width of a panel) is because for every piece of wood, the side or face closest to the bark will shrink more than the other side or face. This difference between faces is greater as the piece is cut from a region in the log close to the center of the log. As the lower grades of hardwoods, No.2 and No.3 Common, typically come from this region, this would be the wood most likely to cup, especially wide pieces. For this reason, wide panels are glued up using narrower pieces, rather than two or three wide pieces. This is also partly why MDF offers a more stable core material than solid lumber. (Note: The idea of reversing the grain of the adjacent staves in a glued-up panel has little or no effect on cupping.)
4) An inserted panel can be mis-manufactured so that it has unbalanced construction; that is, the face and the back will not swell or shrink the same when the MC changes. If the inserted panel has a veneer surface on the face, the back also needs a similar behaving piece of veneer to keep things balanced. Also, the veneer itself can have grain angle issues with enough movement when the moisture changes to warp the core.
KEY: All of these reasons require a moisture change of some sort. If there is no MC change, there will be no warp.
Two more ideas (correct MC is the first idea) for reducing the likelihood of warp.
5) When making the door initially, we can install stiffeners (usually metal) in the rails and stiles that will better help the wood resist warping. This is especially effective on large doors.
6) Use an MDF core with a veneer surface laminate rather than solid wood core with veneer or a solid wood core that is also exposed.
Straightening. I have listed here several ideas on how to straighten a door, but you will likely agree that some are not too practical all the time. Some basic concepts are: once warped, even when the MC returns back to the original value, the warp does not disappear- - that is, some warp is semi-permanent.  Plus, even if the door is flattened, a subsequent moisture change can cause the door to warp again. As a result, most cures will not work too well. Spend time and effort avoiding the problem in the first place rather than repairing the warp once it occurs. Also, the effort involved in straightening can be very expensive…it might be cheaper to make or order a new door.
Method #1. Rout a groove (perhaps ½ inch x ½ inch x nearly full length) in the back of the stile. Then make a curved piece that will fit into the groove that has about twice the curvature of the door, but in the reverse direction. Clamp this piece into the the groove and see if it straightens the door and perhaps even reverse the warp slightly. If it puts in too much reverse warp, then shorten the piece. But, if this piece does work, then glue it into the slot and after the glue dries, remove the clamps.
Method #2. Rewet the wood partially and then bend it flat and continue bending it a bit more. Then dry it in this reversed warp condition. When the bending clamps are removed, the wood will spring back a little bit…hopefully it will spring back enough so that the door is flat. This is somewhat “hit and miss.”
Method #3. Cut a bunch of small grooves crosswise (90 degrees) to the warp in the rail or stile. These would be perhaps 3/8 inch deep- -experiment to find out how deep and how many. These grooves will weaken the rail or stile so it might come back to flat automatically, but if the warp is caused by the inserted pane, the warp will be worse. If not automatically flat, the door, with the weakened rail or stile, can be easily bent into a flat position. Then a brace can be fastened to hold the door flat (see #6) or epoxy glue can be put into the grooves while the door is flat, restoring the strength and keeping the door flat.
Method #4. There are commercially available jigs that can be put on a door, running from one corner to the diagonally opposite corner. This is similar to the diagonal piece of wire that is used to straighten a swinging gate in a fence. It works, but might not look very cute.
Method #5. We can wet the door and then bend it flat plus a little more. It is difficult to know how much to bend the door beyond flat so that when the pressure is removed, the spring-back will result in a perfectly flat door. Repeating: This is not a permanent fix, as a change in humidity can cause the warp to return.


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