Why Wood Warps and How to Prevent and Fix It

So I’m sitting here, staring at a pile of lumber. I made a stack of it a couple months ago before I got pulled off on another project. It now looks like fun house boards. Twisted and sprung. About as useful as a $3 bill. As I stand here looking down at my useless Lumber I ask myself one question. What causes warps in wood?

 

The basic answer is that wood warps because of uneven drying rates within the wood fibers. As my boards had sat there throughout the summer. The ones exposed to more air and heat near the outsides of the pile had dried in unexpected ways. This would caused the surfaces that dried quicker to shrink at a greater rate than the surfaces that are protected.

I also noticed that some of the boards were checked but that’s a different article.

Types of Warping

  1. Bow – Wood warps along the length of the lumber bending the thinnest face in a curve
  2. Crook- Wood warps along the length of the lumber bending the thicker face in a curve
  3. Kink – Wood warps along the width of the lumber producing a pronounced kink in the straight face
  4. Cup – Wood warps along the length of the lumber with the two long edges cupping towards each other
  5. Twist/wind – Wood warps along the length of the lumber with either end twisting in opposing directions

Now of course the wood you are working with may have any and all of the above deformities. This makes warp correction or use of warped wood very difficult.

Factors that Affect Warping

Background:

A saying I heard a lot as a kid was, “I know its hot in hell…but it is a dry heat?” I didn’t understand what that meant. I had never lived in an area with high humidity. Hot? Yes. But dry. After traveling to Japan during their summer, the meaning became clear. It was a brutally hot 115º day. We were out for a walk and it started to rain. I was so excited that it was going to cool down and we would have some respite from the baking cobble stones. It stopped raining.

The cooler temp lasted for 6 minutes. Then the whole place became a sauna. I literally wondered if this was how I was going to die. Choking on steam in the Bikan district of Kurashiki. While that may sound like a meaningless aside it does play into the factors affecting warping.

Factors:

Below is a list of different factors that will affect wood warping. To be clear these are actually affecting the Equilibrium Moisture Content(EMC). EMC is when the moisture outside and inside a piece of wood are at steady state. More ambient moisture will cause the wood to swell. Lower ambient moisture will cause the wood grains to shrink. This acclimation process can months. The below factors also will affect how long EMC will take.

  1. Ambient Temperature – While temperature is not the cause of the moisture in the air, it does affect how quickly the wood will acclimate. A colder climate will slow the moisture exchange, while a warmer one will speed up the process.
  2. Wood Thickness – This can affect both how the wood warps and how quickly it acclimates.
  3. Wood Species – Certain species of tree produce more stable boards. I would consult the free resource produced by U.S. Department of Agriculture, the Wood Handbook.
  4. Grain Orientation – As we will talk about below, where the board was sawn from the log is one of the greatest factors in wood warping

How to prevent warping

  1. Inspection – Probably the easiest is to inspect the lumber upon purchase. This is not always applicable, but if you can choose which pieces you take, inspect each and only take straight grain, non-warped boards. The guys at Loews completely understand and will usually help you sort though the stack to find the best. You can also check the lumbers moisture content with a special tool but I consider this overkill.
  2. Storage – The next easiest step is to make sure your lumber is stacked correctly with adequate ventilation. This is accomplished with ‘stickers.’ Thin strips of wood that separate layers of lumber. Make your boards are sitting flush and there are no deviations between board sizes. This means making separate stacks for different lumber dimensions. Try to locate the wood on a flat dry surface which will not wick moisture.
  3. Wood grain – The most difficult is to control where a board is cut from a log. There are three main ways of cutting boards. Flat-sawn, quarter-sawn, and rift-sawn. I have created a diagram showing how each would be accomplished. Rift-sawn is the most stable but also the most wasteful. It follows the grain of the log always producing boards that are perpendicular to growth rings. Flat-sawn is the least wasteful but but the most unstable. Lumber is cut flat completely ignoring grain. Despite the instability, flat-sawn in the most common technique. The compromise between the two is quarter-sawn. It gets closer to the grain orientation making it more stable and also is much less wasteful.
Commercial Option:
  1. Drying – Wood that is kiln dried will be more stable because the manufactures have much more control over how fast the drying process takes. Surprisingly kiln drying is done more to prevent bug damage. It also allows for more resinous wood to set and become more stable. Even after drying the wood will need to acclimate to whatever environment it ends up in. Wood that quickly regains moisture is much more likely to warp.

How to fix warping

While it is not easy, it is possible to fix warped wood. The main idea is to add moisture back to the wood and then clamp it into shape. That way as the lumber dries it is forced to a specific shape.  Below we listed a few different forms.

  1. Basic – Spray down the lumber and let the wood absorb the water. Once it has done so, clamp the board into position and allow to dry. The warp should now be much less pronounced.
  2. Ironing – Take a wet towel or paper towel. Place it on the position of the warp. This may be a large area so I would do it in sections. Leave a hot iron on the wet towel allowing the board to steam. This will speed up the moisture uptake and relax the wood fibers quicker. Clamp the board in place and allow to dry. Repeat as necessary.
  3. Steam Machine – This is a specialty piece of equipment that economizes the ironing technique explained above. A tank of water is boiled creating steam. This is then fed to the lumber usually through a lattice or hole filled surface. The wood is usually clamped in place over this surface. This allow for maximum lumber surface area to be affected.
  4. Ironing Machine – This functions similarly to the steam machine except it only heats the wood. Usually a wet piece of lumber is placed on the heated metal surface which allows the wood fibers to relax.

The steaming technique is utilized when warping wood on purpose. We will talk more about it below.

How to use wood that is warped

OK say you went over the techniques above and decided that fixing that warped pile of wood was too much of a hassle. You don’t want to waste your time. How do you use your goofy looking wood without too much extra work.

A lot of this will depend on the project you are building and the type of warp that has occurred. Let me give you an example. I was building a workbench. I had a number of 2×4’s that were bowed and twisted. Because I was screwing them down on to a strut and then gluing them to straight boards on either side, I was able to ignore the warping. Glue, screws, and some heavy duty clamps can work wonders. I think I could have even done a crooked board as long as it wasn’t too extreme.

Now if your project doesn’t have the adjacent lumber to clamp to, or is more delicate, you may have to either fix the warping or discard the piece. A method I have used it to cut away the worst of the warp and then find uses for the salvageable pieces. You can also just keep the board for another project. Something will come up that can use the lumber. At least it always does in my shop.

How to warp wood on purpose

Now you may be asking yourself, is there any way to get this warping thing to work for me instead of against me. There is. Woodworkers have being doing so for a long time. This technique can be found in everything from guitar making to boat building. The idea is that you steam the wood and slowly bend it into the desired shape. Depending on the application it may be more delicate(Guitar body) and so you will steam the boards first and then tie them to your form. Others, with usually thicker boards, may steam and ratchet the board down. This allow the fibers to slowly stretch into the desired shape.

 

 

 

Both of these methods usually require some specialized jigs. If you are looking to try this, I would suggest assessing the length of wood you want to bend and then google how to bend a board of that size. There are a lot of very brilliant solutions out there for you to emulate. I have included pictures of some of my favorites in this article.

 

Conclusion

So after all my research I saw was what I could and made my work table. There may be a few Wows in the tabletop but I’m pretty proud of the fact that I didn’t have to get rid of too many boards.

Wood warping is an issue that all Woodworkers have and will face. With a few small adjustments to our storage methods as well as some ingenuity when facing crooked Lumber, I think you’ll find that this type of issue and solution will help you become a better woodworker.

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