What Causes Wood to Warp and How to Fix it?

Have you ever bought 2x4s to work on your DIY project and then realized that the boards are twisted and warped? Especially when you have made all the cuts and tried to assemble the project, the pieces don’t line up. Or sometimes, a few days later after the project is complete, you notice that your project is deformed because the wood is now twisted. So what causes wood to warp or twist? After some research, I found an explanation. 

Regardless of the type or specie of wood, all boards are prone to warping or twisting, but some types are much more stable and far less likely to warp than others. The main cause of wood warping is when the moisture in the wood dries quicker on one side of the board than the other, causing the fibers to contract at different rates, which shifts the overall lay of the fibers. Now your 2×4 will no longer lie flat on the ground because one side has shrunk faster than the other side of your board. 

However, you can avoid warped lumber fairly easily by carefully selecting the best boards from a lumber yard. Then, it’s important to store and dry the boards properly in the right conditions. If the lumber still warps, there are a few different ways to fix the problem.  

What Exactly Is Warped Wood?

Warped wood is wood that has been deformed when the moisture level changes in the wood unevenly. As the moisture leaves a piece of wood, the fibers of lumber will contract slightly in different parts of the lumber, causing it to deform.

The main reason behind a warped piece of wood is the moisture content and the level of humidity in the air around the wood. Wood will shrink as it dries until it reaches EMC (equilibrium moisture content or the proper level of moisture). Or, oddly enough, it will soak up the moisture in the air until it reaches EMC. This process can take anywhere from days to months, depending on the environment and type of wood. Wood can take up and release moisture indefinitely.

Warped wood does not mean damaged beyond repair or unusable by any means. It may take a little extra time to fix, but there is no need to waste your money and throw away wood that could be fixed.

What Causes Wood to Warp?

There are many factors that can contribute to wood warping such as wood species, grain orientation, sunlight, uneven finishing, airflow, and temperature. 

While some types of wood have a relatively high moisture content, other types have very low moisture content. The expanding then shrinking of wood is caused by both the wood’s moisture content and the humidity level in the air. As humidity increases, the moisture content of wood will also increase, and the wood expands. As humidity decreases, the moisture content will also decrease, and the wood will shrink. 

Thicker wood takes longer to both absorb and lose moisture. Quartersawn lumber will shrink and then expand about half as much as lumber that is flat-sawn.

How to Prevent Wood Warping

Here are a few tips on how to store your wood the right way to prevent any of the wood from warping. Even if the wood was improperly stored where you purchased it, if you store it the right way at home, it will be just fine.

Green lumber should be stacked on stickers as soon as possible after sawing. Stickers are narrow strips of wood, usually around 1”x1”, laid between the layers of lumber to allow proper airflow and to help prevent rot. This improves air circulation between the boards, speeds up drying, and prevents warping. Applying an end sealer to the boards will reduce splitting.

Properly stacked lumber dries quickly in warm, dry weather. Softwoods like pine and cedar will air dry to a suitable moisture content in as little as 6 weeks during the spring and summer months. Hardwoods like Maple, Oak, and Walnut will dry in 6 -12 months. Little or no air drying takes place in wet, damp weather.  

Boards should be placed on stickers that must be uniform in thickness and as long as pieces of lumber.

• The stickers (wood layer) must lay flat and be vertically aligned.
• The lumber must rest flat on the foundation, concrete, not grass.
• Heavy weights must be placed on top of the pile to help prevent warping.
• The lumber needs to have plenty of ventilation.
• Store in a cool, dry, and shaded area.

Following these few storing techniques will prevent warping and will save you a lot of time needed to try and fix the warped wood later on.

Source – Delmhorst Insrument Co.

Which Way Does Wood Tend To Warp?

The types of wood warping include:

• Bow – When your wood is bowed, it means the wood deviates from the flatness lengthwise.
• Crook or Vain – This is when there is a warp along the length of the edge of the wood.
• Twist – This type of warp is where the four corners of any face of a board are no longer in the same place.
• Kink – A smaller area of the board has a crook in it, typically due to a knot.
• Cup – When your wood deviates from a straight line across the width of the wood. The edges are either higher or lower than the center of the wood. The edges curl up.

Source – Home Depot

Wood Least Likely to Warp

Although any species of wood could warp, some types of wood are much more stable and far less likely to warp than others. Wood with a close grain has less of a tendency to warp than open grain. The closer the grain, the less space between the wood fibers, reducing the possibility of shrinking.

Hardwoods generally have a closer grain than softwoods, and the densest hardwood species grow in the tropics. Many tropical species also contain natural oils that remain in the grain after the wood has dried. This prevents shrinkage. Because it is so hard, wood from these species is difficult to mill and work. Most of the species have been over-harvested, so the wood is rare and expensive.

The tendency for a board to warp depends on the species of wood and how the board was cut from the tree. The most stable board is cut from a hardwood tree with dense grain in a way to minimize shrinkage rates across the grain. Wide boards, even dense hardwood ones, are prone to warping if they come from the outside of the tree. On the other hand, even a softwood board may resist warping if it is primarily hardwood. Because they have a close, symmetrical grain, quarter-sawn hardwood boards are usually the most stable.

• Fir – This wood is very stable when it reaches the right moisture content or EMC with the moisture in the surrounding air. After the wood reaches EMC or is seasoned, fir will do very little shrinking or warping.
• Cedar – is one of the densest wood species, which really helps prevent cracking due to changes in moisture.
• Redwood – is known to have a very straight wood grain pattern but also has a natural chemical inside that protects the wood against moisture.
• Pine – It is more stable than cherry and maple.

Any of these woods would be great for your DIY projects and are typically easy to work with and easy to fix if they do warp.

How to Pick Lumber That Won’t Warp?

You can avoid warped lumber fairly easily by careful selection. Here are the basic things to look for when selecting lumber (roughly in order of importance):

• Stick with #2 grade or better when labeled kiln dried.
• Obviously, ignore any boards that are already showing signs of warping or are obviously wet in the lumber yard. It can be difficult to tell, so get yourself a moisture meter and take it to the yard with you.
• As you pull each board off the pile, put one end on the ground and sight down it to see just how straight it is. If it already has some sort of curve to it, most likely, it just going to continue warping.
• Start by looking at the ends of the boards on the stack and look for boards that do not have the center rings (sometimes called the pith) of the tree running through them. The first few rings are typically juvenile wood which is usually less dense and more prone to cause warping, as well as splits and checks.
• Select boards that have narrower growth rings (this usually automatically eliminates the pith).
• Give preference to boards where the rings run across the narrow dimension and do so consistently from end to end. This quarter-sawn or rift saw ring orientation is more stable. Plane-sawn boards are okay as long as they don’t show pith or significant diagonal grain.
• Select boards where the grain runs as straight as possible and avoid the ones where the grain runs diagonally from end to end.
• When possible, buy wider and longer boards than the dimensions you need. For example, if you want 2×4s, buy 2×8s or 2×10s and rip them to width. This gives you a chance to cut pith or juvenile wood and knots out of the board. They generally have to use better quality logs to start with to get longer and wider lumber. With the wider width, there is more likelihood of having pith, but you can cut it out, and by doing so, wind up with a board that is more like #1 grade than #2, and you often end up with quarter or rift sawed grain.

Can You Fix Warped Wood?

There are many different ways to fix warped wood. Here are some of the most common ways used by the woodworker.

• Applying Heat – Clamp the board to a flat surface such as a workbench or to the floor. Apply heat and pressure on the affected area. The surface needs to be very hot, but do not overheat. Otherwise, it will damage the wood. Slowly bend the wood in the opposite direction of the warp and wait for it to cool down. 
• Using an Iron – Wrap the affected area in wet towels. The towels should be large enough to cover the board and durable to take the heat of an iron. Place the wrapped board on a flat surface, such as a workbench. Take a steam iron and heat it to the highest point. Once the iron is hot, press on the warped surface and run it through the entire board, applying even pressure. In the most affected area, you might need to hold the iron for 5 to 10 seconds before moving it to the next area. This method might take a few tries before the problem is fixed. Remove the towels and have the wood dry on a flat surface. 
• Using Sunlight – sunlight is another great source of heat to fix warped wood. Similar to the previous method, take moist towels and wrap the warped wood. The towels should be moist and not drenched. Place and clamp the wood on a flat surface in direct sunlight. This method works well during dry, warm weather. You may need to leave the board in the sun for a few days. During this period, spray the covered wood with additional water to keep it moist. 

Why Does Wood Warps or Bows When Ripped

Different types of wood react differently when cut. If a tree grows on a slope, it can build up stresses into its very fibers by virtue of fighting against gravity as it grows. This builds up ‘stress,’ which can be released when the wood is cut. This is a good argument for using riven wood since it allows the woodworker to work with the internal grain structure of the wood etc.

Another problem that can lead to this is case hardening. Case hardening describes lumber or timber that has been improperly kiln-dried. If dried too quickly, wood shrinks much at the surface, compressing its damp interior. This results in unrelieved stress. Case-hardened wood may warp considerably and dangerously when the stress is released by sawing.

Woods like walnut can have a lot of stress and can move – a lot! When cutting walnut, cut it larger than what you need and let it sit for a day or two before milling to size, especially from a thick piece.

There are two approaches to fixing this problem, depending on the need and the expected appearance.

• Overcut the piece by 1/2” (or more if it’s going to be a really long piece) and let it sit. Then re-rip it by cutting off the convex side first and then the concave side. In some cases – although not many, this has just made it curve more.
• Cut the piece into several thinner/narrower strips and glue them back together, clamped as needed. This is how the Glue-Lam beams are made but on a tinier scale.

Conclusion 

Even though there are many details to take into consideration with worrying about wood, in the end, as long as you take a little time to choose the right wood and store it properly, you should not have any problems with warped wood. If you do happen to find your wood is warping, at least now you will be able to take action and then get back to creating wonderful DIY projects.

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