I’ve often come across the issue of cupping or bowing in wood, especially when drying lumber. It’s a common concern that makes many hesitant to dry or expose their lumber to the elements, fearing warping.
But I’m here to tell you that it’s entirely possible to maintain the perfect form of your lumber. As a professional woodworker, I’m excited to share my insights on how to prevent wood from warping, understand its causes, and more. Trust me, with the right knowledge and techniques, keeping your wood straight and true is more than achievable.
Why Does Warping Occur in Wood?
Wood warps because of the uneven moisture content changes in its different parts. The moisture movement in the wood is unpredictable or almost impossible to trace. But by understanding how moisture content works in the wood, you can prevent warped wood.
Wood warping can be monitored by examining the wood cut from a tree. If you see fresh-cut green lumber, it is loaded with water. Its moisture levels can vary from 30% to 250%.
There are two types of water in fresh timber – free water and bound water. You need to remove the latter forcibly from the wood, while you can extract free water quickly.
The drying of wood should reach the right FSP or Fiber Saturation Point. Otherwise, the wood will exhaust all the water content and will begin to shrink.
While the wood is shrinking, there are three possible directions where the wood fibers bend. It could be longitudinal or movement along the grain, radial or perpendicular to the growth rings, or tangential or perpendicular movement to the grain but parallel to the growth rings.
Many reasons can cause the shrinking and swelling of the wood, but the primary reason is because of the moisture content and humidity of the surrounding air.
I’ve learned that two key factors significantly impact the moisture content of wood, or what we call the Equilibrium Moisture Content (EMC). This is essentially the point where the moisture in the air equals the moisture level in the wood.
But that’s not all. I’ve also noticed that other elements contribute to wood deformation. These include the type of coating used, the size of the wood, and even the grain pattern. Understanding these aspects has been crucial in my woodworking projects to ensure quality and durability.
Why Does Wood Bend?
One common reason why wood warps or bends is unbalanced EMC. Imbalance in EMC causes the wood to bend because if the air is too dry, the wood absorbs moisture resulting in its swelling. Moreover, more water on the wood will release moisture into the air, making the wood shrink.
There are many reasons why wood bends, but the ambient air temperature influences the drying process of your wood. Therefore, you should monitor moisture content or the temperature of your wood while drying to attain the correct EMC and FSP of the wood.
Otherwise, your wood will begin to bow, kink, cup, crook, warp, or twist. The deformation of wood will result in your lumber’s density to change in inches.
Types of Wood Warping
To help you steer clear of warped wood and spot potential deformations while it’s in storage, I want to share the types of warping you should keep an eye on. From my experience, being aware of these can make a significant difference in maintaining the quality of your wood.
Bowing wood is a form in which the lumber diverges from flatness lengthwise but not across the face. Bowing could happen to your wood if you leave the ends or edges of the wood unchecked.
Our experts regularly perform end-checking to see if there are evident cracks during wood drying. Wood tends to crack because the outer surface layer of the wood has no moisture, but the inner layers still have more moisture content.
Wood kinks are wood upturned formations near the edge of the wood. It is permanent wood warping that occurs due to improper handling, uneven drying, and poor storage conditions.
If you have twisted wood, the four corners of your timber are no longer in their original state, or the shrinkage is in radial and tangential directions. Softwoods are more likely to twist, but it can also be due to the drying process.
Luckily, twisted wood is reversible depending on the number of twists on your wood. You can reverse it by applying pressure to the concave face of the knotted wood. After the pressure is released, your wood will return to its original shape.
However, this process should be done more carefully because a single mistake can fracture the wood.
When you have cupped wood, there will be a noticeable curve or series of bends in the middle of your wood. Cupping occurs once the outer layer of the board expands faster than the inner layers.
It is common in heartwood which is more challenging to control than sapwood.
Crook wood is a deformation when the fibers on the edge move, making the other end shorter. This is reversible and can be straightened by applying the correct pressure to the concave face.
Warping Prevention in Lumber
From my experience, these types of wood warping can happen at any time, particularly if the wood is exposed or placed outdoors. If you’re looking to learn how to keep your wood from warping, I’m here to share some approaches that have helped me keep my wood in tiptop shape.
Kiln drying or oven drying is better than traditional air drying. However, this process takes longer and makes your wood prone to damaging factors. However, you can properly dry your wood and maintain its EMC to prevent wood warping.
This drying process also allows kiln-dried wood to release moisture and absorb air evenly, giving you control and proper handling of wood drying.
Store the Wood Properly (In a Cool and Dry Place, Laying Flat)
To avoid wood warping, you need proper storage techniques. Otherwise, you leave your wood prone to deformation. After you successfully dry your wood, do the following procedures for proper storage.
Step #1: Store wood horizontally on its edges on a flat surface. Make sure you stack wood with the same size and length.
This storage formation allows your wood to spread its weight evenly and ensure that no wood carries too much weight. Thus, you must separate the lumber accordingly to its thickness, length, and size.
Step #2: Create proper ventilation in your storage facility and in between the wood while stacking it. When there is a lack of air, the piles of wood absorb moisture in the surroundings and affect the wood’s moisture content, resulting in swelling or warping.
Step #3: If you have no storage facility, your only choice is to stack your wood in your backyard or a vacant lot. So in these circumstances, you need to protect your wood from vapor passing through.
You can use tarp or coverings to prevent the water in the air from being absorbed by your wood. Excess absorption of moisture can make your wood warp.
Step #4: You can also counter the bending of your wood by placing a heavy weight on the pile of your wood. If you do this, ensure that the weight is spread evenly across your wood to keep other parts of the wood from bending.
Step #5: Make sure the storage facility or the venue where you keep your wood is clean and dry. The area must be free from dirt, broken branches, and fallen leaves. The temperature of the room should be cool but not too cold and also not too hot.
Monitor the Moisture Levels
I always make it a point to check the moisture of my wood regularly with a calibrated prong-type moisture meter. Keeping a record of these moisture levels is a key practice for me to ensure that my wood stays in good condition. Let me share with you some basic guidelines on how to accurately take the moisture levels of your wood.
Step #1: Check if your moisture meter is working correctly by testing the sensory element.
Step #2: Take two relative humidity readings and get their average value. The result of your readings should be accurate.
Step #3: To maintain the correct moisture levels of your wood, seal the ends so the wood won’t dry up quickly.
Follow this guide in monitoring the moisture content of the wood and relative humidity levels outdoors:
|19 – 25%
|26 – 32%
|33 – 39%
|40 – 46%
|47 – 52%
|23 – 30%
For a better moisture reading, you can install hygrometers in your storage area to get accurate humidity results in the storage area.
It is also essential to consider acclimating wood to match the surroundings before construction. This technique will stabilize your wood before utilizing it, decreasing the chances of your warped wood.
Consider Wood Grain and Sawing Techniques
Grain pattern plays a significant role to keep wood from warping. It contributes to the behavior of your wood because if the wood is cut from the heart of the tree, it is more stable.
The straight grain with 0.79 mm or 1/32 inch between the grain lines tends to warp regardless of the wood species you use.
The sawing technique can also influence wood deformation if the method are incorrect. A quarter-sawn board is less likely to warp because the growth rings are symmetrical. However, the flat-sawn board may result in wood crooks because the edges are equidistant from the core.
Use Stable Wood Types
Choosing the right type of wood is crucial to minimize deformation. I’ve found that woods like redwood and cedar are less prone to warping. Also, opting for plywood can be a wise decision for stable wood. The alternating layers in plywood really help stabilize the material, something I’ve observed time and again in my projects.
Sealing Your Wood
If the ends of the wood are not sealed, it will absorb moisture and release air. Thus, drying and sealing wood prevent warping and protects the wood surface from harsh elements. Waterproofing materials seal the wood, prevent warping, and keep the wood in good condition.
Cure the Wood Properly (Avoid Fast Drying & Excess Drying)
Proper curing affects the condition of your wood, which can also prevent the wood from warping. Before you dry your wood, treat it so the drying won’t take too long or too slow. Also, I recommend using a kiln  or oven to make your wood dry.
Wood Types That Do Not Warp
Redwood does not warp because of its straight grain pattern and natural chemicals called tannins. This redwood composition helps it resist rotting, so it is ideal to use as a building material for decks, patios, and outdoor furniture.
Western red cedar is a soft hardwood resistant to insects, but this wood is known to be sturdy in adverse weather conditions. It is a solid wood that contains natural chemicals protecting its wood fiber.
It is more expensive than redwood, but you will get your money’s worth because this wood does not warp.
Douglas fir is a seasoned wood, making it less likely to warp. However, you need to balance out the moisture absorption of this wood, so kiln drying is recommended for this wood.
Changes in Moisture Content: Why this Happens and How it Affects Wood Warping
To minimize the distortion of your wood, you must understand that wood tends to warp because it loses too much moisture. The standard moisture content of wood should be 9 – 20%.
If EMC is not balanced, the wood won’t remain flat; hence both edges and the surface of the wood warp.
Wood will be damaged if it absorbs moisture above 20% or relatively high moisture content. If the relative humidity is more than 80%, the wood will start accumulating mold.
In my experience, if the moisture content in wood falls below 9%, there’s a high chance it will shrink or warp. To accurately measure and achieve the ideal moisture level, which is around 19%, I always recommend using a kiln drying machine. This tool has been a game changer in my woodworking projects, ensuring the wood stays in perfect shape for use.
How Does Ambient Temperature Cause Lumber to Warp?
The ambient temperature does not directly affect the wood to warp, but it is one of the factors for the relative humidity in the area.
If there’s high environmental humidity, the moisture absorption in the wood increases, making it expand. While if the environmental humidity decreases, the wood shrinks.
How Does Wood Thickness Affects Wood Warping?
Wood thickness affects the deformation of wood because the thicker the wood, the higher the moisture it has. How the wood is harvested, treated, and cut gives us an idea of how it affects the wood to shrink or warp.
Several woodcuts are dependent on their use or purpose. For example, wood with a thickness of 2 inches is used as a board. It is considered dimensions if the wood is cut between 2 to 5 inches, and timbers are usually cut into 5 inches or higher.
How Wood Grain Orientation Drives Wood to Warp
Wood grain pattern orientation drives wood to warp because of how the grain pattern is cut. As mentioned earlier, sawing techniques include flat and quarter. A quarter is less likely to shrink or expand than a flat sawing technique.
Quarter sawed lumber is cut vertically, so the annual rings should have 45 to 90 degrees to the board’s face. On the other hand, flat sawn timber consists of planks with 0-45 degrees annual rings to the board’s face
How Protective Layers Cause Wood to Warp
Protective coating prevent warping in wood, as the layers seal its surfaces. This prevents it from absorbing and releasing moisture content. When you apply protective coats of paint, wood stain, or wood sealer, follow the manufacturer’s steps and instructions.
Keep Your Wood from Warping With These Tips
Steps to Fixing Warped Wood
Now that you know how to keep wood from warping, you can now select the best drying process, storage, sealers, and wooden material for next project. This way, deformation won’t be your concern.
Most importantly, maintaining the moisture content of your wood will prevent wood warping and shrinking before or after your construction.
Robert Johnson is a passionate furniture maker & carpenter, sought after for his knowledge on the craft.
You’ve probably seen his down-to-earth wisdom in USA Today, Bobvila, Family Handyman, and The Spruce, where he has shared commentary and guidance on various woodworking topics.
Robert is the brain behind Sawinery, where he aims to share tips, tricks, and a passion for all things carpentry.