Why Does Wood Warp? (And How Can I Fix It?)

Nothing can ruin a wood project like warped wood. It can affect both function and appearance in a woodworking project. And then, it feels like you wasted time and money on a project that doesn’t do what you intended it to do, or look how you intended it to look.

But it doesn’t have to be that way.

Here are some helpful tips to help make sure your projects don’t succumb to wood warping. And even if they do, we’ve got your back with some simple fixes and some preventative tips to make sure it doesn’t keep happening.

We’ll look at:

• The causes of wood warping
• How to fix wood warping
• How to prevent wood warping
• How wood moisture meters can help

Let’s start with a basic understanding of why and how wood warps in the first place.

What Causes Wood Warping

Wood is hygroscopic, meaning the wood cells can either absorb or release moisture to match the amount of humidity in its surrounding environment. Think of it like a sponge—when a sponge is wet, it expands, and when it’s dry, it shrinks.

After lumber is cut, it naturally starts to acclimate to its environment. In doing so, the wood “moves” when it releases and/or absorbs moisture until it reaches equilibrium moisture content, or EMC.

So it’s a natural process for wood to expand and shrink, though it doesn’t always happen evenly across a single piece of wood.

But some level of wood movement is inevitable. When the moisture changes throughout the wood are uneven, however, warping occurs.

Exposure to sunlight can cause wood to warp for a similar reason. The heat from the sun can lead to uneven moisture loss. One area of the wood can expand or contract differently than another.

Wood grain and growth rings direction compared to the wood cut also matters with warping. The direction of the annual growth rings of a piece of wood affect how it shrinks and expands (more on that below).

Drying wood improperly can also lead to warping. When wood is dried too quickly, it can lead to internal stresses within the wood.

But even properly dried wood can warp sometimes. So it helps to know which factors can lead to several different types of warping.

Types of Warping

Wood moisture, improper drying, or exposure to sunlight can all contribute to warping, like in these hypothetical situations:

• Bowing—If damp wood is left in the sun, the quick heat could curve the board lengthwise along its face, making the ends of the board turn upward, like a ski or bow.
• Crooking—If wood is left standing upright without proper support as it acclimates, it can create a lengthwise curve along the edge of the board, making it look similar to a samurai sword or banana.
• Cupping—If wood is left on a concrete slab, the top of the wood that’s exposed to circulating air will dry differently than the bottom, which will absorb moisture from the slab below and swell, creating curves along the width, making a shape like the brim of a bowl or cup.
• Twisting—Wood that is wet in the middle but dry on the edges will twist as it acclimates to its environment, creating a rope-like shape like a Twizzler.

Straight-grain wood tends to be strong and durable because the wood grains consistently run parallel, or straight down, the vertical axis of the tree it came from. Straight wood that’s been dried properly to its EMC should stay straight unless pressure is applied to it. Ash and hard maple are known for having straight wood.

How logs are cut affects the angle of its growth rings and, therefore, its stability. Which can then affect how likely it is to warp.

Quartersawn wood tends to be more durable against some kinds of warping because of its perpendicular grain. Its grain is more stable and contracts less with sudden moisture changes. Types of wood like oak, maple and cherry benefit the most from quartersawn cuts.

Flatsawn wood, however, is more angled, so it is more susceptible to wood shrinkage and warping. Its grain pattern can shrink and swell more significantly with moisture than quartersawn wood. Types of wood like cedar and redwood are good options for flatsawn cuts.

What this comes down to is that sometimes wood will warp during the drying process (even if it’s dried properly) because of the structure or grain of the wood. Wood with uneven grains or structure is more likely to warp when it dries and may not be able to be fixed.

Warped wood that’s the most likely able to be fixed is straight wood that’s been dried properly. So starting a project with straight wood that’s been properly dried to its EMC is your best bet.

But when wood does warp, whether during the drying process or because of environmental conditions, what can be done about it?

How to Fix Wood Warping

Keeping in mind that uneven grains or structure can determine whether warped wood can be fixed, a safe practice when trying to fix warped wood is to wet and then flatten the wood so that it dries evenly.

For minor cupping, this could be as simple as pointing a fan toward the warped part of the wood or setting it on a dry surface in the sun to dry more evenly (though you should keep an eye on it so it doesn’t overdry and warp in another direction).

If your wood is thin, isn’t severely warped, or is recently warped, here are a couple other methods to try.

Moisten and steam

• Dampen the concave surface of the board with a wet cloth.
• Move a hot iron across the wood to steam away the cupping, but be careful that the wood doesn’t warp in the opposite direction because of too much heat.
• Clamp the wood down flat and let it dry thoroughly.
• This method works best with minor warping on thin pieces of wood.

Moisten and weigh down

• Wet the concave side of the wood.
• Place weights on the concave side of the wood.
• Let the wood sit and dry for 24 or more hours, making sure to check its progress until the wood dries.

With thin wood, even with the above methods, the wood could go back to its warped shape after a while.

More severe twisting or bowing could mean the warp is irreversible. In this case, the wood is beyond repair and needs to be replaced. Lay the wood on a flat surface and use something with a straight edge (like a rule or level) to assess the extent of the warp. For example, wood that’s warped in multiple directions, like in an “S” shape, is probably too warped to fix.

Even more important than how to fix wood warping is how to prevent it in the first place.

How to Prevent Wood Warping

Your best bet for a woodworking project to go smoothly is to choose wood that is straight before starting a project. But otherwise, you can save money, time, and stress levels by using preventative measures to avoid wood warping. With proper attention and care, the process can be pretty straightforward.

Here are some simple tips.

1. Give the wood time to acclimate to the EMC. Let wood sit for several days in the room where it will be used, allowing it to acclimate to the EMC of the environment.
2. Keep wood in a consistent environment. Keep the area you’re storing wood as temperature-controlled as possible, so that the temperature and relative humidity levels don’t fluctuate.
3. Seal the wood. Use an end coating on each end of the wood to prevent more rapid release of moisture on the ends.
4. Store wood properly. Use a dry, well-ventilated area to store wood. Stack wood evenly and use stickers between each board to ensure even airflow along the entire board.
5. Use a wood moisture meter to monitor moisture. Monitor the moisture content (MC) of your wood, keeping track of its movement.

Let’s talk more about the importance of moisture meters.

How Moisture Meters Can Help with Prevention

Using a quality moisture meter to keep track of the MC of your wood is the most convenient way to measure MC and prevent wood from warping.

With readings from a reliable moisture meter, you can match the wood’s MC to its final environment more easily and accurately, making sure that it has indeed reached EMC.

With a simple tool like a wood moisture meter, it’ll only take seconds to detect one of the major causes of warping in wood—moisture. With a wood moisture meter, you can make sure the wood is dry and acclimated properly before you start working with it.

And monitoring moisture in wood starts with having the right tools. A reliable and accurate wood moisture meter, like Bessemeter’s pinless DS500, will provide moisture readings for both hardwood and softwood.

This small investment in an accurate tool, along with putting in the appropriate prep and precautions ahead of time, will save you loads of money and heartache in the end.