Wood is one of the most versatile building materials to exist today, but that doesn’t mean it’s without its fair share of drawbacks. Wood warping is one of the most troublesome challenges of working with wood, especially since it can develop through no fault of your own.
I know that when I receive lumber or other building materials that are warped, it can quickly sour my mood and ruin my plans for the day. If you work with clients or customers, it can disappoint them, too – something you want to avoid at all costs!
In this article, I’ll dive into everything you could ever need to know about wood warping, including how to prevent it, which woods are most and least prone to it, what causes it, and what you can do to fix it.
Contents (Jump to Topic)
What Causes Wood Warping?
When wood warps, it’s due to sudden, uneven changes in moisture throughout the wood. Have you ever spilled water on a book or a piece of paper? The book might look fine at first, but as it dries, the pages become wrinkled and uneven, and they no longer lay together as they’re supposed to.
The same thing happens with wood but on a much larger scale. When wood gets wet, it absorbs water and expands. When it dries, it shrinks back down. However, if different parts of the wood dry at different rates, this can stress other areas of the wood and permanently change its shape.
Usually, pieces of wood and lumber are much larger than paper slices. If a 2×4 becomes saturated, for example, the outside of the board may dry much faster than the inside, resulting in warping.
Equilibrium Moisture Content
How much a piece of wood shrinks as it dries is all based on equilibrium moisture content. When the EMC is stable, it means that the piece of wood in question is neither losing nor gaining moisture.
EMC has no single value – instead, it’s determined relative to the moisture in the air around the wood in question. EMC is not necessarily equivalent to the level of humidity in the air, though. For example, in an environment with zero percent humidity, a board will eventually reach an EMC of zero. However, in an environment with 100% humidity, a board might only acquire a maximum EMC of 30% or so.
You can roughly determine the EMC of a piece of wood with formulas.
Factors that Dictate Moisture Content
Even with formulas, the moisture content of a piece of wood isn’t easy to find. For this reason, I prefer to invest in a wood moisture meter instead of trying to calculate it all! Besides humidity, some factors that can throw a wrench in your calculations are:
- Wood species
- Wood size
- Grain direction
- Protective coatings
- Air temperature
All of the above affect how fast a piece of lumber absorbs or releases moisture. For example, just as a single sheet of paper will dry much faster than a book, a thinner, smaller piece of lumber will reach EMC much faster than a large log. Logically, any wood surface that’s been coated with a hydrophobic substance will also acquire EMC much more slowly (if at all).
Grain direction plays a role, too. If you’ve ever visited an outdoor lumber yard, you may have noticed that the ends of some boards felt wet while the middle sections didn’t. This is because surfaces perpendicular to the wood grain absorb moisture much more quickly than surfaces cut parallel to the grain.
And finally, consider how air temperature works, as well. As temperatures get lower, the air can accommodate less and less humidity, so EMC also changes much more slowly. Conversely, in hot conditions, EMC exchanges will happen faster.
How to Prevent Wood from Warping
Essentially, to prevent wood from warping, you have to protect it from its enemy: moisture. However, more than just that, you need to protect it from sudden changes in humidity.
For example, Kiln-dried lumber has been dried carefully over a short period in highly controlled conditions. A kiln can dry lumber much faster (and more safely) than air drying can, but the final moisture content may be lower than the environment outside the kiln.
However, not everyone has access to a kiln for drying purposes. Fortunately, there are other ways to protect your wood from warping, too.
Selecting the Right Wood for the Conditions
Before you do anything else, it’s a good idea to take some time to research what types of wood are available to you. By selecting a wood species or variety that’s more compatible with your environment or your project, you can save yourself a lot of hassle down the line.
Take cedarwood, for example. Cedar is naturally resistant to warping because of the plentiful oils and acids in the wood. These oils show the exchange of moisture with the air. Redwood and fir wood also contain similar chemicals that help protect against warping.
Keep in mind that wood grain can affect lumber’s propensity to warp in other ways, too. The closer together the wood grain is on a given piece of wood, the less moisture can penetrate it. The way the wood was cut from the tree affects this, too.
Typically, when a log is cut into boards, it’s cut in one of three styles: rift-sawn (the least common), quarter-sawn, and flat-sawn (the most common). Flat-sawn wood is cheap and easy to do but results in low-stability lumber that’s more prone to warping. Rift-sawn wood results in very high-quality boards, but a lot of waste, so it’s seldom done. Quarter-sawn boards are an excellent compromise between the two because there’s little waste, but the boards are more stable than flat-sawn boards.
Finally, consider pre-treated woods, such as kiln-dried woods or pressure-treated woods. I said a little bit about this above – kiln-dried woods go through a highly-controlled drying process, so they tend to have very uniform moisture levels when they emerge from the kiln. As the name suggests, pressure-treated wood is exposed to high pressures where different chemicals are forced into the wood. These chemicals can make the wood more resistant to moisture, insects, and rot.
Controlling Humidity During Storage
If you need some way to store your wood for any amount of time, you’ll need to create a stable area to protect it from fluctuations in moisture. If you live in an area with very high or low moisture content in the air, this storage space can also be used as a controlled “acclimation” area before using the wood for building.
However, the storage building itself isn’t the most important thing; the way you store your lumber plays a role, too. For example, putting a heavy weight on top of your lumber stacks can help prevent warping by keeping the boards flat, and a solid, level foundation will do the same thing. You can use “stickers,” or thin strips of wood, between boards or sheets to give them better ventilation, too.
Lumber and boards should always be stacked with pieces of the same type, length, and thickness when possible.
Acclimating Before Installation
You should always acclimate your wood to get it as close to EMC with your environment as possible before installing it. This will give you the best chance possible of preventing warping down the line.
If you’re building something that will be shipped to another area, or you’ll be transporting your wood somewhere else, consider treating your wood with a stain, paint, epoxy, or another moisture-resistant coating. This is especially important if your project will live outdoors, where it’ll be exposed to fluctuating moisture levels.
How to Fix Warped Wood
If you do end up with a batch of warped wood, all is not lost. While it requires a bit of work to get the wood back into its proper shape, it’s not impossible. Since moisture softens wood fibers and allows them to bend, you can repeat the same warping process in a controlled way to undo the warp.
Consider a book that’s been drenched, for example. If you want, you can let a wet book air dry, but the pages will end up wrinkled and ugly. However, if you treat the book right while it dries – in this case, by weighing the pages down with weights or ironing them to make them resume a flat shape – you can (more or less) undo the damage.
If you want to fix warped wood, your strongest allies are heat and moisture. Heat speeds up the moisture absorption of wood. As you might expect, an iron, which uses both heat and moisture, is a useful tool for fixing warped wood.
Specialized manufacturers even use heat and moisture to bend wood into specific shapes, such as for curved decks or musical instruments. Think of an acoustic guitar, for example. The guitar’s outer walls are made from long, thin pieces of wood that are first bent into the proper shape.
Besides using an iron, some of the other methods you can use for de-warping wood are:
- Sunlight: Wrap your wood in damp towels, then leave it to sit in the sun for two to four days. The sun’s warmth will help loosen the fibers of the wood and help it take on its new shape. You’ll have to moisten it repeatedly throughout the process.
- Pressure: Wrap the wood in wet towels, then use clamps or weights to return it to its intended shape while it’s flexible. Then, allow it to sit for several days under pressure as it takes on the new shape.
- Heat: use an iron, a heat gun, or a similar tool to apply heat to the wood until it’s pliable. Then, bend it back to its original shape using clamps or pressure. Be careful not to damage or burn the wood.
Pressure is widely regarded as the most effective wood warping solution, though it works best (and fastest) when used together with heat and moisture. If you don’t have time to un-warp your lumber, you can always plane it or cut it down to a new shape, too. However, keep in mind that this method involves removing (and wasting) good lumber.
Wood warping is an unfortunate nuisance that can sneak up on even the most experienced woodworker. If you work with wood, you’ll run into the issue yourself someday, without a doubt. However, by taking the right steps, you can protect yourself, your wood, and your construction projects from this nuisance. Trust me – it’s a lot easier to take a few steps to prevent warping in the first place than it is to try and un-warp a large batch of lumber!
What is your experience with wood warping? Do you know of any effective and easy methods to fix warped wood? If so, feel free to leave some advice for others below.