Why is My Lumber Warping After Delivery?

You receive a beautiful load of lumber. But a few days later, you notice something odd. The wood that once seemed straight and even now appears warped and won’t lay flat.

What happened? Is it worth fixing? And how can you prevent it from happening again?

The most common culprit behind wood warping is improper moisture content management. It can happen before, during, or after lumber delivery.

If wood shrinks at different rates during the drying process, it’ll warp. That’s why one of the best ways to prevent it is to learn how moisture content affects wood and what you can control.

There’s no guaranteed fix for warped lumber. But understanding moisture content gives you the best chance for success.

Let’s walk through the basics of wood warping:

• Why wood warps
• How to prevent it
• What to do if your lumber warps
• How to choose stable lumber
• And which tool to arm yourself with

Let’s start with the root cause of warping.

Causes of wood warping

Warping occurs when uneven drying causes a piece of wood to shrink or expand at different rates.

Several factors make wood more likely to warp, including:

• An uneven drying process
• Acclimating wood with high moisture content
• Storage with exposure to moisture, sunlight, or uneven airflow
• Knots, cross-grain orientation, and flat sawn cutting techniques
• Wood species, like softwoods
• Large fluctuations in ambient humidity and temperature

No matter what factors influence warping, it usually happens during the drying process.

That’s why managing and monitoring moisture content is crucial to prevent wood warps.

Let’s talk more about what exactly wood warping is, and how moisture is involved.

What is wood warping?

Warping is any defect or deformity that causes wood to deviate from a flat plane.

The main types of warps include cups, bows, crooks, twists, and kinks.

It’s normal for wood to shrink as it dries (and to expand if it reabsorbs moisture). But uneven drying and large fluctuations in moisture content can distort the shape.

Professors at the University of Wisconsin-Madison, Department of Forestry state,

“When one edge or face or end of a piece of wood shrinks more than the opposite edge or face or end, the piece warps.”¹

Here’s a quick breakdown of the main types of warping and their causes:

Cups occur when one face of a board shrinks (or dries) faster along the width. The edges rise higher than the center, giving it a cup-like shape. This happens most in lumber that’s flat sawn, wide-cut, or milled from an unstable wood species.²

Bows happen when wood shrinks faster along the length of one face of the board. It’ll curve from end to end like an archer’s bow. You’ll usually see it in wood products with weaker, juvenile wood (like in Number 2 Common or below grade wood).³

Crooks occur when one edge of the board shrinks faster than the other, causing it to side bend. It usually results from using juvenile wood and certain sawing techniques.⁴

Twists happen when the ends of a board dry on two different plains. You’ll see it most in wood with cross grains (or wood fibers that run width-wise).

Kinks often result from a knot in the wood. The board will look straight along most of its length, but have a sharp left or right side bend near the end.

Warping is often obvious, but other times it can be subtle.

If in doubt, lay your board on a workbench or table top to see if all sides lay flat. And use a level to verify.

Next, let’s talk about why moisture plays a part in most types of warps.

How does moisture content affect wood warping?

Photo by Mark Stebnicki

Wood is hygroscopic. That means it absorbs and releases moisture from its environment like a sponge.

If the surrounding air is drier than the wood, the wood will release moisture and shrink. If the air is wetter, the wood will absorb moisture and expand.

Wood will continue exchanging moisture with the air until it reaches equilibrium moisture content (EMC).

But it’s not a once-and-done process. If the humidity and temperature around the wood change, it’ll try to reach EMC again.

And that’s crucial to keep in mind if you’re receiving a load of lumber. Especially if it’s coming from an area that’s drier or more humid than where you plan to store or install it.

It doesn’t take much for moisture changes to impact a project:

“In the case of sugar maple, for example, a change of 6 percent in moisture content may result in a dimensional change in a furniture part of about 2-1/2 percent. In general, the amount of shrinking and swelling which takes place is directly proportional to moisture content changes in the wood.”⁵

Let’s take a quick look at the science behind that.

Trees contain so much moisture when they’re first felled that the water weight can actually double the weight of the wood fiber. That’s right—green wood can have a moisture content (or MC) of 200%.

But before you can use the wood in a project, it needs to lose enough water to reach an MC of 6-19%. And that’s true of most softwoods and hardwoods.

The process of losing so much water leaves wood vulnerable to warping. That is, unless you use proper storage, drying, and monitoring methods.

Imagine a board sitting out in the sun as it dries. The top face of the board will get more sun exposure than the bottom. The top will dry faster, causing the board to cup.

Or think of lumber inside a warm and dry kiln, where it releases moisture and shrinks. If you store the wood outside in the humidity once it’s kiln-dried, it’ll reabsorb moisture as it tries to reach EMC again.

It’s normal for wood to shrink and expand by a few MC percentage points. Especially with seasonal changes. That’s why wood flooring installers let boards acclimate in homes before installation. They even install boards to allow for fluctuations.

But if you receive lumber with MC too far from the ideal range, it’ll go through extreme changes to reach EMC. And instead of keeping its original shape when it shrinks or swells, it might begin to warp.

A few other factors can influence EMC, such as wood size, wood coating, and grain orientation. So let’s look at those next.

How to prevent lumber from warping

Lumber yards and delivery drivers can take a few steps to avoid causing wood to warp by offloading it into a position or location where it’s likely to warp. And as a woodworker, you can use some of these same steps once you receive a load of lumber.

Tips for lumber yards, delivery drivers, and woodworkers

Let’s walk through four key tips:

1. Stack, dry, and store lumber using proper methods.

 

• Store lumber 12 inches off the ground to avoid water runoff.
• Stack it horizontally on a level foundation.
• If you store it outside, align the lumber east to west to reduce solar heat.
• Don’t stack wood of different sizes in the same layer.
• Protect lumber from exposure to rain and direct sunlight.
• Store wood for indoor use in 60 to 80 degrees Fahrenheit and 30 to 50% relative humidity.
• Keep at least a makeshift roof over wood intended for outdoor use.⁶

2. Dry wood following proper drying procedures and timelines.

 

Kiln drying can take a day to a few months. Some softwoods like Southern Yellow Pine can be dried in a day. And many other softwoods can take around three days.

 

And for air drying, it’s often said that a piece of lumber takes one year for every inch of thickness.⁷

 

Moisture meters are crucial to have on hand because they can tell you when the wood is truly dry enough to use.

 

For optimal drying, follow these best practices:⁸

• Separate boards with stickers (small pieces of wood) to allow for even airflow and ventilation.
• Place stickers no more than 24 to 28 inches apart (or even closer, for hardwoods like elm).⁹
• Place the boards themselves an inch apart for airflow.
• Don’t let partially dried lumber regain moisture quickly.
• Don’t dry lumber too slowly (or too quickly) to avoid checks and honeycombs.
• Don’t over-dry lumber, since it can warp when you try to equalize MC.

3. Monitor the MC in wood and the EMC of the storage environment.

 

Use a reliable moisture meter to check wood moisture levels at regular intervals. And use a hygrometer to check relative humidity and temperature in your workshop or storage area.

 

Adjust conditions as needed.

 

As you’d expect, high humidity and cool temperatures can cause wood to dry more slowly, while drier air and warmer temperatures can dry wood more quickly.

 

But make any adjustments to the environment in small increments to prevent big fluctuations.

 

4. Transport lumber with a vapor barrier to protect it from the elements.

 

Wrap wood with a waterproof barrier—especially if it’s shipped on green wood pallets or coming from a more wet environment, like the Pacific Northwest.

Inspect lumber deliveries

Let’s talk more about how woodworkers and flooring installers can avoid getting lumber that’s either warped or prone to warping.

1. Don’t unload lumber until you inspect it for early signs of warping. It’s acceptable to unload a few pieces just for inspection.

 

Hold the long end up to your eye with the other end on the floor, and look down the length on all four sides.

 

2. Inspect the MC in an average number of boards with an accurate wood moisture meter. The National Wood Flooring Association (NWFA) recommends checking 40 boards for every 1,000 square feet of wood flooring.¹⁰

 

Most softwoods for construction need to have an MC below 19%.

 

3. Check the end grains and faces. Spiral grain or highly curved growth rings are both prone to warping.¹¹ And avoid defects like knots—a common cause of kinks.

 

4. If you’re receiving lumber shipped by boat, ask to include a device that tracks temperature and humidity inside the shipping container.¹²

 

5. Ask whoever you’re buying lumber from how they dry and store their wood. You want to build a relationship with a lumber seller that provides quality wood and a fair return policy.

If a load of lumber doesn’t meet your quality standards, you don’t have to accept it. Replacing it with lumber less prone to warpage might be a better use of your time and money.

Inspecting lumber beforehand is helpful. But what if you’re reading this with a pile of warped boards in your backyard?

Let’s tackle that now.

What to do if your lumber warps

Wood has a mind of its own. You can do all the right things and sometimes still end up with warped wood. So how do you fix it?

Once wood warps, it’s extremely difficult to reverse. The best use of your time and money is often to contact the seller to ask for a refund or discount.

If that doesn’t work, you can try selling it to someone who seeks unique wood for their crafts.

But if you’re committed to keeping your warped wood, we’ll cover a few things you can try.

Dealing with cupping (usually caused by high humidity levels in the air):

 

• Decrease humidity levels with a dehumidifier.
• Make the room warmer.
• Improve ventilation and promote even airflow.
• Monitor moisture levels.
• If there’s no change, sand down the raised edges.

2. Dealing with bowing (usually caused by low humidity levels/dry air):

 

• Use a humidifier.
• Cool down the storage area.
• Improve ventilation and promote even airflow.
• Monitor moisture levels.

3. Dealing with crooking, kinking, and twisting:

 

These warps usually stem from specific wood grains, sawing methods, or wood deformities. So adjusting moisture content won’t make much of an impact.

 

You’ll see wood forums mention tactics like rewetting wood, clamping it down, and adding weight. But if you’re a professional woodworker or flooring installer, it’s often wise to replace the wood.

 

To avoid wood waste, you can use hand planers, jointers, or bandsaws to at least get the warped wood into usable pieces.

When it comes to fixing warps, one commenter in the Fine Woodworking Forum put it best: “Sometimes the juice is not worth the squeeze.”¹³

If these methods fail, don’t stress. You can start fresh, armed and ready to set up your future projects for success.

Now let’s talk about how to choose lumber that’s less likely to warp in the first place.

How to choose lumber that’s less prone to warping

Naturally, knowing some insider tips on how to choose wood will help you reduce the risk of getting stuck with warped wood.

You can inspect details like wood species, cutting technique, and growth rings. But one of the best things you can do is check the moisture content of lumber before you commit to buying it.

Measure moisture content

Take several moisture readings on every piece of wood you test. It’s quick and easy to do with a pinless moisture meter in hand.

A range of 6-14% MC is acceptable for most indoor projects. But avoid getting wood with over 19%, unless you’re using it for firewood, framing, or outdoor furniture.

Make sure you use a high-quality moisture meter. You can get a basic meter for under $100, but the readings may not be reliable (which defeats the entire purpose of using one).

The features on moisture meters vary between brands and individual meters. But the main difference to consider is whether you want a pin-type or pinless meter.

To take readings with a pin meter, you have to poke two holes in the wood for every measurement you take. And it only measures the area between the pins. You also need to switch out pins to read different depths.

You also risk not driving them deep enough in dense, solid wood.

Pinless meters like the Bessemeter SD500 make it quick and easy to take as many readings as you need. The sensor pad allows you to scan a larger area at dual depths. And it gives you a full picture of the moisture content of the wood without damaging it.

Details to consider

• Kiln-dried lumber stamps:
  The wood stamp KD (kiln dried) shows it’s been dried to a specific moisture level—usually to just below 19% for construction materials. S-GRN is sanded green wood that hasn’t been dried.
• Wood species:
  In general, hardwoods like redwood, oak, and cherry are more stable types of wood. Softwoods like spruce or pine are more prone to warping.
• Wood thickness:
  Thinner wood is less likely to warp. It reaches EMC faster, which should mean fewer (and smaller) moisture fluctuations.
• Wood coating:
  Wood with sealant or finish added takes longer to reach EMC. This can decrease fluctuations in the short term. But keep in mind that it can become susceptible to warping later.
• Pressure-treated wood:
  Wood sometimes gets soaked in chemicals to protect it against pests and rot. But since wood soaks up moisture, it’ll slow down the drying process and increase warping risk.
• Sawing method:
  Quarter-sawn wood might cost you more up front. But it expands and contracts less along its width, making it more stable than flat-sawn wood.
• End grain:
  Inspect the ends of boards. Straight, consistent grain patterns make wood less likely to warp.
• Growth rings:
  Avoid wood with highly curved growth rings. This indicates sapwood, which is much more prone to warping.

Now you have the knowledge you need to guard against future warping frustrations.

But do you have the right tool?

Arm yourself against wood warping with a moisture meter

We’ve covered the nitty-gritty of why wood warps. And it’s clear that there’s more power in prevention than in trying to fix it.

You can reduce the risk of wood warping by proper storage, drying methods, and careful wood selection. But monitoring moisture content is crucial throughout.

With a quality wood moisture meter, you’re empowered at every step of the process.

It helps you:

1. Choose wood that’s less prone to warping.
2. Check wood for even drying while it acclimates.
3. Make an informed decision about when it’s safe to start your project.

It’s the best insurance for your time, your money, and your reputation!

1. Wengert, Eugene M., and Dan A. Meyer, “Causes And Cures For Warp In Drying,” Forestry Facts, November, 1993. (↑)
2. Ibid. (↑)
3. Ibid. (↑)
4. Wengert, Gene, “How Do I Check the Quality of Incoming Lumber?” Woodworking Network, March 15, 2016. (↑)
5. Eckelman, C. A., The Shrinking and Swelling of Wood and Its Effect on Furniture, Purdue University, p. 3. (↑)
6. Ibid. (↑)
7. Weikert, Scott, “Air Drying Lumber from Your Own Forest.” PennState Extension, June 2023. (↑)
8. Wengert, Eugene M., and Dan A. Meyer, “Causes And Cures For Warp In Drying,” Forestry Facts, November 1993. (↑)
9. Reeb, J. E., & Brown, T. D., Air- and shed-drying lumber, OSU Extension Service, September 1995. (↑)
10. Installation Guidelines, National Wood Flooring Association (NWFA) via AuroraHardwood.com, November 2008. (↑)
11. Wengert, Gene, “How Do I Check the Quality of Incoming Lumber?” Woodworking Network, March 15, 2016. (↑)
12. Wright, J., Moisture Problems: An Unseen Hurdle of Wood Transportation, Wagner Meters, September 26, 2024. (↑)
13. “How to fix wood warp?” Fine Woodworking, May 22, 2023. (↑)