How Does Moisture Affect Wood?

The science of moisture and wood

Wood shrinks or expands to be in equilibrium with the moisture of its environment. Scientifically speaking, it’s hygroscopic. This moisture level in wood is called its moisture content or MC%.

MC is the weight of the moisture in the wood, expressed as a percentage of the dry weight of the wood. The formula looks like this:

   

“Weight before” refers to the weight of a sample piece of wood before it’s been dried in an oven, and “weight after” is its weight after being completely dried.

When a tree is first cut down, it contains two types of water:

1. Free water
2. Bound water

The National Wood Flooring Association’s publication Moisture and Wood explains that free water is in the cell lumina (cavity), while bound water is trapped within chemical or hydrogen bonds of the wood’s cell walls.² When a piece of wood dries, it releases all its free water first before releasing the bound water.

The point at which all the free water has been released is known as the fiber saturation point, or FSP. The FSP is around 25–30% MC.

Wood will continue to release moisture after this point until it matches the moisture of its environment, something known as the equilibrium moisture content (EMC).

But as it dries below the FSP, it’ll also experience more changes dimensionally. We call it “wood movement.” Let’s look at the effects of this movement on the wood itself.

How moisture affects wood dimensions

As wood releases moisture, it experiences three kinds of dimensional shrinkage:

1. Tangential shrinkage takes place parallel to the growth rings.³
2. Radial shrinkage occurs across the growth rings, radiating out from the center of the log.
3. Longitudinal shrinkage happens lengthwise in the log.

Typically, the greatest amount of shrinkage happens tangentially with anywhere from a 5 to 15% change. Radial shrinkage is less—about 2 to 8%. And the longitudinal shrinkage is about 0.1 to 2%.⁴

The radial and tangential shrinkage make up the volumetric shrinkage of the wood, and the ratio of the two is called the T/R ratio. The goal is for both of these numbers to be as low as possible.⁵

The amount of shrinkage will depend on the wood species.

Hardwoods tend to experience greater movement radially and tangentially. Those with higher amounts of shrinkage include southern red oak, white oak, elm, birch, basswood, and sugar maple.⁶

Softwoods and imported hardwoods, on the other hand, experience a lot less movement.

Another factor that plays into wood movement is the angle at which the wood is cut. For example, quartersawn wood, which is “cut perpendicular to the growth rings,” will be less affected by moisture than flatsawn wood.⁷

How moisture affects wood appearance

The change in the wood’s size can damage its appearance, particularly if the wood loses moisture too quickly.

Some possible damage includes:

• Warping
• Checking/cracking
• Gaps

If wood has dried too much and then begins to gain moisture, it can experience other defects like cupping and buckling.

And this potential for damage is what makes MC an important topic for woodworkers.

Why moisture content matters to woodworkers

If you don’t know the moisture content of the wood you’re working with, you’re setting yourself up for problems like the ones we mentioned in the previous section: warping, cupping, buckling, gaps, and more.

Imagine this scenario for a moment:

You’ve built a beautiful oak bookshelf for your office—from a tree that fell in your yard. Maybe you let the wood air dry outdoors for a while but didn’t check the MC before building with it.

The problem is that after a few months of enjoying your bookshelf, you notice cracks in the boards. What’s going on?

Your oak boards hadn’t reached the EMC of your location and continued to lose moisture, shrinking in the process.

Now, your oak bookshelf has cracks that won’t be easy to hide or repair.

Other problems due to an incorrect MC include:

• Warping of the wood’s shape
• Nails coming loose
• Glue not holding together

Without knowing and working with the MC, you risk wasting your precious time on projects that will end up ruined. At the same time, you lose money on the tools and materials used for that project.

And finally, if you sell your products, you risk hurting your reputation among your customers if their items eventually show moisture damage.

So, what are some simple ways to prevent these challenges?

How to avoid moisture damage

Avoiding moisture damage is all about knowing the MC of your wood and allowing that knowledge to guide the decisions you make in your woodworking project.

It looks like:

1. Testing your wood with a wood moisture meter
2. Allowing the wood to acclimate to the EMC
3. Protecting the wood
4. Keeping the wood in a consistent environment

Keep reading for more details on each of these steps.

1. Test your wood with a moisture meter

Use a quality moisture meter to check the moisture content levels of your wood. This is the most convenient way to measure MC—the other way being the time-consuming oven-dry method.

With a good pinless meter, it takes seconds to get moisture readings! A pinless moisture meter, unlike a pin-type meter, won’t require you to push pins into the wood, leaving ugly holes behind. All you have to do is press the sensor plate on the wood, and you’ll have a reading.

Now, compare that reading to the EMC of your location. If you’re building something for the indoors, you’ll aim for about 7–9%. (Use an EMC chart to determine the exact number.)

If your wood’s MC is at or near that number, you’re set to go. Otherwise, the next step is acclimation.

2. Allow the wood to acclimate to the EMC

If your wood is still far from the EMC, allow it to acclimate to that number. Leave the boards in their final environment and check them periodically with your moisture meter.

The MC will be the key indicator for knowing when the wood is ready.

3. Protect the wood

Use paint, varnish, or another type of finish on your wood to help protect it from moisture changes in the environment.⁸

After all, even when you get your wood down to the right MC, slight moisture changes can still happen in the environment—a leak or seasonal changes, for example.

However, we don’t recommend solely relying on any type of wood finish to protect your wood. The first line of defense against moisture is having your wood at the right MC before you begin working.

4. Keep the wood in a consistent environment

Whether you’re storing your wood for later use or putting it in its final environment, aim to keep the surrounding air conditions—temperature and relative humidity—at in-service levels. This will prevent changes in the EMC, which could cause your wood to shift too.

Consistency in an indoor environment usually means somewhere between 60 and 80° F for temperature and 30 and 50% for humidity levels.

Knowing moisture content starts with having the right tools

Knowing about the effect of moisture on wood allows you to work with the MC, rather than against it.

But the knowledge won’t be helpful without a quality moisture meter to show you the MC of your wood.

So, why not look into getting one for yourself? Learn what to consider when looking for one.