Everyone loves the beauty and longevity of wood flooring. Unfortunately, both are put at risk from moisture-related problems if the moisture content of the wood isn’t checked at multiple points throughout the installation process. You need to know the moisture content of the wood before you begin your work. If you sand or finish when the wood hasn’t reached equilibrium moisture content (EMC), you may very well see the boards cupping or crowning before you’ve even completed the work.

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Doing any work on the wood floor when the wood’s moisture content is outside its specific EMC range doesn’t guarantee trouble, but it is asking for trouble. If you can’t be sure that the wood has the right moisture content before you sand, you’re taking the risk of compromising the floor’s durability and looks.

Understanding Equilibrium Moisture Content (EMC)

Before getting into the specifics of how moisture-related problems develop and some best practices to avoid them, it’s important to understand wood’s relationship with moisture.

The moisture content (MC) of a piece of wood is expressed as a percentage. This percentage represents the amount of moisture in the wood relative to the wood’s weight if no moisture were present at all. However, the MC of any piece of wood doesn’t remain static.

Wood is hygroscopic, meaning it absorbs and releases moisture in response to its environment. Specifically, as the relative humidity changes, so can the moisture content of the wood, which in turn can change the physical dimensions of the wood. As the relative humidity in the air increases, the wood absorbs more moisture and expands. When the relative humidity decreases, the wood releases moistures and shrinks. When the dimensions of the installed wood change, problems like cupping, crowning, shrinking and cracking can occur.

Since environment has such a direct effect on wood, it’s critical for wood to acclimate to its final moisture content. As wood acclimates, its MC will eventually reach a point of balance with the relative humidity in the air surrounding it. This point of balance is when the wood reaches its equilibrium moisture content (EMC). When EMC is reached, the moisture content of the wood has stabilized. The wood is no longer releasing or absorbing moisture. However, if the air humidity changes again, the wood dimensions will start changing again.

Theoretically, if the wood is no longer releasing or absorbing moisture, its dimensions won’t change due to moisture content, and you don’t have to worry about all the ugly problems that can occur if the flooring didn’t have the proper MC before you started sanding.

Here’s the kicker: There’s no final end game with EMC. As the relative humidity in the air changes, the MC for the wood flooring will change. This is why it’s absolutely critical to the success of your work that you know exactly what that average relative humidity is for where the wood flooring has been installed. Based on the average relative humidity and some of the wood’s attributes (more on this later), you can determine what the ideal EMC is for that wood.

Only once you know the ideal EMC for that specific wood flooring installed in that specific area can you determine whether the MC in the wood makes it ready for sanding – or vulnerable to unsightly damage. For instance, if the moisture changes there will be a change in dimensions that will cause problems. Generally, whatever the ideal EMC is for a specific project, you want to measure the wood’s MC to make sure that it is within two percentage points of the EMC.

It is OK to work on the wood with improper MC. However, it is easiest to have wood at the proper MC prior to installation. The ideal situation is when you narrow the range of the wood floor’s MC swings over time, as it reacts to the normal fluctuations in relative humidity of its environment, then you work on it only when the wood’s MC is close to its EMC. The less the wood floor’s MC varies from its EMC over the course of its lifespan, the less likely the floor will suffer from moisture content-related damage.

Potential Problems

Why should you care? Because the moisture content of the wood at the time you do your sanding and finishing work directly affects how the wood floor looks when you’re done and in the future. Here are some of the problems that can occur when you sand or finish a wood floor when it’s too far outside its EMC range.

Cupping

Too much moisture below the board can cause its edges to raise up into a cup shape. If you sand it at this point, you’ll probably sand down these edges too far so when the MC stabilizes, the boards will now crown.

You might also create cupping damage if you apply an impervious surface finish when there’s excess moisture. This finish will limit the evaporation at the floor’s surface, resulting in uneven moisture within the wood.

If the cupping remains after the floor is finished drying, it’s unlikely that the sides will recede on their own. The result is that the floor will probably need to be re-sanded and finished.

Crowning

The opposite of cupping, when the center of the board is raised above its edges. You can create crowning as explained above. Crowning can also occur if there’s too little moisture on the bottom side of the board. This is caused by the sides shrinking down. As you might guess, if you sand the floor at this point you’ll likely create the cupping effect that’s only apparent once the MC stabilizes and the sides expand again.

Sunken Joints

The joints can swell with moisture from the adhesive used. If you sand the floor before this swelling goes down, you’ll create a cupping effect at the joint once it equilibrates back to the environment.

Attributes Affecting How Wood Reacts to Moisture 

As noted earlier, there are a number of attributes, some specific to the wood installed and some to the environment where it’s installed, that directly affect how the wood flooring reacts to moisture. Here are a few to stay aware of.

Board width

Wider boards will usually show greater dimensional changes than narrow boards. It’s also important to note that the changes in length and width in most wood species don’t react to the same degree.

Grain

Flat-sawn lumber reacts more aggressively to changes in moisture content than does quarter-sawn lumber, as much as double the rate.

Species

Each wood species has its own particular makeup that defines how reactive it is to excess or low moisture levels. As a result, every species has a unique EMC in any given environment specific to that species.

Geography

The geography of the end-use location has great impact on the expected average relative humidity. Even if everything about the wood installed was the same, its EMC will be different in a coastal area than it would be for that same wood when installed in the American southwest.

One important note on geography—the average temperature in an area has considerably less impact on how the wood will react to moisture levels than does the relative humidity. If the relative humidity percentage is the same anywhere within a fairly wide temperature range, between 30 and 110 degrees Fahrenheit, the temperature itself will have minimal impact on the wood’s moisture content.

Controlled conditions

In addition to the local climate, how well (or not) the end-use location controls its own relative humidity also affects the EMC. Most interior rooms will have a relative humidity ranging somewhere between 30 and 60 percent, even if they’re not climate controlled.

In contrast, if the wood floor is installed in an outdoor or semi-outdoor (e.g. enclosed sunroom) space, the average relative humidity will be the result of a much broader swing of potential relative humidity rates. This means that your EMC is necessarily a bit more imprecise, and you can expect moisture-related shrinking and swelling to some degree over time.

Best Practices

This may all sound like you’re at the mercy of the elements, but you’re not. Determine the wood floor’s EMC and use a wood moisture meter to measure for the actual MC before you begin sanding or finishing. As long as you have this information, you can make the decision whether the time is right for you to begin your work.

Each wood and project has its own specific variables and EMC, but you can use these best practices to protect your work and yourself.

Check the moisture content measurements taken upstream. 

The moisture content of the wood should be measured at least three times before you get to it. The manufacturer will have its MC percentage at which it sells the wood. The MC should have been specifically measured during this project when it first arrived and right before it was installed.

Review these records to understand both how this batch of wood is reacting to local conditions and to determine whether it had an acceptable MC when it was installed. If it didn’t, you can’t fix that, but it is a flag that will affect what you look for before you begin sanding.

Do a visual review of the floor for signs of moisture damage. 

If you already see some gaps, splits, cupping, or crowning, you know there are already some moisture issues. The cupping or crowning may be just a temporary reaction to current MC levels and will even out. Signs of splits, cracks, or gaps are more problematic as this is a sign that the board dimensions have shifted considerably since being installed.

Let the floor settle after installation.

Letting the floor settle will let the wood return to its EMC state, especially if adhesives were used. If you see some cupping or crowning, let it settle a few days more and see if it changes. You can also start measuring the MC at this point.

Measure the MC of the floor with a high-quality moisture meter. 

Once the floor has had a chance to settle, do get more precise than just a visual appraisal. Use a reliable wood moisture meter that’s calibrated for the floor’s wood species to measure the MC level.

Measure the MC in more than one spot. 

As a natural material, wood is beautifully unpredictable. Variations in depths, grains, and other properties of the wood mean that it can have different MC levels in different spots at the same time. When you take multiple readings in a good cross section of areas on the floor, you can get a more accurate reading of the overall MC level.

Record the MC levels. 

For your own use and protection, add your MC readings to the appropriate paperwork. Seeing the MC levels over time will help in your decision-making. It will also protect you down the road in case moisture damage occurs. If your readings indicate that you only did the sanding and finishing when the wood was within an acceptable range to its EMC, then you can show you did all you could to prevent moisture-related issues from occurring.

Measure the MC again if time passes between sanding and finishing. 

Even if you measured the MC prior to sanding, you can’t assume the wood floor is still at that same MC if you’ve let some time pass before putting on the finish. Wood does still acclimate back to the EMC once the finish is applied, but it will probably do so at a slower rate.

In addition, if you put the finish on while the wood’s MC is too far away from its EMC, you’re forcing the wood into larger than necessary swings in MC levels, which also sets the stage for future damage like cracks and splits. Knowing the MC level before applying the finish also provides a baseline you can use to determine when the finish is dry.

Preventive Measures

As you can see, taking some time to determine the EMC for a project and measure actual moisture content in the wood before sanding helps you protect the floor’s longevity. Arming yourself with this critical information—and a good wood moisture meter—is all you need.