How to Identify and Fix Moisture Problems in Your Concrete Subfloor

hardwood floor buckling from moisture

Wood floors may buckle as they absorb excess moisture from concrete subfloors.

You’ve done everything you can think of to prep your wood flooring for installation.

You checked moisture levels.

You let them acclimate to the environment.

So why is your client calling you about evidence of moisture damage?

The problem might run deeper than you think. Perfectly acclimated wood flooring can still be damaged if exposed to moisture later on. This can be due to circumstantial things like leaky pipes or windows. But one of the sneakiest culprits can come from deeper within your floor—namely, your concrete subfloor.

Concrete subfloors could release moisture, which would then be absorbed by the flooring above. If the concrete subfloor contains high moisture levels, your floors may experience swelling, buckling, cupping, or discoloration.

Fortunately, there’s a way to avoid this damage. Here are the steps you can take to protect your floors, including:

How to Identify High Moisture Levels

Placing the Rapid RH L6 in the testing hole

The Rapid RH L6 must be placed into a hole that is drilled into the concrete to get a more complete picture of the concrete’s moisture levels.

Before installation, you want to check the subfloor.

Some moisture issues with subfloors are more obvious, such as sweating, dampness or the presence of mold. Or you might notice efflorescence, a white, powdery residue that shows up when high moisture levels carry the concrete’s salts to its surface.

Unfortunately, more often than not, high moisture levels in concrete can go unnoticed. A concrete subfloor can appear dry when it isn’t. There could still be excess moisture beneath the surface.

So the only way to truly determine a subfloor’s moisture content is with the right tools and procedures.

Let’s start with step one.

1. Check calibration and prepare the area

First off, make sure all your relative humidity (RH) in-situ probes are calibrated per the ASTM requirement. You will want to make sure you have documentation of that fact.

Once you have that, ASTM standards require you to keep the room you’re testing in at service conditions for 48 hours. Then you can begin to test for moisture.

2. Take moisture readings

Using a C555 concrete moisture meter

Our C555 Concrete Moisture Meter can help you identify and compare high
moisture levels within a slab of concrete.

Per ASTM F2170, install three Rapid RH probes for the first 1,000 square feet of your floor. After that, install one probe for each additional 1,000 square feet.

Want to find the best spots to place your probes?

You can use concrete moisture meters, like our C555 concrete moisture meter. These meters are able to determine which areas of the subfloor have the highest surface moisture levels in comparison to the rest of the subfloor.

This makes them a great companion to RH probes. However, some flooring professionals may choose to use concrete moisture meters as the final say on whether their concrete is dry enough to safely accept a floor covering. Some flooring manufacturers will also say that using a concrete moisture meter is sufficient.

If you do choose to use a concrete meter by itself, it’s important to find one of the more accurate ones available, like the C555. This will help you make more accurate judgments about the concrete’s moisture levels.

However, even though a concrete moisture meter alone may be acceptable to some installers and manufacturers, keep in mind that it won’t be in compliance with ASTM standards as the final say on whether concrete is ready for a floor covering.

So let’s go back to how they can be used to work with RH probes.

In identifying moisture hotspots with a concrete moisture meter, you can place your RH probes in the areas with the most moisture. That way, when those areas reach the appropriate moisture level, you can be more confident that the other areas of a slab have as well.

However you choose to place your probes, you should know that ASTM F2170 requires you to drill holes for your probes at a certain depth. Drill 40% into the slab if your concrete is drying from one side. If your concrete is drying from two sides, drill 20% deep.

Place your probes in the drilled holes and wait 24 hours before taking the probes’ reading.

While you’ll still want to wait this long to meet industry standards, you should know that our Rapid RH probes can give you an idea of your moisture content within the first hour. This sneak peek can give you extra time to plan how you’re going to deal with any moisture-related issues.

3. Evaluate moisture results

So now that you have your moisture readings, how do you know if they’re too high?

ASTM standards recommend the concrete to be below 75% relative humidity, or whatever the manufacturer specifies. If your readings are higher than what they recommend, wait for the concrete subfloor to acclimate before installing your floor covering.

You can usually find this information on the flooring manufacturer’s website.

How to Correct High Moisture Levels

If you identify high moisture levels in the concrete subfloor, here are some measures you can take to correct them:

1. Stop moisture before it reaches your floor covering

Before doing anything else, you’ll want to identify if there could be any sources of moisture intrusion.

If not, the high moisture levels may be a result of free water left over from when the concrete was first mixed and poured.

But if a leaky window or pipe caused the subfloor’s high moisture levels, it must be sealed to prevent future moisture intrusion.

2. Dry the subfloor

Now, we just have to dry the subfloor to help lower the concrete’s moisture levels. Use fans to improve the room’s airflow and adjust the room temperature as needed. Adding dehumidifiers to the area will help draw out excess moisture.

Some slabs, especially older slabs where moisture vapor retarders were never installed, may never fully dry out. In this case, the only option may be installing a moisture mitigation system over top of the concrete.

3. Monitor moisture levels

Keep an eye on your RH probes while you’re waiting for your subfloor to dry.

You can streamline this process by upgrading to our True Remote Monitoring system. With this system, you can view RH readings anytime, anywhere—without traveling all the way to the job site.

This means you’ll be able to keep a watchful eye over your RH readings even if you don’t travel to the jobsite. You’ll be able to know when your subfloor has reached the right moisture level to start floor installation.

What Ensuring Floor Quality Means for You

Making sure your concrete subfloor is at the appropriate moisture level before you install floor coverings can help you avoid a lot of headaches down the road.

For one, you won’t have to worry about moisture damaging your floor covering. You also won’t have to worry about letting your clients down.

Checking the concrete subfloor can help you build a reputation as a careful, thorough flooring installer who does whatever it takes to provide high-quality floors.

And it all begins with accurate moisture testing.

Achieve higher quality flooring today with our fast C555 concrete moisture meter and our state-of-the-art Rapid RH in-situ probes.

Find them in our online store.

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