Don’t Let Moisture Sabotage Your Concrete Subfloor

You’ve put in a beautiful hardwood floor, following proper procedures.

Not long after the flooring installation, however, you notice the boards have begun twisting upward and cupping. What happened? You carefully checked your floorboards before you put them down, so it can’t be that.

Perhaps your wood floor is installed over concrete. If so, you might have a concrete moisture problem. Moisture in concrete subfloors poses a major issue for the wood flooring above it. This is why we want to equip you with the knowledge to address the issue and prevent it.

You’ll learn about:

Keep reading to avoid disasters with your projects.

Causes of common problems in concrete subfloors

Moisture is one of the biggest concerns with concrete subfloors under wood floors. It can lead to problems such as cupping and buckling, discoloration of the floor, shrinkage, and mold and mildew growth.

Usually, this excess moisture comes from1:

  • The concrete mix
  • The environment around the slab

The concrete mixConcrete blocks

Sometimes, concrete contractors end up with concrete that has too high of a water-to-cement ratio.

Because of this high water content, the concrete needs more time to dry beyond the several months usually required. If the construction crews are in a rush, they may not wait long enough for this to happen.

The result?

Moisture in the concrete slab can migrate into the wood flooring placed above it.

The environment around the slab

Moisture beneath the concrete slab can also seep in, affecting the concrete and eventually the wood floor. All slabs are required to have a moisture vapor under the slab when it is poured. However, sometimes it’s improperly installed or damaged during installation.

Extra moisture can come from the environment due to the grading of the property. An improperly graded plot or changes due to landscaping can cause water to collect under or around the concrete slab.

Poor drainage systems can have the same effect, allowing water to pool in areas it shouldn’t.

As mentioned above, a proper moisture vapor barrier can go a long way to mitigate the effects of the additional water.

Then, there’s the climate and humidity of the air. High humidity and temperatures can cause condensation that migrates into the concrete. Concrete can also absorb moisture directly from the surrounding air.

So, have you identified the cause of your problem?

If so, you’re probably wondering what to do next.

How to address moisture issues in a concrete subfloor

When you have moisture issues in your concrete subfloors, you’ll need to do a couple of things to address them.

First, you’ll need to stop moisture from coming in through the sources mentioned above. And then, you’ll need to dry the concrete if possible.

We’ll walk through the following steps:

  1. Stop moisture problems at the source.
  2. Test the moisture of the concrete.
  3. Dry the subfloor.
  4. Repair or replace damaged wood flooring.
  5. Install moisture mitigation systems if required.

1. Stop moisture problems at the source.

A hand tightening a leaking pipe that is causing a concrete subfloor moisture problemTrying to dry concrete without dealing with the moisture is like trying to mop up a sopping floor without turning off a faucet. You have to get to the source of the issue first. That will determine the way you solve it.

If the moisture is due to improper drainage, find ways to redirect the water so it doesn’t pool under the concrete slab.

If a plumbing leak has caused moisture to seep down into the subfloor, fix the broken pipes.

If water is coming up from the soil, you may need to put a moisture mitigation system on the slab before putting hardwood flooring back on top. More on that in Step 5.

2. Test the moisture content of the concrete.

Knowing the moisture content of the concrete is essential to a successful job.

Begin by measuring its moisture content. Then test again throughout the drying process until it reaches the target number—under 75% relative humidity (per ASTM F2170 standards) or whatever the manufacturer recommends.

There are three different ways to test moisture in concrete:

A concrete moisture meter: A concrete moisture meter is best used to give a general idea of where moisture is located in a piece of concrete, but it doesn’t provide a highly accurate picture of the concrete’s true moisture condition. Thus, a concrete moisture meter is best for showing you where to place the following two tests.

Calcium chloride testing: This test is also known as a moisture vapor emission rate (MVER) test. It involves placing calcium chloride on top of the slab and seeing how much moisture it absorbs. After 72 hours, the calcium chloride is weighed, and the difference in weight determines the amount of moisture.

Though approved by ASTM F1869, this method is time-consuming and may not provide the most accurate results since it only tests the surface of the concrete.

In situ relative humidity testing: This final method is a newer approach approved by ASTM F2170. It involves drilling holes into the concrete and placing sensors at 40% of the depth of the concrete. Then, results are available within 24 hours.

Per the ASTM standard, place three sensors for the first 1,000 square feet of subfloor and one more sensor for every additional 1,000 square feet.

3. Dry the subfloor.

To dry the subfloor, you will most likely have to pull up the wood floorboards above it. Place fans and a dehumidifier near the opening.

Moisture testing will be the key to determining progress.

4. Repair or replace damaged wood flooring.

A flooring specialist replacing damaged hardwood flooringUnfortunately, excess moisture in concrete subfloors can wreak havoc on hardwood floors. Depending on how severe the damage is, you may need to repair or replace the damaged boards.

5. Install moisture mitigation systems, if required.

Placing a moisture mitigation system between the concrete subfloor and the floor covering can prevent moisture from traveling between them.

Since moisture mitigation systems are expensive, this step is the last resort when the concrete vapor barrier has been compromised and the concrete won’t completely dry. The moisture mitigation system will seal the moisture into the slab and stop it from moving into the wood floor.

Preventing moisture issues in concrete subflooring

Preventing moisture issues in concrete subflooring is not much different from fixing them.

Here are some tips:

  • Ensure proper drainage of water from the job site.
  • Put a vapor retarder under the concrete slab. ASTM E1745 is the standard for vapor retarders under concrete. It provides permeance ratings, which express how much water a vapor retarder will allow to permeate through it. The lower the number, the less water that can permeate. According to this standard, the vapor retarder you place under the concrete slab will need to be 0.1 perms or less.
  • Make sure the concrete has a low enough water-to-cement ratio when it’s being poured.
  • Allow the concrete to dry properly, using dehumidifiers and fans if necessary.
    Test the moisture content and relative humidity of the flooring materials, especially the concrete, with ASTM-approved test methods (see above). In situ relative humidity testing will be your best bet for the concrete.
  • Put moisture mitigation systems in place if you’re unable to stop the moisture any other way.

Some of these precautions may seem unnecessary, but they’ll be worth it when you don’t have to worry about moisture issues in your wood floor months down the road.

Deal with concrete moisture issues and save your hardwood floor.

You’re on your way to solving the moisture problem in your concrete slab and having a successful project. You also have the knowledge you need to prevent concrete subfloor problems from happening again.

But what about that cupping on your hardwood floor? Needing some more guidance in dealing with that?

  1. Youngworth, Kendall, “Signs a Concrete Floor Has Moisture Issues,” Buildings,, April 15, 2019. ()