Concrete slabs appear to be solid and hard, but in reality, concrete is moist stuff. It begins as a liquid slurry of water and aggregate, and even after it hardens, moisture continues to pass out of the concrete for years. Concrete can also absorb and transfer ground moisture.
Why does this matter? While this quality of concrete doesn't make much difference for outdoor applications, such as patios or walkways, it matters a great deal indoors, especially when you intend to install residential flooring on the concrete slab.
Concrete Moisture Levels Can Affect Flooring
Moisture can be a problem when a floor that looks dry really isn't. Because concrete is permeable, ground moisture can slowly seep upward in the form of vapor and adversely affect anything that rests on it. If given ample space, warmth, and ventilation, water vapor will simply evaporate. But when flooring is laid directly on the concrete, it effectively creates a water-tight, air-tight barrier. Moisture builds up and has nowhere to go.
Trapped moisture creates havoc that you do not need:
- Moisture is conducive to the growth of mold and mildew.
- Moisture can cause organic flooring materials (such as solid hardwood, engineered wood, cork, or bamboo) to degrade.
- Moisture can cause any flooring to bow, warp, and bubble.
Three Methods for Testing Moisture Levels
It is vitally important to test your concrete slab's moisture before installing flooring over it. The definitive test is with a calcium chloride kit or a relative humidity meter. Testing with either method will tell you how precisely how much moisture is present, making it possible to choose a surface flooring that is appropriate. Flooring manufacturers rate their products based on the moisture levels they can tolerate, and testing your slab can tell you if a particular flooring material is likely to perform adequately.
A simple DIY test can give you a general idea of the moisture in your concrete.
Tools and Supplies You Will Need
- Clear plastic sheeting
- Duct tape
Cut clear plastic sheeting into several 24 by 24-inch squares.
Inspect the Concrete
Make sure that the concrete where you intend to place the plastic is free of standing water, as this will skew your test results.
Secure the Plastic
Tape the plastic squares securely to the concrete with duct tape or any other kind of waterproof tape. Firmly press down all sides of the tape to prevent moisture from escaping.
Wait 48 Hours
Wait at least 48 hours, then check the plastic to see if any moisture has beaded up on the inside of any of the plastic squares.
Check If It's Wet
Moisture on the inside of the plastic, either in the form of water droplets or fogginess, indicates the slab is expelling moisture in measurable amounts. Before choosing any surface flooring to install on such a slab, a more precise test using a calcium chloride test, or a measurement with a relative humidity meter, is necessary.
Concrete Moisture and Flooring Materials
Concrete moisture is more problematic in below-grade (basement) locations where conditions are naturally humid, but it can also be a problem for on-grade slabs as well, especially when they are new. One method for mitigating the problem is to raise the surface flooring slightly off the slab by first laying rows of "sleepers" topped with a plywood subfloor, so that the surface flooring material does not rest directly on the concrete. This is a very common technique where carpeting is desired.
Generally speaking the best surface flooring materials for a concrete slab that is still transmitting some amount of water vapor include:
- Ceramic or porcelain tile
- Resilient floorings such as luxury vinyl flooring (LVF)
- Rubber tile
Flooring materials that do not perform well on concrete that is emitting water vapor include:
- Solid hardwood
- Engineered wood
However, flooring manufacturers now market some flooring materials as being "moisture-resistant." Some laminates, engineered woods, bamboo, and cork flooring materials identified as moisture-resistant can be used successfully on concrete slabs. Manufacturer's instructions will indicate if concrete slab installation, below grade or above grade, is regarded as appropriate.
Where an above-ground concrete slab is sufficiently cured and is no longer transmitting water vapor, just about any flooring material can be used.