Test Concrete Floor For Moisture With This Simple, Cheap Method

Laying Plastic Sheet on Flooring
Erik Dreyer/Getty

Concrete is moist stuff. It begins moist, as a slurry of water and aggregate, and even after it hardens, moisture remains for an incredibly long time.  While this is inconsequential for outdoor applications, like patios or walkways, it matters a great deal indoors, especially when you intend to install flooring on the concrete.

Invisible Moisture:  When a Floor That Looks Dry Is Not Really Dry

Even after concrete has hardened, moisture can be a problem.  Because concrete is permeable, ground moisture can slowly seep upward in the form of vapor and adversely affect anything that rests on it.  

Vapor, given ample space, warmth, and ventilation, will evaporate.  But flooring laid on top of the concrete 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:

  • Mold and mildew will grow.
  • It also begins to affect the flooring, degrading organic materials (such as solid hardwood or engineered wood).
  • Flooring bows, warps, and bubbles.

Testing:  Necessary Step Before Flooring

It is vitally important to test your concrete slab's moisture before installing flooring.

The definitive test is with a calcium chloride kit or a relative humidity meter. But here is a simple test that gives you a general idea of the moisture in your concrete.

Difficulty: Easy

Time Required: 2 days or more.

Here's How:

  1. Cut clear plastic sheeting into a 24" by 24" square.
  2. Make sure that the concrete where you intend to place the plastic is free of standing water as this will skew your test results.
  3. Tape the plastic firmly 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.
  4. Wait 48 hours.
  5. Check the plastic to see if any moisture has beaded up on the inside of the plastic.


  • Extending Test:  No beading does not necessarily indicate that there is no moisture, as this is just a simple test.  Longer tests over a variety of climactic conditions might result in moisture building up on the inside of the plastic.  If you have time, longer times will return more accurate results.
  • Positioning:  Placing multiple squares of plastic in various positions around the floor, too, will return better results.  
  • Target Problem Areas:  Cracks are avenues for the passage of moisture.  Be sure to place plastic over any kind of visible imperfections (cracks, holes, etc.) in the floor.
  • Best vs. Worst Flooring:  Ceramic or porcelain tile stand up well against concrete moisture, as do any type of resilient flooring such as luxury vinyl tile.  With laminate flooring, solid hardwood, or engineered wood, it is critical that the concrete base have no moisture present.

What You Need:

  • Clear plastic sheeting at least 24" by 24"
  • Masking tape