How to Test for Moisture and Reduce Job Failures

I have seen every side of a moisture problem. These include the legal side -- no one wins; the floor covering contractor's side -- general contractor tells you, "You start the installation or we'll get someone else"; the estimator's side -- he tells you, "This is one of my best customers and I don't want to upset him"; the installer's side -- you are told, "Just get the job done."

And, worst of all, the installation fails and everyone is upset with the floor covering industry. These are all emotional responses and have nothing to do with a professional response. If a subfloor is too wet to install a resilient material, it is too wet and needs to be dried. If you proceed with the installation, you will be held accountable for that decision.

The fact of the matter is that the moisture condition is not a floor covering problem, it's a concrete problem. If there is a solution, it must take place either when the concrete is specified or when it is poured. But, how often are the concrete people at the job site when there is a problem, and why not?

Who said that this is a floor covering problem? All that we are saying is that the concrete is too wet to install resilient materials over. So now we are the "bad guys." So be it! I would rather be a so-called "bad guy" than one who is financially responsible for a bad installation.

So where does it all start? It starts with education! The same old story: educate installers, educate salespeople, educate contractors, and educate yourself. After all, if you don't know the answer, how can you expect to classify yourself as a professional?

The questions are:

How do you do a moisture test?
What type of moisture test works best and when?
What are the margins of error with each of the available tests?

How Do You Conduct a Moisture Test?

Whenever I ask this question, I get all sorts of looks and comments. The bottom line is that there are only a few professionals in this industry who can answer this question correctly. First, understand the laws of moisture migration: moisture always migrates from a cool environment to a warm environment, from a wet environment to a dry environment, and moisture travels as a heavily moisture-laden gas vapor.

The constants are as follows:

1.The temperature on the job site at the time of the moisture test should be comparable to the temperature at the time the floor covering is to be installed (60-70°F). Never conduct a moisture test with temporary heating or cooling.

2.Humidity on the job site must be stabilized to normal conditions. High humidity produces minimal moisture migration; low humidity promotes increased moisture migration.

3.The concrete must be clean and free of all types of sealers, curing compounds, or anything that can serve as a vapor barrier to the test area. I normally abrade the area to be tested to ensure that the floor is in a good, porous condition. Sealers and curing compounds retard moisture migration.

4.The time required to do a test will vary according to type, so be sure you know the limitations of each type of test used. Remember that nothing of any consequence happens within the first 48 hours when doing mat, bond, or calcium chloride tests.

Know the Types of Tests Available

Moisture conditions for concrete fall into two categories; Static and Dynamic. A static moisture condition is a function that determines how wet the internal structure of the concrete is at present. A dynamic moisture condition is a function of how much the moisture in the concrete is currently migrating from the surface.

Moisture testing for concrete also falls into two categories: Subjective and Objective. A subjective moisture test relies upon the person testing to decide whether or not it is safe to install the floor covering. An objective moisture test quantifies the moisture condition in terms of percentage or pounds of emissions.

Brief Descriptions of Moisture Test Procedures

Visual Appearrance Test
The weakest of all moisture tests, it is widely used on an assumption basis. The installer or contractor walks through the job site and looks for signs of moisture. Those signs include damp areas under buckets, boxes and plastic sheets lying on the subfloor and signs of alkali salts deposits. Even the best and most knowledgeable floor covering professionals cannot visually detect a moisture problem.

Concrete Age Test
While assumed age is an indicator of a dry slab, many second-time installations fail as a result of moisture problems. As a structure ages and settles, some drain fields fail to carry off excessive moisture. Many experts agree that concrete less than two years old should be checked for moisture.

The Rubber Sheet/Plastic Mat Test
This is the most commonly used test. However, its margin of error is very great. For this test, you place a 36" x 36" mat on a clean concrete surface, secure the perimeter with duct tape and leave in place for a minimum of 48 hours. After removing the mat, inspect the concrete's surface for darkening, any signs of moisture droplets, surface drying (whitening) and how well the duct tape stuck to the concrete. Some feel this test can be done overnight or in 24 hours. It has been my experience that nothing starts to happen for 48 hours.

The Mat Bond Test
This is similar to the mat test except you adhere to the subfloor a 36" x 36" piece of the actual material you'll be installing. Use the proper adhesive, applied to specifications, and rolled and secured around the perimeter with duct tape. Leave the material adhered to the concrete for a minimum of 72 hours and then tear it up. You have a moisture problem if the material is easily removed, the adhesive is damp and stringy, or if it releases from the concrete and stays on the back of the material.

The Moisture Meter Test
Moisture meters are growing in popularity because there is no waiting for the results. But sometimes the results can be misleading. If there are coatings on the concrete or conductive fibers in the concrete, you can get inaccurate high moisture readings. There are two basic types of moisture meters used: conductivity and impedance. The conductivity moisture meter works on the principle that water is conductive -- the more water, the better the conductivity. Thus, a high reading shows on the moisture meter. The concern is that there are coatings and additives on or in concrete that will cause false readings. The non-destructive impedance type of meter, which is more accurate, uses low-frequency radio waves that travel down into the concrete. The meter measures the speed at which the waves travel and the result is the percentage of moisture by volume present in the concrete. These meters, too, are very sensitive and should only be used on a clean, residue-free surface. While moisture meters are expensive, they are well worth the cost.

The Chemical Tests
These include the calcium carbide bomb, phenolphthalein and litmus paper (pH) tests. The calcium carbide bomb is an airtight container in which calcium carbide crystals are enclosed in a glass vial. Chips of the subfloor concrete are put inside. Then a cap placed on tightly and the container is shaken, which breaks the glass vial. When the crystals come in contact with moisture they produce acetylene gas which is measured by a pressure gauge on the bomb. This device is also very expensive.

Both the phenolphthalein and litmus paper tests measure the alkalinity of the surface of the concrete. Alkaline salts are a whitish-gray residue left on the surface of the concrete after the moisture has escaped. These tests can be misleading because not all moisture problems carry high quantities of alkali to the surface.

The Calcium Chloride Test
This is the most widely recognized and used by resilient manufacturers because you can get a quantitative amount of the moisture that is migrating. It is measured by the weight of the water (8.3 lbs. per gallon) per 1,000 square feet per 24 hours.With this information you can speak intelligently to the general contractor, architect, or owner about just how high the readings are and where they need to be for a successful installation.

Most manufacturers publish the acceptable amount of moisture that their products will withstand. That amount will vary between 3 and 5 lbs. With future testing you can measure the amount of progress the drying process is making.

As far as my own testing preference, I use a combination of methods. First is the impedance moisture meter, which gives me the amount of moisture present in the concrete by volume. If I get marginal readings, I will then resort to a series of calcium chloride tests (the moisture meter will help me determine where to place the calcium chloride test kits).

I have a good reason for using this particular series of tests. I know that when I meet with the end users they will be skeptical about what I have to say. Therefore, I want all of my information gathered, organized, and easily understood. Like it or not, the facts speak for themselves. Presenting them in a professional manner not only takes a lot of stress out of the situation, it lets the people at the meeting know that you are a professional.

Contributed by Ray Thompson, Jr
Reprinted from Fall 1996 Floor Covering Installer Magazine.
Copyright 1996 Business News Publishing Co., II, L.L.C.

Ray Thompson can be reached via email