Like many place I go in my travels, concrete moisture issues are never NOT discussed at FCICA events, either in an educational session or just in informal discussions.

FCICA, The Flooring Contractors Association, is meeting this month in Savannah for our annual Convention and celebration of our 30th anniversary. I’ve been attending these events every year for ten years – I would never miss them for the great education and networking with a “who’s who” of the commercial floor covering industry.

Like many places I go in my travels, concrete moisture issues are never NOT discussed at FCICA events, either in an educational session or just in informal discussions. It’s a topic that experienced installers and newcomers to our industry always seem to want to learn more about.  In the almost 20 years that I have been on the sales and technical side of resilient floors, most of the complaints I have been involved with can be traced to concrete issues. People contacting me on the Internet ask about concrete more than any other topic.  The same holds true for my past eight-plus years writing Let’s Talk Resilient for in FCI.  My first column in 2003 and several since have gone into this topic in depth.

  You’d think I’d covered it enough but you, the readers’ are always asking for more. Well, we aim to please!  For those of you who haven’t read much about this topic before or those who need a refresher, here is a very basic understanding of the issue that will hopefully give you some talking points for discussions with your colleagues and your customers.


First, let’s get the words right.  If you want to be a professional, use the right language. Here are some terms that many in the industry get wrong.

Cement is the grey powder that goes into concrete, not the finished product. Concrete is the finished product.  Cement is to concrete as flour is to cake – an ingredient.  There is no such thing as a cement floor. It’s concrete.

Curing is the chemical reaction between cement powder, water, sand and rocks that binds them into concrete. However, it’s very important to know that curing and drying are not the same thing.  Drying happens after curing. Don’t assume just because it’s cured that it’s ready for a floor. Curing a 4-inch slab takes about 28 days.  Drying that same slab can take four months or more and it’s the drying that really matters to the floor covering.

Hydrostatic pressure can only happen in a basement where the water table outside the building is higher than the basement floor. Hydrostatic pressure is liquid water coming through the floor. Any other moisture problem is just that – moisture.  Call it a moisture problem, not hydrostatic pressure.

Water is a liquid; moisture is a vapor that can be invisible.  You often can’t see moisture problems until you put a floor over it. 

Why Moisture Problems?

Old concrete: For old concrete slabs it’s easy to assume that it dried out a long time ago so you don’t have to worry, right? Wrong!  If you are dealing with a slab on or below grade that is in contact with the earth, that can be a dangerous assumption because there may or not be a vapor retarder or barrier under the slab.  Moisture naturally wants to move up from the ground into the air.  Without something under the slab to stop it, moisture from the ground can pass through the slab and into the adhesive layer, often bringing alkalinity with it that dissolves the glue.  This can be a problem on a slab of any age, even if there was a floor down before.  Even if you are in the desert, moisture moves from the ground to the sky.  ALL slabs are suspect for potential moisture problems.

New concrete: For new concrete, there is a lot of extra water in the mix that is called “water of convenience.”  The extra water makes it possible to move concrete in trucks and “pour” it in place.  However, of the total concrete mix, more than half the water is not needed for the structural integrity of the concrete, so that water needs to go.  Allowed to dry naturally, this excess water will evaporate out over time.  However, if the concrete is covered with a floor before it’s dry, then there can be a problem later on.

Above grade concrete: There are a lot of moisture problems on concrete floors from the second floor and up so don’t assume that moisture problems can’t occur on these floors just because they are not in contact with the ground.  The reason is that these slabs are often made using lightweight concrete or they are on a metal deck that only dries from one side.  The industry standard, ASTM F 710, calls this out, saying “Floors containing lightweight aggregate or excess water, and those which are allowed to dry from only one side, such as concrete over a moisture vapor retarder or concrete on metal deck construction, may need a much longer drying time and should not be covered with resilient flooring unless the moisture vapor emission rate or the percentage of internal relative humidity meets the manufacturer’s installation specifications*.”

Other causes of moisture in concrete: External factors such as grading of the ground outside the building, landscaping sprinklers, downspouts and other situations outside the building can allow water to run towards the building instead of away. This water can get under the slab and move upwards in the form of water vapor. Sometimes, maintenance people can cause a problem with too much wash water, especially on a brand new floor, but these problems are rare and often misdiagnosed.  Broken pipes beneath a slab, while rare, can also cause issues.

Can You See, Feel or Smell Moisture in a Concrete Slab?

If you are looking at a concrete slab and see water, that’s a problem.  If you are below grade in an area with a high water table, maybe it’s a hydrostatic pressure issue, in which case the slab needs to be waterproofed and coated with a moisture mitigation system.  An engineer should be consulted to drill into the concrete and see what is going on underneath. 

More often, water on the slab can be from a leak, rainwater, testing the fire sprinklers or some other source. It’s not enough to just mop up the water and install the floor once the slab looks dry.  That water is absorbed by the concrete and will come back later to cause problems so you need to deal with it before the floor goes down. 

If you are looking at a slab and see, smell and feel nothing, that’s not an assumption of dryness. Moisture is invisible, so what you don’t see can still be a problem.

What Does Moisture Do to an Installed Floor?

On an installed floor, moisture problems can look a lot of different ways. Adhesive oozing between the seams of resilient tile often get blamed on the installer or the maintenance team. It’s rarely caused by either – it’s usually moisture from below, not water from cleaning or an installer mistake. Mold in carpet or padding, ceramic tile cracking, resilient flooring with bumps, bubbles, blisters, indentation, cracking of floor coverings, or splitting seams can all be traced to moisture. It’s not pretty.

What Can Be Done?

I could go into all kinds of details about how the right mix, curing methods and drying time can prevent moisture problems.  However, that’s rarely the floor covering installer’s responsibility. Our job is to supply and install the floor. If you know there is a moisture problem before the floor covering is installed, action can be taken to prevent problems.

How do you know? Test, test, test! The industry standard, ASTM F 710 is very simple: “All concrete slabs shall be tested for moisture, regardless of age or grade level,*” and most manufacturers say the same.

The floor covering industry recommends that moisture testing be done independently, per the 2001 Floor Covering Industry White Paper Position Statement on Moisture Emission Testing, which says “It is therefore our recommendation that concrete moisture vapor emission testing be performed by qualified independent agencies#.”

I also recommend that flooring installers and project managers become aware of the different test methods and how to do them correctly, because in many cases independent testing is not done so it falls to the floor covering dealer or installer.

There are only two methods accepted today, the Calcium Chloride test and the Relative Humidity test. It is NOT acceptable to use moisture meters or a plastic sheet taped to the floor. Those methods do not provide an accurate reading that will be accepted by the floor covering, patching compound or adhesive manufacturer. In order to do testing you need some fairly serious tools and an understanding of the test methods.

Calcium Chloride Test

ASTM F 1869 is the Calcium Chloride test that measures vapor emissions from the top ¾” or so of a concrete slab. It’s for concrete only and should not be used over patching or leveling compounds, lightweight concrete, gypsum underlayment, moisture mitigation systems or other coatings.  It’s for bare naked concrete only.

This method has been around since the 1950s and is fairly widely accepted. It takes 3 or 4 days to do the test.  The kits are widely available and contain everything you need except for tools.  The premise behind this test is simple - salt absorbs water vapor from the air.  Think about a shaker of salt on a humid summer day when the salt gets all clumped up and a lot of restaurants put rice in the shaker to absorb the excess moisture. In the case of F 1869, a dish of salt (calcium chloride) is weighed, placed under a plastic dome on top of clean concrete for 60-72 hours and then weighed again.  The weight gain is from moisture that is absorbed.  This number goes through a formula and the results are expressed in pounds of moisture vapor per 1,000 square feet per 24 hours, or just “pounds.”

The keys to the test are preparing the slab and making sure site conditions are right.  To prepare the slab, you need a grinder. A light grinding of a 20” x 20” area should produce naked concrete with a texture similar to course sandpaper.  It is not enough to scrape, sand or wire brush the surface; it must be cleaned by grinding.  If there were no floor coverings installed for at least 30 days before you grind, you can put the calcium chloride kit down right away.  If there was a floor covering in place, you must wait 24 hours after grinding and then place the kit. 

As far as site conditions, temperature and humidity are critical.  “The test site shall be at the same temperature and humidity expected during normal use. If this is not possible, then the test conditions shall be 75 +/-10°F (23.9 65.5°C) and 50 +/-10 % relative humidity. Maintain these conditions 48 hours prior to, and during testing.+” If you can’t do the test under these conditions, don’t bother because the results may not be accurate.

Relative Humidity Testing

While the F 1869 Calcium Chloride test, done correctly, is a good test for surface moisture but it does not measure past about ¾” down.  This is a serious limitation because the concrete could be dry on top and very damp inside, and this moisture tends to move up to the surface.

Relative Humidity (RH) testing measures moisture inside the slab at 40% of the depth. This method has been used in Europe for decades and is now gaining some popularity in North America.  It is known here as ASTM F 2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using In Situ Probes. With this method, holes are drilled to 40% of the slab thickness and a plastic sleeve is inserted.  After a recommended waiting time, a relative humidity probe is used to determine the equilibrium relative humidity in the cavity at the bottom of the hole.

RH tests are more accurate than Calcium Chloride, take less time and are easily repeated days, weeks or even months after the original test.  The test method calls for a three-day waiting period after you first drill the hole, but after that you can leave the sleeve in place indefinitely and come back to insert a probe into the hole for a result in less than an hour. 

Also, there is equipment available now that the manufacturer claims will provide accurate results in a matter of hours.  ASTM F 2170 is the latest technology for moisture testing and is very quickly being recognized by flooring manufacturers throughout the world.  Another RH method, ASTM F 2420, “The Hood Method,” measures RH on the surface of the concrete slab and is a good alternative when drilling is not an option.

Why Bother? I've Never Had a Problem!

If you have been installing over concrete slabs without moisture testing and never had a problem, consider yourself very lucky.  Moisture-related problems cost the floor covering and construction industries over a billion (with a b) dollars a year in complaints.  If you consider yourself a professional in our industry you need to get up to speed on this testing.

There are two excellent certifications I’d recommend for people who want to learn more about concrete and how to test. The Clean Trust (formerly IICRC) approves instructors and schools that offer “Introduction to Substrate/Subfloor Inspection (ISSI)”, a two-day course. Also, the International Concrete Repair Institute (ICRI) offers a two-day Moisture Testing Certification that includes a “hands on” portion for moisture testing technicians.