Garage Detailer

Moisture Testing

Garage Floor Moisture Testing Methods

Concrete moisture vapor emission is a natural occurrence of any concrete slab.  Concrete is a porous substance that absorbs and wicks moisture from either above or below the slab.  Most garage floors are poured with a vinyl vapor barrier underneath, however they can be damaged during pouring and tamping of the concrete, or even non-existent.  If concentrated moisture is present it will create a hydraulic pressure under your top coating making it lift, bubble and peal.

Prior to coating a garage floor, it is highly recommended you test the garage floor for moisture before applying a top coat.

Freshly poured slabs have a high water content that must evaporate during the curing process.  New concrete should be allowed a minimum 30-60 days to cure before a coating is applied. This pertains to all floors whether the slab is above or below grade. Besides the curing process of new concrete, long established floors can also wick moisture from sources below the slab, even when approved moisture barriers were installed. This moisture will naturally wick to the surface and can easily break the bond of floor coatings causing your garage floor coating system to fail.

Moisture Vapor Pressure

Humidity and temperature both have a direct effect on the pressure exerted on the concrete surface measured in “pounds per square inch”. Inside your garage the static vapor pressure is often one half the pressure seen inside or below the concrete garage floor slab.  This indifference causes the moisture to literally be sucked upward, often trapped just beneath the floor coating materials. The hydraulic pressure exerted can easily lift epoxy and other floor coatings causing them to bubble and peel. It is crucial to use even the most basic testing methods to determine if your floor has an unacceptable moisture issue prior to coating.

Moisture Tests

There are 3 tests primarily testing methods used to determine moisture level content in your garage floor concrete. These three garage floor moisture testing methods include:

1) The calcium chloride test involves measuring the amount of absorption of moisture by first placing a covered Petri dish of calcium chloride on the concrete slab.  This dish should remain in place between 60 and 72 hours. The calcium chloride will absorb any moisture vapors that transmits through the concrete floor and then you can measure the amount using a gram-weight scale with a gradation of 1/10th (0.1) gram.  Be sure to weigh the calcium chloride container before and after exposure to the concrete slab on the same scale so you have a baseline. The test results are stated in pounds per 1000ft squared in a 24 hour period.  This test should be conducted in interior conditions when the building is closed with air conditioner and systems turn on simulating actual use conditions.  The test is not recommended for exterior use.  Since most garages are normally unconditioned space with wide temperature fluctuations, this is still and acceptable method of testing the area.

Calcium Chloride Test Kit with Digital Pocket Scale

Inexpensive Calcium Chloride Test Kit with Digital Pocket Scale

Here’s where you can purchase the Calcium Chloride Test Kit and an inexpensive Digital Pocket Scale.  Spending less than $40 on this simple test can save you from the frustration of trying to remove a coating which failed.

2)  Relative Humidity using In-situ Probes is the most accurate method to determine moisture levels. This requires placing probes in the concrete and taking readings with a hygrometer. A relative humidity reading of 75% or below is acceptable for most floors. These testers are used by professionals with starting costs around $400 and up.  The Tramex Concrete Encounter Moisture Meter is a non-invasive tester that professionals may also consider that does not require the drilling of holes to install the probes.

Digital Concrete Moisture Tester

Home user may find this Digital moisture tester (shown on the left photo) with probes to do a satisfactory test.  You may also find this tester handy to have around the home for other purposes such as testing your  windows sills for moisture to prevent wood rot and checking how dry the firewood is before you purchase.

3)  Plastic Sheet Method is the cheap and dirty test for a quick test.  This involves simply taping the edges of a 2′ x 2′ (600 mm x 600 mm) square piece of vinyl plastic sheeting to the concrete and allowing the plastic to remain in place for 24 hours. When the plastic is removed, check for any moisture accumulated under the plastic. If moisture is visible, you have a moisture problem that needs to be addressed.  Using this method you should test near the four corners of the garage and in the middle of the floor.  You may find an issue with exterior drainage around the garage that only affects a corner of the garage you may miss otherwise.

What do the Test Results Mean?

Most flooring manufacturers and ASTM set the vapor transmission limit at 3 lbs/1,000 sq. ft./24 hours. If it is greater than this, you may wish to consider applying a moisture barrier product such as Aqua Lok II before applying any floor coatings. Aqualok II reacts with available alkali to form a crystalline structure, densifying concrete at the depth of penetration, reducing vapor and moisture drive, and allowing application of coatings and sealants to the concrete floor.

Alkalinity Testing

Alkalinity is another important factor to test for prior to coating concrete. The internal alkaline chemistry of concrete prevents reinforcing steel (rebar) from rusting. However, when the surface of a concrete slab has an alkalinity over 9 on a pH scale, adhesive and bonding systems will fail. Some folks use muriatic pool acid to etch the floor so the coatings will stick good.  Doing so requires neutralization of the acid and testing the PH before coating.  Baking soda and rinsing with plenty of water usually does the trick.  You can use a super Economical pH Test Strip with a Range 1.0 to 14.0 to check where the vapor testing is performed. A PH reading in the 7 range is ideal. Readings over 9 will cause devastating damage to your bond.


NOw you know the importance of some basic preliminary tests to conduct before you coat your floor system.  Be sure to perform a moisture test, even if its just by taping some plastic to the floor and also testing the PH of the concrete (shoot for a PH of 7).  We hope this information has been helpful.  Feel free to visit the following links for additional information on this topic.

Learn more DIY tips and garage floor moisture testing methods

Here’s Why Garage Floor Coatings Fail

Garage floor moisture testing methods
Article Name
Garage floor moisture testing methods
Why and how you should test your garage floor moisture level prior to coating a garage floor with epoxy or another type of garage floor coating.
Coating Works / Garage Detailer