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Don’t Epoxy Until You’ve Done This Simple Moisture Test First

efflorescence on garage floor

Efflorescence due to moisture in the concrete

How much moisture does your concrete transmit?  Before you decide to coat your garage floor with epoxy or paint, this is the question that needs to be answered before you make your final decision.  If you don’t know the answer to this question, then you need to perform a simple moisture vapor transmission (MVT) test of the surface of your floor before you attempt to apply any coating to it.  If you don’t, all your hard work may be destined for failure.

As hard and dense as concrete seems to be, it is actually very porous.  Water vapor and moisture beneath the slab can permeate to the surface of the concrete.  Because coatings such as epoxy and paint are impermeable, the water vapor can’t pass through the coating and will cause it to delaminate.  In some cases if the epoxy has a good mechanical bond, the hydrostatic pressure of the moisture can actually cause the epoxy to pull up part of the surface of the concrete with it, creating even more problems for you in the future.

You may already have signs that you have a moisture problem and you don’t even realize it.  If there are any areas of your garage floor that always seem to have a powdery residue, this is called efflorescence.  It is created when moisture travels through the concrete, condenses and evaporates, and then leaves a residue of calcium hydroxide.  Or, you may have an area of your garage floor that always seems damp.  If so, then you most likely have a moisture issue.

So, where does this moisture come from?  In simplified terms, the porosity of concrete causes it to act like a sponge.  If the ground that your slab was poured on is naturally damp due to climate or other factors such as water drainage or irrigation issues, then your slab will naturally draw in moisture from the ground.  If the surrounding air on the surface of your slab is drier than what is underneath, then the moisture that your slab has collected will be transmitted to the surface of your floor as it tries to equalize.

This reaction can be enhanced if your slab was poured below grade, which is lower than the surrounding earth.  If you live in a naturally humid area, your concrete can also absorb the moisture in the air and then release it as the humidity drops.  That is why it is good to coat your garage floor in the afternoon or early evening.

The easiest way to determine if you have a moisture problem is to test for it.  A simple way is to perform the ASTM-D-4263  Plastic Sheet Test. You can do this simply by cutting a 16”x16” piece of plastic sheeting and taping down the perimeter with duct tape to the surface of your garage floor.   After letting is sit for 24 hours, peel it up and look for any condensation on the plastic or dark spot on the surface of the concrete.  This dark spot is created from moisture.  If none exists, you should be fine.  If you do have some moisture, then you may want to consider different flooring for your garage or you can take it a step further and do a calcium chloride test.

Great information on how to perform a calcium chloride test as well as check your concrete’s PH level.

A calcium chloride test is used to determine exactly how much moisture you have emitting from your concrete.  It does this by placing a pre-weighed petri dish of calcium chloride under a sealed plastic container on your slab.  After a predetermined amount of time, you remove the petri dish, seal it, and mail it off to be tested.  As long as your results are less than 4 lbs of moisture flow per 1000 square feet per 24 hours, then you can continue.  Don’t worry; they do the math for you.  These kits are self-contained and can be purchased for around $8 a piece.  If you like, you can purchase them from here.

Just remember, of all the things that need to be considered before you decide on an epoxy or paint coating for your garage, determining if you have a moisture issue is the first step.  It’s easy and fairly quick to do and can save you from the disappointment and anguish of a failed flooring.


  1. If you are adding water to the concrete when cleaning and efflorescence forms afterwards, is this a possibility of moisture vapor problems? Or is it just because I added water to the surface. Thanks

    • Hello Michael. If you did not have efflorescence before cleaning, then the water caused a reaction when it dissolved excess lime on the surface of the concrete and created calcium hydroxide. When it dried, the calcium hydroxide reacted with the carbon dioxide in the air and created your efflorescence. It wouldn’t be a moisture vapor problem. A solution of vinegar and water scrubbed on the concrete and then rinsed well with fresh water under high pressure will usually remove it.

  2. Hi Shea,
    I have a garage floor that has been getting efflorescence during the rainy season, along with some collection of water in areas where the floor is cracked. Is there any way I can seal this up so that it stops happening.

    • Hi Barry. The first thing you want to do is make sure water drains away from the foundation of your garage and not towards it. After that, you need to repair your cracks to prevent water from coming up through them. Larger cracks should use an epoxy patch repair with or without a sand slurry depending on the size, while the smaller ones can be repaired with a polyurethane or polyurea crack repair product. Once that is done, lightly acid etch the concrete to clean up the efflorescence and open up the pores of any dense areas of the concrete. The last step would be to apply a moisture blocking penetrating sealer to prevent any moisture from coming up through the remainder of your slab.

      • Great advice. Know anyone who does this kind of work in the SF Bay area?

        • Sorry Barry, we are not familiar with your area. We suggest contacting a couple reputable concrete contractors in your area and ask them who they can recommend that can tackle the job for you.

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