How to test concrete for moisture before you buy epoxy

How to test concrete for moisture comes down to one free test you can run tonight. Tape an 18-inch square of clear plastic to the bare slab, seal all four edges, and leave it at least 16 hours. If you find water on the underside, or a dark patch on the slab where the sheet sat, vapor is moving up through the concrete. A store-bought coating kit won’t hold on that floor.

That’s the whole test. It costs nothing, and it needs no tools. And it answers the one question the kit in your cart never brings up on its label. A store kit runs $2 to $5^[1] a square foot in materials, and the supplies the box leaves out cost extra. Sixteen hours of plastic and painter’s tape is a cheap way to find out if that money will still be on the floor in two years.

What does the plastic sheet test actually measure?

The plastic sheet test has a standard, ASTM D4263. It’s a screen, so it gives you a yes or a no, never a number. You seal clear plastic to bare concrete, wait, and look. If water vapor is coming out of the slab, the sheet traps it. You’ll see it as drops on the underside, or as a dark patch on the concrete under the plastic.

What it won’t tell you is how much. A slab with heavy drops and a slab with one faint dark ring both read “yes.” To put a real number on it, you need a calcium chloride test (ASTM F1869) or a relative humidity probe set into the slab (ASTM F2170). Both are contractor work, and both take 60 to 72 hours.

If you’re a homeowner deciding whether to open a kit, the yes or no is enough. Coating makers set their limit at about 3 pounds per 1,000 square feet per 24 hours on the calcium chloride test. On the probe test it’s 75 percent humidity inside the slab. A slab that fails the plastic sheet isn’t always over those limits. But a store kit goes on at 2 to 3 mils, with no moisture barrier primer (bids often call it a moisture mitigation primer) under it, so it has nothing to spare. Treat a yes as a stop sign, no matter where the number would have landed.

How do you run the test?

There are five steps, and only the waiting takes real time.

1. Clear and clean the spot. The plastic has to sit on bare concrete. Sweep, then scrub off any dust film or old sealer with water and a stiff brush. Let it dry a few hours first. You’re trying to read moisture coming up from below, not the mop water you just left.
2. Cut three 18-inch squares of clear plastic sheet. Clear matters, because you need to see the underside without lifting it.
3. Tape all four edges down. Seal every edge the whole way around, with no gaps at the corners. An open edge lets the trapped vapor out, and then the test reads dry when it isn’t. This is the step people skip, and it’s why the test gets called unreliable.
4. Leave it at least 16 hours. Overnight covers it. Don’t park on it or point a heater at it. And don’t open the overhead door and let a January wind blow across it.
5. Read it before you move it. First look through the plastic at the slab. Then peel up one corner and check the underside. Both reads matter, and they don’t always agree.

Where do the three squares go?

One square in the middle of the floor tells you almost nothing. That’s why so many people run this test, get a clean result, and still watch a coating lift at the edges eight months later. Vapor doesn’t rise evenly across a slab, so test where it gathers.

The center of the parking bay. This is your baseline, and it’s usually your driest read. If this square is wet, the whole slab is wet and you’re done deciding.

Within 2 feet of the overhead door. The edge of a slab poured on the ground sits closest to the dirt outside, and to wherever the gutters dump roof water. It’s also the coldest strip of the floor in winter, so water from the garage air condenses on it. Coatings fail at the door more than anywhere else, and this is why.

Against the wall you share with the house. The heated and cooled room on the other side changes the temperature and humidity through that edge of the slab. This strip is also where a plumbing leak or a bad downspout tends to show up first.

Run all three at once. Three squares of plastic and one roll of tape cost a lot less than a second coating job.

How do you read the result?

You read two surfaces, and each one counts on its own. Drops or fog on the underside of the plastic mean vapor left the slab, hit the plastic, and had nowhere to go. A dark patch on the concrete itself means moisture built up in the pores at the surface. You’ll see it as a square of deeper gray after you lift the sheet. Either one counts as a yes. Both together is a strong yes. If the slab is dry on both surfaces after 16 hours, in all three spots, it passed.

Two things can make a damp slab read dry. A slab that already has a sealer or a curing compound on it blocks the vapor from reaching the plastic. Then you’re testing the sealer, not the concrete. And a cold slab in winter gives off vapor more slowly than the same slab in April. If you test in January and plan to coat in spring, run it again in spring.

Which slabs are worth testing before you spend anything?

The test is free, so anyone about to put a coating on concrete should run it. It matters most when you see one of these warning signs:

• A slab poured on the ground before about 1990. Plastic vapor barriers under home slabs didn’t become standard until well after that. A slab without one has an open path for water from the soil.
• No plastic sheeting showing at the slab edge. Look where the slab meets the foundation wall, or where the apron steps down at the door. Sometimes you can still see the edge of the sheet.
• A garage floor that sweats in spring. Water on the floor in March and April is a warning sign, so test before you coat.
• White powder along a wall. That’s efflorescence, salt that moisture carried up through the concrete and left on the surface.
• An old coating that peeled. Many peel jobs trace back to what was under the film, not the film itself.
• A slab poured this year. New concrete holds its mix water for months after it reaches full strength.

Clay soil holds water against a slab longer than sand does. Clay covers most of the ground from central Oklahoma across the Ozarks and down into North Texas. It’s the reason moisture failures bunch up in this part of the country. It’s also why a dry read here is really good news, not something you could take for granted.

Three reads, three different next moves

You run the test before you buy because each result points to a different purchase. One of them is no purchase at all.

Nobody in a store aisle will steer you to that third row. Say the floor’s job is to hold a truck and a tool chest. Then a damp slab with a penetrating sealer costs a fraction of a coating. It can’t fail the way a film fails, because there’s no film. Plenty of good shop floors are just that.

What the contractor tests measure that this one does not

The plastic sheet is the screen. A coating maker’s data sheet is written against the two tests that give a number, because “damp” isn’t a spec. When a slab shows a warning sign before an epoxy job, a contractor tests the concrete under the coating (the substrate) one of these two ways.

• Calcium chloride, ASTM F1869. A sealed dish of drying salt sits on the prepared slab for 60 to 72 hours. The weight it gains turns into the moisture vapor emission rate, or MVER, in pounds per 1,000 square feet per 24 hours.
• Relative humidity probe, ASTM F2170. A probe goes in a hole drilled to 40 percent of the slab’s depth and settles over the same window. It reads the humidity inside the concrete, not at the surface. So it catches what the dish misses over a vapor barrier.

You don’t need either number to open a store kit. You need them when someone is about to put a written warranty on workmanship over your slab. Our concrete floor surface preparation guide covers both tests at contractor depth. The vapor barrier guide for slabs covers what belongs under the concrete in the first place. And if a failed floor is already in dispute, how to check an installer’s moisture claim walks through what a fair test looks like.

The cheapest outcome is the one where you buy nothing

Run the three squares tonight. If they come back dry, go buy the kit with a clear head, and put your weekend into prep. Acid etching leaves a shallower, smoother surface than the coating needs to grip, and that’s the next thing that will cost you. The garage floor paint and professional coating comparison lays out the gap in thickness and grip. The DIY and professional cost comparison shows that stripping a failed film runs about $1.80 a square foot, if it comes to that.

If they come back damp, you just saved the price of the kit plus the price of getting it off. That’s a good night’s work for a roll of tape.