The Concrete Moisture Test Every Epoxy Installer Should Run

Before any coating goes down, the slab should test under 75 percent relative humidity by an ASTM F2170 (1) in-slab probe, or under 3 pounds per 1,000 square feet per 24 hours by the ASTM F1869 (2) calcium chloride test. A slab that fails either number needs a mitigation primer or should not be coated at all.

The short version

• Overview: Two lab-grade tests set the number, one plastic-sheet test tells you whether to bother. The number, not the look of the slab, is what decides.
• My take: If an installer will not test a slab on or below grade, that is the whole conversation. I would not sign.
• How we price: Testing and mitigation are not flat per-square-foot line items. Anyone who quotes them before touching your slab is guessing.

The short answer

Concrete is never as dry as it looks. A slab on or below grade pulls water up from the ground and pushes it toward the surface as vapor every day, and that upward push has a name: vapor drive is water vapor moving from the wet underside of a slab toward the drier top. Seal a coating over a slab that is still driving vapor and the water collects under the film, builds pressure, and lifts the coating into blisters. That is the failure behind most of the epoxy floor peeling and blistering people write to me about.

So the job before the coating is to find out how much water the slab is moving. Two standards do that with real numbers, and one cheap test tells you whether the slab is wet enough to worry about at all. Here is what each one measures, what passes, and where they disagree.

Test one: the ASTM F2170 relative humidity probe

ASTM F2170 (1) is the relative humidity method, and it is the one I trust most. A relative humidity probe is a sensor sealed into a hole drilled into the slab, left to equilibrate, then read for the humidity deep inside the concrete rather than at the surface. The test drills to 40 percent of slab depth for a slab drying from one side, sits capped for at least 24 hours, and reports a percentage.

Surface moisture blows off in an afternoon and tells you nothing about what the slab will do six months after it is sealed. The probe reads the water actually stored inside. Most epoxy and urethane systems want the slab under 75 percent RH before anything goes down, some tolerate 80 to 85 percent, and dedicated mitigation primers push that ceiling to 95 percent. The honest con: the probes and meter are not cheap, and the slab has to sit undisturbed for a day, so a rushed crew skips it. That day is the one worth waiting for.

Test two: the ASTM F1869 calcium chloride test

ASTM F1869 (2) is the older calcium chloride test, and it measures a different thing: the moisture vapor emission rate, or MVER, which is the weight of water coming out of the top of the slab, reported in pounds per 1,000 square feet over 24 hours. A pre-weighed dish of dry calcium chloride is sealed under a plastic dome on the clean slab for 60 to 72 hours, then weighed again. The salt gains weight as it absorbs the vapor, and the gain converts to the pounds figure.

Many epoxy systems cap acceptance at 3 pounds per 1,000 square feet per 24 hours, some flake and industrial builds allow 5, and above that you are into mitigation or a hard no. The con is worth knowing: the calcium chloride test reads only the top half inch or so of the slab, so it can pass concrete that is wet deeper down. That is why ACI 302.2R (3), the concrete industry’s own moisture guidance, leans toward the RH probe for coating decisions. When the two disagree, believe the probe.

The threshold table

Here is the whole decision on one line each. This is the table to bring to a walkthrough.

Thresholds are typical acceptance values; always defer to the coating manufacturer’s own data sheet, because manufacturers and standards do not always agree, and the number on the bucket governs your warranty.

Test three: the plastic sheet DIY test, and its limits

The plastic sheet test is the one you can run yourself tonight, and it answers a yes-or-no question, not a how-much one. Tape a two-foot square of clear plastic sheeting flat to the slab on all four edges, ASTM D4263 style, and leave it 16 to 24 hours. Lift it and look. Condensation on the underside, or a slab that has gone dark under the square, means the slab is driving moisture and you have your answer: do not coat until someone runs a real test. This is the same overnight check I point people to on any slab in a basement or ground-floor room.

What it cannot do is clear a slab to be coated. A dry-looking square is not a passing number: the plastic test reads only the surface on the day you ran it, misses moisture deeper down, and swings with the weather. Use it to decide whether to spend money on a probe, never to skip one. This is also why epoxy is not a waterproofing membrane: the coating does not stop water in the slab, it just gives trapped water a lid to push against.

What passing and failing actually mean: the mitigation primer path

A pass means the slab is dry enough to move straight into surface prep, grinding to profile and then coating. A fail does not automatically kill the job. It moves you onto the mitigation path.

A mitigation primer is a high-solids epoxy primer rolled onto the bare, profiled slab that forms a vapor-tight barrier, so the moisture the slab is still driving stays under the primer instead of reaching your finish coat. A product like Tnemec Series 208 Epoxoprime MVT (4) is rated to hold back vapor up to 15 pounds per 1,000 square feet and relative humidity up to 95 percent, measured against the same F1869 and F2170 tests. That is the mechanism: it does not dry the slab, it caps it.

Here is the net-new point most pages skip. There is no honest flat price for the test or the mitigation primer before the slab is read, and you will not find one on this site. The number of coats and the primer spec depend entirely on what the slab tested at, so a contractor who quotes a flat mitigation price sight unseen is guessing at your slab’s moisture, not measuring it. This is why a basement floor runs $6 to $9 per square foot installed at the low end and climbs from there: the coating is the cheap part, the slab is the variable. See the full cost breakdown for where prep sits against the coating itself.

What to ask an installer who does not test

If a bid on a slab-on-grade or below-grade floor has no moisture test in it, that is not a discount, it is a risk transfer onto you. Ask these three questions and listen for the dodge.

• “What will this slab test at on an F2170 probe, and can you show me the reading before you coat?” A pro gives a plan to test. A gambler tells you the slab “looks fine.”
• “If it fails, what is your mitigation spec, and is that in this price or a change order later?” You want it named now, not sprung on you after the grind.
• “Which of these numbers does your coating manufacturer require in writing?” The right answer cites the data sheet. Vagueness here is your warning.

An installer who tests will have crisp answers, because the test is routine to them. One who does not will get cloudy, and that cloud is the tell. This matters most on new concrete, where a slab can read wet for months after it looks cured.

I stopped taking coating jobs a while back, but the one habit I would give any homeowner from all those years is this: the moisture test is not the expensive part of the job. It is the receipt that tells you the expensive part will hold. Skip it and you are not saving money, you are pre-paying for the redo.

Get the number before you sign

If you are holding a quote that does not mention a moisture test on a slab that touches the ground, have me check the quote before you sign it, or get a real range for your floor with testing and mitigation named as their own lines rather than buried. The rest of the moisture and prep story lives in the Learning Center.

Frequently asked questions

How do you test concrete moisture before epoxy? Two standard tests. The ASTM F2170 relative humidity probe seals a sensor into a drilled hole and reads the humidity deep in the slab after 24 hours. The ASTM F1869 calcium chloride test weighs how much vapor comes off the surface over 60 to 72 hours. A plastic sheet taped down overnight is a free screening test that tells you whether to run the real ones.

What moisture level is OK for epoxy? Most epoxy systems want the slab under 75 percent relative humidity on an F2170 probe, or under 3 pounds per 1,000 square feet per 24 hours on an F1869 calcium chloride test. Some builds allow 80 to 85 percent RH or 5 pounds. Always follow the coating manufacturer’s own data sheet, because the number on the bucket governs the warranty.

Can you epoxy a damp concrete floor? Not directly. A slab that is actively driving moisture will blister the coating from underneath as vapor collects and pushes up. If the slab fails a moisture test, it needs a vapor-mitigation primer that caps the moisture first, or it should be left uncoated until it dries and passes. Coating over a wet slab without mitigation is the classic setup for peeling.

What is the plastic sheet test? It is a free screening check. Tape a square of clear plastic flat to the slab on all four edges and leave it 16 to 24 hours. Condensation on the underside or a darkened slab means the concrete is driving moisture and needs a real test before coating. A dry square does not clear the slab to be coated, it only tells you the moisture is not obvious.

What happens if you epoxy over wet concrete? The vapor coming up through the slab hits the sealed underside of the coating, has nowhere to go, and collects into blisters that grow into peeling sheets. The coating itself is usually fine. The problem is the slab underneath, and no coating on top fixes a slab that is still pushing water. That is why the moisture test comes before the grind.

How this page is maintained

Written and reviewed by Nick Shushan, founder of EpoxyTimes, a flooring contractor with roughly 25 years in the trade, most of it in hardwood in New York, working in epoxy since 2019. I installed epoxy floors myself for years and now advise rather than take jobs. Thresholds are drawn from ASTM F2170, ASTM F1869, and ACI 302.2R, with acceptance always deferring to the coating manufacturer’s data sheet. Testing and mitigation are quoted by a contractor who has read the slab, never guessed from a photo. Last updated September 6, 2026.