Ask why a coating failed in this part of the country and the answer is almost always the same: nobody measured the slab. The product gets blamed, the installer gets blamed, but the failure was written before the first coat — in concrete pushing more moisture than the system could tolerate. This is what checking looks like, and why we treat it as non-negotiable.
Why Louisiana slabs fail coatings from below
Concrete is porous, and vapor moves through it from damp toward dry. A slab-on-grade foundation — the standard across the Baton Rouge metro — rests directly on soil that stays wet, fed by heavy rainfall and a water table that sits shallow through much of the area. Many older slabs here were also placed without a polyethylene vapor retarder underneath, so nothing interrupts the path from soil to surface.
On bare concrete, that vapor evaporates harmlessly. Under an epoxy film it has nowhere to go, so pressure builds at the bond line until the coating blisters or peels — often in the first summer or two, when heat accelerates the vapor drive. None of this is visible from the top of a dry-looking slab, which is exactly the trap. The full mechanism, along with the application-side humidity problems, is covered in how Louisiana humidity affects concrete floors. This page is about the measuring.
The tests that produce numbers
A coating decision needs data, and only two common tests produce it. The third earns its place as a screen:
| Test | What it tells you | Typical pass line | Time it takes |
|---|---|---|---|
| In-slab relative humidity probes (ASTM F2170) | What the concrete is holding deep inside — probes read RH at 40 percent of slab depth after equilibrating in sealed sleeves. | Commonly a limit near 75–80% RH, set by the coating manufacturer. | Drilled and sleeved one visit, read at least 24 hours later. |
| Anhydrous calcium chloride test (ASTM F1869) | How much moisture vapor is actually leaving the surface — measured in pounds per 1,000 square feet per 24 hours under a sealed dome. | Commonly a limit near 3 lbs / 1,000 sq ft / 24 hr. | Domes sit sealed for 60 to 72 hours, so plan on about three days elapsed. |
| Plastic sheet check (ASTM D4263) | A quick screen — taped-down polyethylene left overnight shows condensation or darkening if the slab is visibly wet. | Pass or fail only. No number, so it qualifies nothing by itself. | Overnight. Useful for deciding where the real tests should go. |
The limits shown are the figures most commonly specified across the industry; the binding number always comes from the data sheet of the system being installed.
The two quantitative tests answer different questions, which is why careful installers like seeing both. Calcium chloride reads only the top half inch or so of concrete — a surface that dried out over a mild week can pass while the body of the slab stays wet. The RH probes see that deeper story. A dry surface over a wet core is precisely the slab that passes the quick check and blisters in year two.
What a proper test sequence looks like
Map the slab first
A surface moisture meter sweeps the floor to find the wettest zones — near the door apron, along plumbing walls, wherever drainage history suggests. Quantitative tests go where the readings say, not where it is convenient to kneel.
Place enough tests to mean something
One reading on a two-car garage is an anecdote. Proper practice scales test count with area — on the order of three locations for the first thousand square feet — so a wet corner cannot hide behind a dry middle.
Let the tests run their full clock
RH probes need at least a day to equilibrate; calcium chloride domes sit for about three. A crew that grinds on Monday and coats on Tuesday did not test — the schedule left no time for the concrete to answer.
Get the numbers in writing
The readings, where they were taken, and the limit they were compared against belong on the quote. That paper separates a measured decision from a guess with a price on it.
What high readings change: the primer, not the project
A failed reading rarely kills a job. What it changes is the first layer. Standard epoxy primers are rated for slabs within normal moisture limits; when readings run above them, the system starts instead with a moisture-tolerant or vapor-barrier primer — a purpose-built epoxy that bonds through elevated moisture and throttles vapor transmission down to what the decorative coats above it can live with. The rest of the build — the body coat, the flake, the polyaspartic topcoat — goes down on a foundation that is no longer fighting the slab.
On a quote, that appears as one added line, and it is worth reading generously. Against a two-car garage (roughly 400–500 sq ft) planning at $2,500–$5,500, mitigation is a modest share — and it is the share protecting all the rest. The Baton Rouge cost guide shows where moisture work sits among the other line items, and polyaspartic vs. epoxy cost covers how the reading can tilt the system choice itself.
The price of skipping the test
Osmotic blistering
Vapor collects under the film faster than it can escape and lifts it into fluid-filled domes. The repair is not a patch — the affected coating comes off, and the moisture problem still has to be solved before anything goes back down.
Sheet delamination
Sustained vapor pressure defeats the bond across whole areas, which peel like a sticker. It tends to appear in the wettest zones of the slab — exactly the places a mapped test would have flagged first.
A warranty argument instead of a floor
When an untested floor fails, the installer blames the slab and the homeowner holds a coating nobody will stand behind. Readings taken before work, in writing, are what make a warranty conversation short.
Paying for the floor twice
Removal, retesting, mitigation, and a second install cost more than the first job did. Against that, quantitative testing is a rounding error — which is the entire economic argument for doing it every time.
The asymmetry is the argument. Moisture failures are the expensive kind — fixed with a grinder, not a mop — and also the most predictable failures in the trade. A test costing a small fraction of the project routinely predicts the exact failure that would have consumed all of it. Skipping it is not saving money; it is declining information the slab was offering. It is also how DIY kit floors end early — no kit includes a moisture test — as laid out in how long a garage floor coating lasts.
Where testing matters most around Baton Rouge
Some slabs argue for testing louder than others. New construction is one: concrete cures in days but dries for months, so a garage in a young subdivision — common in Prairieville — can look bone dry and still read far too wet to coat. Flood history is another: slabs that took standing water in the 2016 flood, particularly around Denham Springs and Central, can read elevated years later where drainage never changed. And older neighborhoods carry the vapor-retarder question — many mid-century slabs, like those in Broadmoor, were poured decades before plastic under concrete was standard practice. In every case the test is the same, and so is the logic: the slab already knows the answer; testing just asks.
Testing is what prep-first actually means
Everything we install rides on the same sequence: measure the slab, grind it to an honest profile, repair what the grinding reveals, then choose and apply a system. Moisture testing comes first because it is the only step that can quietly invalidate all the others — perfect grinding under the wrong primer still fails. That is the reasoning behind every garage floor coating we quote, and it scales straight up to commercial floors, where wash-down adds water from above — commercial kitchens being the extreme case. When you compare bids, the moisture question is the fastest filter there is: an installer who cannot tell you how they test has already told you how they prep.
Frequently asked questions
How do you test concrete for moisture before epoxy?
Two quantitative methods do the real work. In-slab relative humidity testing (ASTM F2170) reads moisture deep in the concrete through probes set at 40 percent of slab depth, and is the most reliable picture of what a slab-on-grade floor will push at a coating over time. The calcium chloride test (ASTM F1869) measures the vapor actually leaving the surface over about three days under a sealed dome. A taped plastic sheet is a useful overnight screen, but it produces no number and cannot qualify a floor on its own.
What moisture level is acceptable before epoxy goes down?
The pass line belongs to the coating manufacturer, not the installer. Commonly specified limits sit near 3 pounds per 1,000 square feet per 24 hours on a calcium chloride test and near 75 to 80 percent on an in-slab RH test, but the number that matters is the one on the data sheet of the primer being used — moisture-tolerant products are rated for readings that standard primers are not. What should never happen is coating with no reading at all.
What happens if my slab fails the moisture test?
Failing the test does not mean the floor cannot be coated — it means the system changes before anyone opens a bucket. High readings are usually answered with a moisture-tolerant or vapor-barrier primer rated for the measured level, which locks transmission down to what the finish coats can live with. If water is actively entering at a wall or threshold, that source gets fixed and the slab retested first. The only truly wrong response to a failed test is proceeding as if it had passed.
More answers on prep, testing, and finishes are on our concrete coatings FAQ.
Ask your slab before you pick a color
If your garage has flood history, if a previous coating ever bubbled on you, or if the concrete is simply young, the cheapest step in the project is finding out what the slab is doing before money goes on top of it. Tell us the square footage and the age of the concrete, and we will start with the measurement — not the sales pitch.
Serving Baton Rouge and the surrounding metro, including Prairieville, Gonzales, Denham Springs, Zachary, Central, Baker, Walker, Port Allen, Addis & Brusly, and St. Gabriel.