When a floor coating fails in Miami-Dade, the owner almost always blames the product. Nine times out of ten the product was fine. The concrete underneath it was never measured, and the water in that concrete did exactly what water does when you seal it in.

Nobody sells you on this part, because it is a small kit sitting on a slab for two days instead of a finished photo. It is also what separates a floor you forget about from a floor you argue about. Here is what happens under your slab, what the two real tests measure, and what to ask before anyone opens a bucket in your building.

Your Slab Is Connected to the Water Table

South Florida sits on porous limestone with groundwater close to the surface. A slab-on-grade poured here is not a lid on dry dirt. It is a wick sitting on damp ground, and concrete moves water by capillary action whether or not anything is spilled on it.

Newer construction gets a polyethylene vapor retarder under the pour to interrupt that path. A large share of Miami-Dade’s housing stock, plazas, warehouses, and shop bays went in during decades when that detail was optional, and plenty of the sheeting that did go down got punctured by rebar chairs and foot traffic before the trucks arrived. Those slabs move ground moisture upward continuously, and there is no top-side repair that changes it.

That is why a slab in Miami can look chalk dry and still be pushing vapor the entire time. Evaporation off an uncoated surface keeps it looking innocent. Seal the surface and the evaporation stops, but the drive from below does not.

What Vapor Actually Does Under a Coating

Once a coating goes down, moisture arriving from underneath has nowhere to leave. Pressure builds at the bond line between the resin and the concrete, and it shows up three ways.

• Osmotic blistering. Moisture collects at the bond line and dissolves salts in the concrete. The coating film behaves like a membrane, water is drawn toward the more concentrated side, and the film domes up into fluid-filled bubbles. Open one and it is wet inside.
• Delamination. Sustained vapor pressure overcomes the adhesion outright. The coating releases in sheets, usually starting at a low spot, a slab joint, or a section that was ground least aggressively.
• Whitened patches. Moisture trapped inside the film scatters light and hazes a clear coat, and alkaline moisture attacks some resin systems chemically. Milky areas under an intact topcoat are water reporting in.

None of this is installation sloppiness. The crew can grind, mix, and roll perfectly and still lose the floor, because the failure is driven from the side of the slab nobody can see.

The Two Tests That Produce a Number

There are two measurements a professional relies on, and they answer different questions. Good specs often use both.

The Calcium Chloride Kit

A dish of dry calcium chloride is weighed, sealed to the slab under a dome, and left for a set period, commonly two to three days. The salt absorbs whatever moisture the concrete emits into that space, and the dish gets weighed again. The weight gained, converted against the dome area and elapsed time, gives a moisture vapor emission rate.

It tells you how much vapor is leaving the top surface right now. It is the older and more widely referenced method, and it is genuinely useful. Its limit is that it reads only the top layer, and only the conditions during the test window.

The In-Slab Relative Humidity Probe

A hole is drilled to a specified depth, a sleeve is set, and the hole is capped and left to equilibrate before a probe reads the relative humidity of the air inside the concrete at that depth.

This one predicts rather than describes. It measures the moisture condition of the concrete body, which is what the slab presents to a coating once the surface is sealed and the top layer can no longer breathe. On a slab-on-grade over damp ground, it is the more honest forecast of year three.

The Plastic Sheet Is a Screening Trick, Not a Test

Taping a square of plastic to the floor overnight is not nothing, and it is not a test. Lift it and find beads of condensation or a dark shadow, and you have learned something real: that slab is wet enough to be a problem, go get a proper reading. If it comes up dry, you have learned almost nothing, because a slab can pass the sheet and still blister a coating within a year.

The sheet produces an impression. The kits produce a number. Nothing can be specified from an impression, and no manufacturer’s warranty language will accept one.

What the Numbers Do to the Spec

The readings are not pass or fail on their own. They get compared against the tolerance the coating manufacturer publishes for the system being installed, and those tolerances differ substantially between a thin consumer-grade product and a high-build industrial one. That comparison produces one of three outcomes.

Comfortably inside the product’s limit: the standard system goes down over normal prep, no mitigation needed. Any contractor who prices a mitigation primer into every job without testing is being as lazy as the ones who never test at all.

Near the limit: the spec shifts to a more vapor-tolerant resin, a different primer, or mitigation as cheap insurance rather than a requirement. This is where experience with local slabs matters more than a chart.

Over the limit: the slab gets a moisture-mitigation primer, and it is not optional. Skipping it here is how a floor fails in month three.

What a Moisture-Mitigation Primer Actually Is

A mitigation primer is a high-solids epoxy engineered to bond to damp concrete and form a barrier the vapor cannot push through, rated to a specific emission or humidity level. It goes down directly on the prepared slab, before any body coat, and the rest of the system builds on top of it.

It adds a coat, typically a day to the schedule, and real material cost. It is also a fraction of what tearing out a blistered floor and starting over runs. When the reading calls for it, we put it in the spec and show you the number that put it there. Full system details are on our epoxy flooring page.

Why Skipped Tests Fail in Month Three

The timing is what fools people. A coating over an untested wet slab looks flawless on handover, because vapor pressure needs time to build against a film still gaining full hardness. Weeks pass. The first blisters are small enough to be mistaken for grit under the coating. By the second or third month there is a cluster, and after heavy rain recharges the ground, it spreads.

By then the crew that installed it is often unreachable, and if they are reachable, the conversation turns to whether you spilled something. There is no top-coat repair here. The failed coating comes off, the slab gets ground back and tested properly, and the system goes down again with mitigation matched to the reading. You pay for the test either way. The only variable is whether you paid for one floor or two.

What to Ask Before Anyone Coats Your Slab

You do not need the chemistry to protect yourself. You need four questions, and the answers tell you everything.

• “Which moisture test are you running, and when?” You want a method named, calcium chloride or an in-slab RH probe, and a time: before the quote, not on install morning.
• “What did my slab read?” A professional gives you a number. Reassurance that the slab looked dry means nobody measured.
• “What is the moisture limit for the system you are proposing?” This forces the reading to be compared against a published tolerance instead of a feeling.
• “If mitigation is needed, is it in this price?” Settle it in writing up front, so mitigation cannot reappear later as a change order.

A contractor who answers all four without hesitating has done this on South Florida slabs before. One who gets impatient at the second question is telling you how your floor ends.

We test every slab we quote across Miami-Dade, Broward, Palm Beach, and the Keys, hand you the reading, and put the whole spec in a fixed written quote before anything gets ground. The test is the cheapest part of the job and it decides all the rest.