Insights — 11 August 2026

What a concrete moisture test actually tells you

A floor coating that blisters six months after installation looks like a product failure. It almost never is. In most cases the coating did exactly what chemistry says a coating must do when it is bonded over a slab holding more moisture than the system can pass or resist. The product gets blamed, the installer gets called back, and the one number that would have predicted the whole event was never measured.

This article explains where the moisture in a slab actually comes from, what the three common tests measure — and do not measure — and how the results change what gets specified. It is the difference between a floor selected on evidence and a floor selected on hope.

Where the water is

Dry to the touch means almost nothing

Every concrete slab is mixed with far more water than the cement needs to cure. Hydration locks up part of it; the rest has to leave, and it can only leave through the surfaces the slab is allowed to dry from. A slab drying from one side — which is every slab-on-grade and every suspended slab with a membrane or metal deck below it — pushes that excess upward through the full thickness, slowly, for months and sometimes years.

Construction water is only the first source. A slab-on-grade also sits in contact with ground that never stops supplying moisture. Where a functioning vapour retarder was installed under the slab, that supply is throttled; where it was omitted, punctured or degraded — common in older Lower Mainland stock — the ground feeds the slab indefinitely. In municipalities with a high water table, the vapour drive through an unprotected slab is continuous, which is why age alone never certifies a slab as dry.

The surface, meanwhile, is the worst possible place to judge any of this. The top few millimetres exchange moisture with the room air within hours, so a slab can feel bone dry on a warm afternoon while sitting at damaging humidity two centimetres down. This is the specific trap: the zone you can touch tells you about the weather; the zone you cannot touch tells you what happens after the coating goes on.

A coating does not stop a slab drying. It relocates the drying front to the bond line — and then has to survive whatever arrives there.

The measurements

Three tests, three very different answers

The trade uses three tests, and they are not interchangeable. Two produce numbers a specification can stand on; one is a screening check that gets treated, wrongly, as a verdict.

Common concrete moisture tests compared
Test	What it measures	How deep it looks	What it is good for
Plastic sheet (ASTM D4263)	Whether condensation forms under a taped polyethylene sheet left at least 16 hours	Surface only	A quick screen. It can prove a slab is damp; it can never prove one is dry enough. Not accepted for warranties.
Calcium chloride (ASTM F1869)	The rate of water vapour leaving the surface, in pounds per 1,000 ft² per 24 hours	Roughly the top couple of centimetres	The historical benchmark many datasheets still quote. Sensitive to room conditions, which the standard requires to be controlled before and during the test.
In-situ relative humidity (ASTM F2170)	The relative humidity inside the slab, from a probe sealed in a drilled hole	40% of slab depth for a slab drying from one side	The modern reference method. Reads the moisture that will redistribute upward after the surface is sealed — which is the moisture that attacks a coating.

Depth placement and test counts are set by the ASTM standards themselves, not by preference — that is what makes the results defensible if a floor is ever disputed.

The depth difference is the whole story. Surface tests report what the slab is doing this week under this week's conditions. The in-situ probe reports what the slab is holding at depth — and once a coating seals the surface, moisture through the thickness redistributes toward equilibrium. The number at forty per cent depth today is close to the number the bond line sees after sealing. That is why the RH test predicts coating survival in a way no surface reading can.

The failure mechanism

How moisture actually takes a coating off

The dramatic version is the blister. Water arriving at the underside of a low-permeance coating dissolves salts at the bond line and sets up an osmotic cell: water keeps moving toward the concentrated solution, pressure builds under the film, and the coating lifts into fluid-filled domes. Osmotic pressures can exceed the adhesion of a well-installed system, which is why a blistered floor is rarely an installation defect — the film held on until physics outbid it.

The quieter version is chemical. Moisture moving through concrete arrives at the bond line strongly alkaline, and sustained high pH degrades some binders and adhesives from below — the industry term is saponification. The floor does not dome; it simply lets go, in patches, over a year or two, and by then nobody connects the failure to the moisture reading that was never taken.

Both mechanisms share one property: they are invisible at handover. A moisture-driven failure almost always presents after the deficiency period has started running, on a floor that looked perfect on day one. That is what makes the test disproportionate to its cost — it is a small number that predicts an expensive event months before any visual inspection could.

On a real project

What a proper moisture survey looks like

Testing is not one hole and a photograph. A survey that will stand behind a specification runs in a sequence:

Establish the slab's situation first.
Slab-on-grade or suspended, what is underneath it, whether a vapour retarder exists, and the building's history with dampness. This decides how much testing the slab warrants and where the risk concentrates.
Test at the density the standard requires.
The ASTM methods set minimum counts — on the order of three locations for the first 1,000 ft² and one for each 1,000 ft² after that — spread to include perimeters, low spots and any area with a history.
Place probes at the prescribed depth and let them equilibrate.
For in-situ RH on a slab drying one way, that is forty per cent of the slab's thickness, with readings taken once the probe has reached equilibrium — the current standard permits this at twenty-four hours.
Read under conditions that resemble service.
A reading taken in an unheated shell in February does not describe the slab under occupancy. The calcium chloride standard requires the space to be conditioned before and during the test for the same reason.
Compare the numbers to the datasheet of the actual system proposed.
Every manufacturer publishes its own moisture ceiling for each system. The reading is meaningless in isolation; it is the reading against the specific product's limit that produces a decision.

The last step is where the value lands. A reading over the limit does not kill a project — it changes it. The options are to wait and retest, to select a system with a higher tolerance, or to install a moisture-mitigating primer as a designed layer rather than a rescue. Which of those is right is an assessment question — slab, use, schedule and budget together — which is exactly why we price floors by assessment rather than blind per-square-foot rates.

On industrial and commercial floors, and equally on suspended parkade slabs before a traffic membrane, the moisture survey is part of how we scope the work — the reading happens before the system is named, not after the first blister. How the candidate systems differ is laid out in our coating systems library.

Questions we get asked

Moisture testing FAQs

How long does concrete moisture testing take?

In-situ relative humidity probes need to reach equilibrium with the slab before they are read — the current edition of ASTM F2170 permits readings twenty-four hours after installation. In practice a moisture survey adds a day or two to the front of a flooring project, which is trivial against the weeks a blistered floor costs to strip and redo.

Can a damp slab be coated at all?

Often, yes — but only once the moisture condition is measured, not guessed. Some systems tolerate higher slab humidity than others, and moisture-mitigating primers exist for slabs that read above a system's published ceiling. The test results decide whether the answer is proceed, wait, change the system, or mitigate. What fails is coating first and finding out afterwards.

My slab is decades old — surely it has dried out by now?

Age only settles the construction water. A slab-on-grade sits on ground that keeps supplying moisture indefinitely, and many older Lower Mainland buildings were built without an effective vapour retarder beneath the slab. In areas with a high water table, such as Richmond and Delta, a fifty-year-old slab can still read wet enough to blister a coating.

Is the plastic-sheet test good enough?

The taped plastic sheet (ASTM D4263) is a screening check, not a measurement. It can tell you a slab is visibly damp; it cannot tell you a slab is dry enough, and a passing sheet is not accepted by coating manufacturers as the basis for a warranty. Quantitative testing — in-situ RH or calcium chloride — is what a specification and a warranty stand on.

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