Moisture Testing Before You Coat

Where the moisture comes from

Concrete on grade is in contact with soil, and soil holds water. Unless there is an intact vapor barrier under the slab, that water moves upward as vapor and exits through the surface.

Older buildings frequently have no vapor barrier at all, because it was not standard practice when they were built. Many Toledo industrial buildings fall into this category.

New slabs carry their own mix water. Concrete takes a long time to release it, which is why a coating installed too early can fail even in a building with a perfect vapor barrier.

How it is measured

ASTM F2170 uses relative humidity probes drilled into the slab to a specified depth. It is the more reliable method because it measures conditions inside the slab rather than at the surface.

ASTM F1869 is the calcium chloride test, which measures how much moisture emits from the surface over a set period. It is still used but is more sensitive to surface conditions.

Either way, the readings are compared against the coating manufacturer's stated limits. If they exceed them, the system has to change.

What high readings mean for the project

High readings do not mean you cannot have a floor. They mean the specification needs a moisture-tolerant primer or a full mitigation membrane before the build coats go down.

A mitigation system adds real cost per square foot. It also converts a floor that would have failed into one that will not, which makes it the cheaper option over any reasonable time frame.

Skipping mitigation to save money on a slab that tested high is how a floor ends up blistering in year two with no warranty coverage.

Signs you probably have a problem

Previous floor coverings that failed, especially glue-down tile or carpet that released.

White powdery efflorescence on the slab surface.

Dark damp-looking patches that appear and disappear with the weather.

An existing coating with blisters that contain liquid, or that lifts in sheets rather than chipping.

What the numbers actually mean

Relative humidity testing under ASTM F2170 reports a percentage measured inside the slab at a specified depth. Most coating manufacturers publish a maximum, commonly somewhere in the eighties or low nineties depending on the system, above which their product should not be applied directly.

Calcium chloride testing under ASTM F1869 reports moisture vapour emission rate in pounds per thousand square feet per twenty-four hours. It measures what leaves the surface rather than what is inside the slab, which makes it more sensitive to ambient conditions on the day.

The two are not interchangeable and do not convert reliably between one another. Where a manufacturer specifies a limit, it will state which test that limit refers to.

Conditions during testing matter. Slabs should be tested at the temperature and humidity the building will actually operate at, because readings taken in an unconditioned building in a wet season do not represent the finished environment.

How mitigation works

A moisture mitigation system is a barrier applied to the prepared slab before the floor build-up begins. It is typically a high-solids epoxy formulated to tolerate and resist vapour pressure that would push an ordinary primer off the concrete.

It is applied over the same mechanically prepared surface as any other primer, at a film thickness the manufacturer specifies for the vapour level measured. That thickness is not optional, and it varies with the reading, which is another reason the test result matters.

Mitigation adds meaningfully to the square foot cost. Against the cost of a floor failing and being replaced in an operating building, it is inexpensive, and it is one of the few decisions in commercial flooring with an unambiguous answer.

Where readings are only marginally high, some systems are formulated to tolerate elevated moisture without a separate membrane, which can be a sensible middle path. That decision belongs to the manufacturer's technical guidance rather than to a judgement call on site.

Testing practicalities

Probes need time to equilibrate in the slab before a reading is meaningful, typically around twenty-four hours for relative humidity testing. Calcium chloride tests run for a set period, commonly sixty to seventy-two hours. Neither is an instant answer, and building that time into the programme prevents pressure to skip it.

The number of test locations scales with area, and the standards give guidance on minimums. Testing one spot in a large building tells you about one spot; slabs vary, and the variation is frequently what matters.

Where a building has a known history of moisture problems, or where the slab is below grade or was poured without a barrier, more testing rather than less is warranted.

Record the results with dates and locations. That record is the evidence base for the specification and the first thing any manufacturer asks for if a warranty claim ever arises.

Who does the testing and when

Testing is only useful if it happens at the right point in the programme, and that point is before the specification is finalised rather than after the contract is signed.

In practice this means the assessment visit rather than the mobilisation day. Testing at mobilisation, with a crew on site and materials delivered, means any high reading becomes an immediate crisis with commercial pressure attached. Testing weeks earlier makes it a specification decision instead.

Testing can be carried out by the flooring contractor, by an independent testing agency or by a manufacturer's technical representative. Independent testing costs more and is worth it where the project is large or where a dispute is foreseeable, because the results are not produced by a party with an interest in the outcome.

Whoever does it, ask for the results in writing with locations, dates, ambient conditions and the method used. A verbal assurance that the slab tested fine is worth very little a year later.

Where a building is being conditioned for the first time, or where the slab is new, retesting closer to installation is sensible because the readings move. A slab tested in an unheated building in November will not read the same in a conditioned building in March.

Where a manufacturer's warranty is being relied upon, check what testing they require and in what form. Warranty claims are routinely declined for want of documentation that would have taken an hour to produce at the time.

Vapour barriers under new slabs

For new construction, the most effective moisture control happens before the concrete is placed, and it is worth knowing what should have been specified.

A below-slab vapour retarder is a sheet membrane installed over the prepared sub-base, beneath the concrete. Its job is to stop water vapour from the ground entering the slab from below. Where one is correctly specified and correctly installed, most of the moisture problems described elsewhere in this guide simply do not arise.

Specification covers thickness and permeance, and the relevant standards set out classes for both. Thin construction polythene used as a substitute is a common shortcut and a poor one: it punctures easily during placement and its permeance is not adequate for a floor that will later receive a moisture-sensitive covering.

Installation is where most failures originate. Seams need adequate overlap and taping, penetrations for plumbing and conduit need sealing, and the sheet needs to survive the reinforcement and the pour without being torn. A membrane with holes in it performs like no membrane at all in the places that matter.

There is a longstanding argument about whether to place concrete directly on the membrane or over a layer of granular fill. Both approaches have advocates and the choice affects how the slab dries and cures, which is a decision for the structural designer rather than the flooring contractor.

None of this removes the need to test. A correctly installed barrier makes a good result likely rather than certain, and testing is what converts likely into known.

Common questions