Moisture

Is this slab dry enough to coat?

Water vapour can move up through a slab, and a coating that blocks it can lose its bond when moisture builds up underneath. This page compares what you measured with the limit your own product publishes, because that limit is the one its maker sets for it.

This page holds no threshold of its own, on purpose

F1869 Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride and F2170 Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes are TEST METHODS. Neither publishes a number a coating must be under. That limit belongs to the manufacturer of the specific product you buy and is printed on its technical data sheet. Publishing "three pounds" or "seventy-five percent" here as though a test method said so would give a standard-shaped answer that no test method contains, so this tool asks for your product's number instead.

One qualification, because it is quoted further down this page. A federal construction specification does set a ceiling of its own for work built to that specification, and it names one of these methods to measure against. That is a specification imposing a requirement on a contract, not a standard publishing a limit for coatings in general, and it is still not the number your product will be warranted against.

Verdict

Clear to coat on moisture

Under your product's published limit by 0.8 lb per 1,000 sq ft per 24 h.

Margin to your product limit 0.8 lb per 1,000 sq ft per 24 h

As a share of the limit 16%

Test locations expected 3

Locations short 1

Days short of required cure 0

Mitigation layer needed No

What the numbers you entered actually say

The reading is under the limit your product publishes by at least the 15 percent margin this tool asks for.
Whether there is a vapour retarder under this slab is unknown, which is a reason to test rather than to assume.
This tool's counting rule gives 3 test locations for this floor size, and 2 were run. A slab can vary from place to place, so fewer locations can miss a wetter area.

What to do next

Record the readings, the date and the conditions, and keep them; a warranty claim can turn on that record.
Take the reading to the product data sheet rather than to a general rule: the limit is the manufacturer's, and so are the warranty terms.

The moisture rules that ARE published, and what they cover

Two rules we can quote come from one federal construction specification, UFGS 09 90 00 in its August 2026 edition, and both are cheap to comply with. Until 2026-10-05 this table also printed a 12 percent handheld-meter ceiling from UFGS 09 96 00; that clause belonged to the superseded November 2014 edition, the current edition has no such clause, and the site removed it rather than present an old limit as a test for your floor.

Rule	Value	Source
Cure before coating a slab on grade	90 days	UFGS 09 90 00 (August 2026) ARCHITECTURAL PAINTS AND COATINGS, paragraph 3.6.1 a (USACE / NAVFAC / AFCEC)
Do not coat damp concrete	Plastic sheet on a wall, emission test on a floor	UFGS 09 90 00 (August 2026) ARCHITECTURAL PAINTS AND COATINGS, paragraph 3.6.1 d Allowable Moisture Content (USACE / NAVFAC / AFCEC)

The second row is worth reading in full rather than in summary. A shorter sentence in the same specification, a flat prohibition on coating a damp surface decided with a taped plastic sheet, sits in paragraph 3.6.2.3, under 3.6.2 Gypsum Board, Plaster, and Stucco. The paragraph covering concrete says something more specific than that.

Do not apply epoxies to damp vertical surfaces as determined by ASTM D4263 or horizontal surfaces that exceed 169.0 micrograms moisture per second, square meter 3 lbs of moisture per 1000 square feet in 24 hours as determined by ASTM F1869.

UFGS 09 90 00 (August 2026) ARCHITECTURAL PAINTS AND COATINGS, paragraph 3.6.1 d Allowable Moisture Content (USACE / NAVFAC / AFCEC) — source, copy saved 2026-10-05; first retrieved 2026-08-24

What that changes for your floor A wall and a floor are judged by different tests inside one sentence: vertical concrete by the plastic-sheet method, horizontal concrete by a quantified emission test against a stated ceiling. The two quantities in that ceiling are the same limit written twice, metric and then English, because a UFGS section is a template a designer tailors down to one unit system. For a slab, the consequence is blunt — the plastic sheet is not the test this clause asks for.

Be exact about what that ceiling is. It is the limit ONE specification imposes on work built to it, and it binds your job only if your job is built to it. It is not the coating's limit and it is not the test method's: the method named in that sentence publishes no pass or fail value of its own, and the number your product will actually be warranted against is printed on your product's data sheet. Where a specification and a data sheet disagree, plan against whichever is tighter.

Note the specification's own distinction on cure: thirty days for concrete, stucco and masonry, ninety for a slab on grade. The specification gives no reason for the difference; ours is that a slab on grade can dry only upward, through the surface you are about to seal.

The cheapest test, and exactly what it does and does not prove

Tape a square of clear plastic sheet to the slab on all four edges and leave it overnight. Condensation underneath, or concrete visibly darker than the surrounding floor, counts as a fail. That is the method named as ASTM D4263 in the clause quoted above, and in that clause it is the test for vertical concrete, not the test asked for on a floor.

A dry sheet shows only that the surface is not visibly wet. It puts no number on the vapour drive, and a slab can pass a sheet test while failing a quantified one. Treat a wet sheet as a stop and a dry sheet as permission to run the quantified test, not as the result.

It is still worth an evening, because it costs little and it fails fast. On a horizontal surface it is a first screen, not the last test.

How many places to test

A slab can vary from place to place: wetter fill, drainage paths, thinner sections and cold edges can each change a reading. This tool uses a counting rule of 3 locations for the first thousand square feet and one more for each additional thousand. That rule is our planning allowance, not a quotation from a standard; your test method's instructions or your product data sheet may set a different count. On a floor of 620 square feet it gives 3 locations.

Test with the building conditioned the way it will be in service. A reading taken in an unheated building describes conditions the floor will not live in once the heating is on and the doors are shut.

Biggest swing factor Whether there is a vapour retarder under the slab. Without one, moisture from the ground can keep reaching the slab, and a single reading is one moment rather than the whole year.

If this reading does not clear

When not to coat a floor lists a failed or missing moisture test first, ahead of the other stop conditions on this site.
Permits and federal rules quotes the federal painting specification's concrete-moisture clause (UFGS 09 90 00, paragraph 3.6.1 d) used in the rules table above, and what that specification does and does not bind.