Is this slab dry enough to coat?

What the numbers you entered actually say

What to do next

The moisture rules that ARE published, and what they cover

Most of what circulates about slab moisture limits is manufacturer guidance repeated without attribution. Three rules we can quote come from federal construction specifications, and all three are worth knowing because they are cheap to comply with.

The middle row is the one the trade routinely gets wrong, and it is worth reading in full rather than in summary. The sentence usually quoted — a flat prohibition on coating a damp surface, decided with a taped plastic sheet — sits in the paragraph covering 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.

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 generally, ninety for a slab on grade. A slab on grade can only dry upward, so it takes three times as long to reach the same state.

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

Tape an eighteen-inch 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, is 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 proves only that the surface is not visibly wet. It puts no number on the vapour drive, and a slab can pass a sheet test comfortably while failing a quantified one. Treat a wet sheet as a stop and a dry sheet as permission to run the real test, never as the result.

It is still worth an evening, because it costs nothing and it fails fast. What it cannot be is the last test you run on a horizontal surface, which is exactly the use it usually gets.

How many places to test, and why one is never enough

A slab is not uniform. Vapour drive concentrates where fill is wetter, where drainage runs, where a slab is thinnest, and along cold edges. The usual protocol is three locations for the first thousand square feet and one more for each additional thousand — on a floor of 620 square feet that is 3 locations.

Test with the building conditioned the way it will be in service. Readings taken in an unheated building in October describe a building nobody will occupy, and the number you need is the one that applies when the heating is on and the doors are shut.