Ask five flooring subcontractors whether they moisture-test a slab before installing, and you will often get five different answers about what "testing" even means. This is the one that matters, and it is worth understanding before it shows up as a line item — or worse, does not — on a bid.
What ASTM F2170 actually measures
ASTM F2170 is the standard test method for relative humidity (RH) in concrete floor slabs, and it measures moisture vapor emission from inside the slab, not on its surface. Small probes are inserted into drilled holes at a depth of 40% of the slab thickness (or 20% for a slab drying from both sides), sealed, and left to equilibrate for a minimum of 72 hours before being read. That depth and dwell time are not arbitrary — they are what the standard determined actually predicts how much moisture will reach a bonded floor covering over time, which a surface reading cannot.
Why a surface moisture meter gets it wrong
A handheld surface moisture meter — the kind that gives an instant reading by pressing pins or a plate against the slab — measures only the top few millimeters. Concrete dries from the surface inward, so a slab can read dry at the surface while still holding significant moisture in its core, moisture that migrates upward once flooring and adhesive seal the surface and stop it from evaporating freely. A surface reading is a fast, cheap check that tells you almost nothing about whether flooring adhesive will hold over the following year.
RH probes versus calcium chloride
The other commonly specified test, calcium chloride (ASTM F1869), measures moisture vapor emission rate at the surface over a set period rather than relative humidity at depth. Both are legitimate methods and some specifications call for one specifically, but RH probe testing per F2170 is increasingly the industry standard and the method most flooring manufacturers reference in their own warranty requirements — worth confirming which one your flooring product's warranty actually specifies before testing, since using the wrong method can mean redoing it.
What the reading means for your flooring spec
Most flooring adhesive and floor covering manufacturers set a maximum RH tolerance, commonly in the 75–85% range depending on the product, above which their warranty does not apply regardless of how carefully the floor was installed. A reading above that tolerance does not necessarily mean the project stops — it usually means a moisture mitigation system or coating rated for that reading gets installed before flooring goes down, which is a real, planned line item rather than a surprise discovered mid-installation. See our subfloor leveling and moisture mitigation service for what that step involves.
Who should be paying for this on a job
This is worth raising explicitly during bidding, because it gets skipped more often from ambiguity about whose scope it falls under than from anyone deciding to cut a corner. On new construction, the general contractor or concrete subcontractor sometimes tests before flooring is even bid. On a renovation or tenant improvement, it is frequently left to the flooring subcontractor by default. Either is fine as long as it is stated in writing — what causes disputes is nobody testing because each side assumed the other had it covered.
What we do before we bid
On any slab-on-grade commercial project, we run ASTM F2170 RH probe testing before finalizing a bid, not after award — because the reading can change the flooring recommendation, the prep scope, and the price, and none of that belongs in a change order once the job has started. Every test comes with written, dated results for your files or your architect's. See our commercial and trade page for how we scope this kind of work, our insurance and licensing details for vendor onboarding, or request a commercial bid with your project's scope and timeline.
