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 the relative humidity inside the slab, not moisture on its surface. A sleeve is set in a hole drilled to 40% of the slab thickness (or 20% for a slab drying from both sides), capped, and left to acclimate for a minimum of 24 hours before the reading is taken. NWFA's installation guidelines say the method predicts what the equalized relative humidity will be through the entire slab once it is covered with flooring, which a surface reading cannot.
Why a surface moisture meter is not enough
A handheld electrical moisture meter — the kind that gives an instant reading when pressed against the slab — reflects only roughly the upper inch of the slab, according to NWFA. That means it cannot show what is happening deeper in the slab, and NWFA notes that moisture readings are likely to rise once a slab is covered when there is no effective vapor retarder beneath it. NWFA describes these meters as useful for mapping where to place quantitative tests, and says they should not be used on their own to decide whether a floor should be installed, unless the flooring manufacturer directs otherwise.
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. NWFA lists both as quantitative tests, and some specifications call for one specifically, so confirm which one your flooring product's installation instructions and warranty name before testing, since using the wrong method can mean redoing it.
What the reading means for your flooring spec
Flooring adhesive and floor covering manufacturers set their own maximum RH tolerance for each product, and NWFA tells installers to follow the adhesive and flooring manufacturers' instructions for test methods, limits and moisture control. As one published reference point, NWFA's wood flooring guidelines strongly recommend waiting for the slab to dry further, or installing a class 1 impermeable vapor retarder, when the RH reading is above 80%. A reading above that tolerance does not necessarily mean the project stops — it usually means a plan: more drying time, or a moisture mitigation system or coating rated for that reading 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 You can book a free quote when you are ready.
Looking for flooring in a specific town? We install it throughout Monmouth and Ocean County, including flooring in Brick, flooring in Manasquan and flooring in Long Branch, same crew and same warranty wherever your home is.
Frequently Asked Questions
What is ASTM F2170 concrete moisture testing?
ASTM F2170 is the standard test method for relative humidity (RH) inside a concrete floor slab, not on its surface. Sleeves are set in drilled holes at 40% of the slab thickness, or 20% for a slab drying from both sides. They are capped and left to acclimate for at least 24 hours before reading. NWFA says the result predicts the equalized relative humidity through the whole slab once flooring covers it.
Do I need to test my concrete slab before installing flooring?
For wood flooring, NWFA's installation guidelines call moisture testing extremely important on every job, and say testing specified by the flooring or adhesive manufacturer is the only way to identify whether the slab's moisture level is adequate. The reading can change the flooring recommendation, the prep scope and the price, so testing before the bid keeps those changes out of a change order.
Why can't I just use a handheld moisture meter on my concrete?
According to NWFA, a handheld electrical meter only reflects roughly the upper inch of the slab, so it cannot show what is happening deeper down. NWFA says these meters are useful for mapping where to place quantitative tests, but should not be used on their own to decide whether flooring goes down unless the flooring manufacturer directs otherwise.
What is the difference between an RH probe test and a calcium chloride test?
An RH probe test (ASTM F2170) measures relative humidity at depth inside the slab, while a calcium chloride test (ASTM F1869) measures moisture vapor emission rate at the surface over a set period. NWFA lists both as quantitative tests, and some specifications call for one specifically. Confirm which method your flooring instructions and warranty name before testing, since using the wrong one can mean redoing it.
What RH reading is too high for flooring?
There is no single number that fits every product. Adhesive and flooring manufacturers set their own limits, and NWFA tells installers to follow their instructions. For wood flooring, NWFA strongly recommends waiting for further drying or installing a class 1 impermeable vapor retarder when the reading is above 80%. Check the limit for your specific flooring. Putting hardwood on a slab? See our hardwood over concrete guide.
What happens if my slab fails the moisture test?
A high reading does not necessarily stop the project. It usually means a plan: more drying time, or a moisture mitigation system or coating rated for that reading installed before the flooring goes down, which makes it a planned line item rather than a surprise discovered mid-installation. See our subfloor leveling and moisture mitigation service for what that step involves.
Who is responsible for concrete moisture testing, the general contractor or the flooring installer?
It depends on the job, and it should be stated in writing. 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. Disputes come from nobody testing because each side assumed the other had it covered.
