An ASTM F2170 probe measures the relative humidity of the air trapped inside a hole drilled into concrete — not the surface, not a quick moisture-meter guess, but the actual humidity of the pore air deep in the slab. Probes go in at 40% of the slab’s thickness because that depth predicts what the concrete will do to a floor for years after it looks and feels bone dry.
That distinction matters more in Utah than the marketing on a moisture meter’s box suggests. The state’s housing stock is young — median build year 1992, with 38.6% of it built in 2000 or later — which means a large share of Utah floors go down over concrete that is still relatively new. New concrete does not dry the way people assume. It can look finished and feel hard within days, while the water used to mix it keeps migrating out for months. ASTM F2170 exists because that hidden water is what buckles hardwood, lifts vinyl plank seams, and fails adhesive long after the installer has left.
What does the test actually measure?
ASTM F2170 measures in-situ relative humidity — the RH of air equilibrated inside a sealed hole drilled to a specific depth in the slab. It reports a percentage, the same unit as a household hygrometer, but it is reading the internal condition of the concrete rather than the room.
This is different from a surface moisture meter, which only samples the top few millimeters and reacts to whatever the weather did that morning. A slab can read dry on the surface and still be releasing significant water lower down. The in-situ test is built to catch that gap, because a floor covering and its adhesive don’t sit on the surface reading — they sit on top of a slab that keeps equilibrating with them for the life of the floor.
Why do probes sit at 40% of slab depth?
Concrete dries unevenly through its thickness. The surface loses moisture to the air quickly; the interior holds onto it much longer. A probe at 40% depth (measured from the top of a slab drying from one side only) sits at the point that best represents the moisture the floor covering will eventually equilibrate with, once it seals the surface and stops that top layer from drying further.
If the slab can dry from both the top and the bottom — for example, a suspended slab with an open crawl space underneath — the standard drops the probe depth to 20%, because moisture is escaping from two directions and the profile through the slab is different. Getting this wrong in either direction produces a reading that looks reassuring but does not describe what the finished floor will actually experience.
What else does the test require to be valid?
The depth is only one part of the procedure. A handful of other requirements are just as easy to skip and just as capable of invalidating the result:
- A minimum 24-hour equilibration period after the probe is inserted, before any reading is taken
- At least three test locations for the first 1,000 square feet of slab, plus one additional probe for every 1,000 square feet after that
- At least one probe within three feet of every exterior wall, where slabs typically hold more moisture
- Testing done under the ambient temperature and humidity conditions the finished space will actually have — not with windows open, HVAC off, or a job-site heater running
Skip the equilibration window and the reading will run high or unstable. Test too few points and a dry patch near the middle of a room can mask a wet strip along a foundation wall. The full protocol exists because a single number from a single hole tells almost nothing about a slab that can vary meaningfully from one corner to another.
What number counts as a pass?
Most wood flooring manufacturers and adhesive manufacturers set an RH limit somewhere between 75% and 80% before their product warranty applies. Above that threshold, the manufacturer typically will not stand behind the floor if it cups, crowns, or the adhesive fails — regardless of how the installation itself went.
That threshold is set by the flooring or adhesive manufacturer, not by ASTM F2170 itself; the test only produces the number, it doesn’t set the pass/fail line. Reading the specific product’s technical data sheet, or asking a testing lab to interpret the result against the product being installed, is the step that actually matters here — a generic “under 80%” rule of thumb can still leave a specific adhesive outside its warranty terms.
How long does a slab take to reach that number?
A commonly cited rule of thumb is roughly 30 days of drying time per inch of slab thickness, under normal conditions. That figure is useful for rough planning and nothing more — it is not a specification, and it assumes typical ambient conditions that a given Utah winter or a given job site’s HVAC schedule may not deliver.
The only way to know where a specific slab actually stands is to test it. A slab that reads well under the 30-days-per-inch estimate is not unusual, and one that runs well past it isn’t either. The full mechanics of slab drying timelines, and how a schedule should account for them, are covered on how long a new Utah slab needs before flooring goes down.
Does every Utah floor need this test?
Not every floor covering is moisture-sensitive to the same degree, and not every slab is new enough to be a live concern. Solid and engineered hardwood, laminate, and many tile adhesive systems carry manufacturer RH limits that make ASTM F2170 testing a real prerequisite on new or recently poured concrete. Sheet vinyl and some resilient products are more tolerant, though still not immune to a genuinely wet slab.
Existing slabs that have been in place and dry for years, with no known water intrusion, generally don’t need this level of scrutiny before a straightforward re-floor. The test earns its cost on new construction, additions, and any slab poured within roughly the last year — which describes a meaningful share of Utah’s newer subdivisions given how much of the state’s housing stock was built after 2000. The testing and sequencing questions specific to new-build concrete are gathered on new construction flooring in Utah.
What happens after the slab passes?
Passing the RH test doesn’t mean the concrete is done changing — it means it has reached a moisture condition the chosen floor and adhesive are rated to handle. Utah’s climate keeps working on the house after that point. Once heating season arrives and indoor humidity drops toward the 30–40% range that a heated Utah home commonly sits at in winter, wood flooring gives up moisture to the dry air and shrinks. Gaps that open between boards in January are a separate, later-stage effect of the same slab-and-climate relationship that F2170 testing addresses at the front end.
Below-grade rooms carry their own version of this problem, since basement slabs sit closer to the ground and often retain moisture longer than a main-floor slab. Material selection in that setting is covered on choosing a floor for a Utah basement slab.
Where does this fit into a project’s cost and schedule?
Moisture testing itself doesn’t appear in the price ranges this site publishes, since a lab’s testing fee is a separate line item from the flooring installation cost and varies with the number of probes and the lab doing the work. What it does affect is the schedule and the material choice — a slab that fails the RH threshold either needs more drying time, a moisture mitigation system, or a floor covering with a higher tolerance.
Installed flooring costs by material and by city, along with a sense of where a specific project is likely to land, are gathered on Utah flooring cost ranges and can be run through the flooring cost estimator. Material-by-material tolerances for moisture, including which floors handle a marginal slab better than others, are laid out on the flooring materials guide. Broader prep questions — subfloor leveling, moisture mitigation, and what typically happens before installation day — sit in the installation and prep archive.
UTFloors is an independent Utah flooring resource. We do not sell or install flooring, we do not quote jobs, and nobody from this site will call you. Cost figures here are published planning ranges for Utah, not quotes — what you actually pay depends on your rooms, your subfloor and the material you choose, and the final price depends on your project. Photography is licensed stock used to illustrate materials and rooms; it is not work by anyone connected to this site. Building code and permit requirements are enforced locally in Utah, so confirm anything regulatory with your own municipality before you rely on it.
Featured image: Photo by Curtis Adams via Pexels.