Concrete
Concrete slab moisture testing for flooring field guide
Measure how wet the slab is before flooring goes down, test deep with the RH probe, condition the building first, and meet the flooring manufacturer's number or mitigate.
Direct answer
Slab moisture testing measures how much moisture is inside a concrete slab and coming out of it before flooring goes down, because excess moisture debonds adhesive, cups wood, and blisters coatings months later. The in-situ relative humidity probe under ASTM F2170 is the modern method. The flooring manufacturer sets the pass limit.
Key takeaways
- The in-situ relative humidity probe under ASTM F2170 is the modern slab moisture test, and the flooring manufacturer's published limit sets the pass.
- ASTM F2170 requires drilling to 40 percent of slab thickness on a one-sided-drying slab, 20 percent if the slab dries from two sides.
- Let each F2170 RH probe equilibrate at least 24 hours before reading; test only after the building holds service conditions (HVAC running) about 48 hours.
- Many resilient and wood floors cap in-slab RH around 75 to 85 percent, solid wood often below 75 percent; calcium chloride caps 3 to 5 lb per 1000 sf per 24 hr.
- Run three F2170 test locations for the first 1000 sf plus one per additional 1000 sf, and the wettest representative reading governs, not the average.
What slab moisture testing is, and why skipping it owns you the failure
Slab moisture testing measures how much moisture is in a concrete slab and how much is still coming out of it, so you know whether the slab is dry enough to take flooring before you glue anything down. Concrete holds water. It holds the mix water that has not finished hydrating or evaporating, and on grade it can keep pulling water up from the ground underneath it for the life of the building. A floor goes down on top of that and seals it in. The test tells you, in a number, whether the slab is ready or whether it is going to push that moisture into your adhesive.
Here is the part that costs people real money. The flooring failure does not show up the day you install. It shows up months later, after the slab equilibrates under the sealed floor and the trapped moisture has had time to do its work. By then the crew is gone, the building is occupied, and the floor that looked perfect at closeout is lifting, cupping, or bubbling. The most expensive flooring failures trace back to a slab that was never tested, or tested wrong.
The order is simple and it does not bend. Test first. If the slab passes the flooring manufacturer's limit, install. If it fails, mitigate or wait, then retest. Skip the test and assume the slab is dry, and you have signed up to pull the floor back out at your own cost. Two related calls feed this one: whether there is a working vapor barrier under the slab, and whether the cure left anything on the surface that skews the read. Both are covered in their own guides.
What excess slab moisture actually does to the floor
Excess moisture attacks the floor in a few different ways depending on what you put down, and every one of them is a callback you eat after the job closes. The mechanism is worth knowing, because the symptom tells you what the slab did.
Moisture debonds the adhesive. Most flooring adhesives are water-based, and they hold because the bond cures and stays cured. Push moisture and high alkalinity up through the back of that bond line and the adhesive re-emulsifies or breaks down, and the floor lets go. You see it as tile that slides underfoot, sheet goods that bubble, or planks that pop loose at the seams.
Moisture cups the wood. Wood flooring takes on moisture from below, the bottom of each board swells more than the top, and the board curls up at the edges into a washboard you can feel through your shoes. Glue-down engineered and solid wood are both exposed to this, and once it cups, sanding it flat does not fix it because the moisture is still coming.
Moisture blisters the coating. On a sealed or coated slab, vapor pressure builds under the film with nowhere to go and lifts it off in blisters and delamination. And across all of it, the alkalinity rides along. Fresh concrete sits around pH 12 to 13, and that high-pH moisture chemically burns adhesives and feeds mold under the floor. The failure is always delayed, always expensive, and almost always preventable with a test.
Where the moisture comes from
Slab moisture comes from two sources, and a good test and a good plan account for both. Confuse them and you mitigate the wrong problem.
The first source is the concrete still drying. A fresh slab is full of mix water. Some of it is chemically bound as the cement hydrates, but a lot of it is free water that has to work its way out through the surface over weeks and months. A new slab reads wet because it is wet, and it dries on its own schedule if you let it. This source has an end. The slab finishes drying and stays dry, assuming nothing keeps feeding it.
The second source is ground water wicking up from below. A slab on grade sits on damp soil and a granular base, and the vapor pressure difference between the wet ground and the conditioned space above drives moisture up through the slab continuously. This source has no end. If there is no vapor barrier under the slab, or the barrier was punctured or lapped badly, the slab keeps delivering moisture forever and no amount of waiting dries it out. A test on that slab never passes, because there is nothing to wait for. The under-slab vapor barrier guide covers how that layer is supposed to stop this at the source. When a slab will not dry down no matter how long it sits, suspect the barrier before you suspect the mix.
How do you test a concrete slab for moisture?
The modern method is the in-situ relative humidity probe under ASTM F2170. You drill a hole into the slab, set a sleeve and a sensor, cap it, let it equilibrate, and read the relative humidity inside the slab at depth. That RH number, a percentage, is what the flooring manufacturer compares against its limit.
Depth is the whole point of the method. For a slab drying from one side, which is most slabs on grade over a vapor barrier, you drill to 40 percent of the slab thickness. For a slab that can dry from two sides, you drill to 20 percent. The standard fixes that depth because it is the point that predicts where the slab will equilibrate once the floor seals the top. A reading taken anywhere shallower tells you about the surface, not about the slab.
Then you wait. ASTM F2170 calls for the probe to equilibrate at least 24 hours before you take the reading, reduced from the old 72-hour wait after a precision study showed the readings settle by 24 hours, so the air in the hole comes into balance with the concrete around it. Some equipment claims a faster read, but the standard's equilibration period is what a spec and a manufacturer warranty will hold you to, so confirm the version of the standard and the manufacturer's instruction before you shorten it. Read the RH, log it, and compare it to the number the flooring maker published. RH is the method most manufacturers now write into their specs, because it measures deep where the trouble lives instead of skating across the top.
- RH (relative humidity)
- The moisture in the slab as a percentage, read by an in-situ probe at depth under ASTM F2170
- 40 percent depth
- Where you drill on a slab drying from one side; 20 percent if it dries from two sides
- Equilibration
- The wait, commonly at least 24 hours, while the probe air balances with the concrete
Why does the RH probe beat a surface test?
The RH probe beats a surface reading because it measures where the slab is going to end up, not where the surface happens to be the day you test. This is the single fact that explains most floors that fail after a slab passed a surface check.
A bare slab dries from the top, so the surface is always the driest part of the slab while the deeper concrete is still wet. Test only the surface and it can look ready while there is a reservoir of moisture sitting below it. Now put a floor down. The floor caps the surface, the slab can no longer dry upward, and the moisture that was deep redistributes until the whole slab reaches one equilibrium. That equilibrium is wetter than the surface was when you tested. The deep moisture rises into the back of your floor.
The F2170 probe reads at 40 percent depth precisely because that depth approximates the equilibrium the sealed slab will reach. It is a prediction of the future condition, which is what you actually need. A surface test answers a question nobody is asking, which is how wet the very top is on an open slab that is about to be covered.
Calcium chloride and MVER under ASTM F1869
The older quantitative method is the anhydrous calcium chloride test under ASTM F1869, which measures the moisture vapor emission rate, or MVER, off the surface of the slab. You weigh a dish of dry calcium chloride, seal it under a clear dome on the slab for a set period, commonly 60 to 72 hours, weigh it again, and calculate how much moisture came out, reported in pounds per 1000 square feet per 24 hours.
The number you will hear is 3 pounds. ASTM F710 references the F1869 result and commonly puts the cap at 3 pounds per 1000 square feet per 24 hours unless the flooring or adhesive manufacturer specifies otherwise. Some adhesives are rated higher, in the 3 to 5 pound range, so the manufacturer's published limit is the one that governs, not the round number.
The limitation is the same one that makes RH the preferred method. Calcium chloride measures only the top half inch or so of the slab, the same shallow zone that reads dry while the slab below stays wet. It is also sensitive to the air conditions over the slab during the test. Many flooring manufacturers now prefer or require the F2170 RH probe over calcium chloride for exactly this reason. If a spec still calls for calcium chloride, run it to the standard, but know what it is and is not telling you.
The surface pH test
Run a surface pH test alongside the moisture test, because moisture and alkalinity travel together and the alkalinity is what chemically attacks the adhesive. As Portland cement hydrates it forms calcium hydroxide, and the surface of fresh concrete commonly sits in the pH 12 to 13 range. That is strongly alkaline.
The test itself is quick. You wet a spot of bare slab with distilled water, let it sit briefly, and read it with pH paper or a calibrated pH meter against the manufacturer's acceptable range. A high pH reading means the adhesive bond is going to be attacked from below even if the moisture number looks borderline acceptable. Many adhesive makers publish a pH ceiling along with the RH and MVER limits, often somewhere around 9 to 10, but the manufacturer's number governs.
High pH and high moisture usually show up together, since the alkaline solution rides up on the same moisture. Treat a failed pH the way you treat a failed moisture number: do not glue down over it, and follow the manufacturer's direction on mitigation or surface treatment.
Why must the building be conditioned before the test?
The building has to be at its normal service conditions before and during the test, or the result is junk. This is the step that gets skipped under schedule pressure, and it invalidates more tests than any other single mistake.
Moisture movement in concrete depends on the temperature and humidity of the air above it. Test a slab in an unconditioned shell, with no HVAC, the doors open, and the space at outdoor conditions, and you are measuring a slab that is nothing like the slab the floor will live on. ASTM F2170 commonly requires the slab and the air above it to be at service conditions for at least 48 hours before testing, which in practice means the permanent HVAC running at the design temperature and humidity. Some specs and manufacturers ask for longer, so confirm the requirement.
The reason is simple. The slab equilibrates to the conditioned space it will actually serve. Condition the building first, hold those conditions through the equilibration and the reading, and the number means something. Test cold and open and you get a number that describes a building that will never exist. Then the real building dries differently, the slab equilibrates to conditions you never measured, and the floor finds the moisture you missed.
How many test locations do you need?
One test does not represent a floor. Slabs dry unevenly, the wet spots are local, and a single probe in a lucky dry corner passes a slab that fails ten feet away. The standards set a minimum count for this reason.
ASTM F2170 commonly calls for three test locations for the first 1000 square feet, plus one additional location for each additional 1000 square feet. The calcium chloride method under F1869 uses a similar density of tests across the floor. Those are minimums, so confirm the count against the current standard and any tighter number in the project specification.
Where you put them matters as much as how many. Spread the locations across the floor and deliberately include the spots most likely to be wet: near grade, against exterior walls, low areas where water pools, slab placed late in the schedule, and anywhere the vapor barrier might be compromised. The worst location is the one that governs, not the average. A floor passes only when its wettest representative reading passes the manufacturer's limit.
Who should run the test
Have the moisture test run by someone trained in the method and, on work that matters, by an independent third party with no stake in the result. The ICRI offers certification for concrete slab moisture testing technicians, and a certified tester is the credential a spec will ask for.
The independence is not a formality. The flooring installer who tests the slab they are about to cover has a reason to want it to pass, and so does the GC staring down a closeout date. A third-party tester has no floor to install and no schedule to make, so the number they report is the number the slab gave. When the floor later fails and the fingers start pointing, an independent test record is the document that survives the argument.
If the installer does run the test, follow the standard to the letter and document everything, because the installer's own test is the first thing that gets challenged when the floor fails. Certification, calibration records, and a clean procedure are what make a self-performed test hold up.
What RH is too high for flooring?
The flooring manufacturer sets the limit, and that number is the only one that controls whether your slab passes. Meet it and you have a warranty. Exceed it and install anyway, and the warranty is void from the moment of installation, no matter when the failure shows.
As a frame of reference, many resilient and wood flooring manufacturers publish an in-slab RH ceiling in the 75 to 85 percent range under ASTM F2170, with a lot of products landing around 75 to 80 percent. Solid wood often wants below 75 percent. For calcium chloride, the common ceiling is 3 to 5 pounds per 1000 square feet per 24 hours, with ASTM F710 referencing 3 pounds unless the manufacturer says otherwise. Those are reference ranges, not your limit. Your limit is the number printed in the installation instructions for the exact product and adhesive going down.
So the rule is short. Find the manufacturer's published RH, MVER, and pH limits for the specific floor and adhesive, test against those, and meet them. Do not test against a remembered rule of thumb and assume it covers a product you have never installed. The limits move by product, and the warranty follows the published number.
| Flooring type | Common RH ceiling (F2170) | Note |
|---|---|---|
| Resilient / sheet vinyl / LVT | 75 to 85 percent | Manufacturer's published limit governs |
| Glue-down wood | Often below 75 percent | Cups from moisture below; verify product |
| Coatings / sealers | Per manufacturer | Vapor pressure blisters the film |
| Calcium chloride (F1869) | 3 to 5 lb / 1000 sf / 24 hr | F710 references 3 lb unless mfr differs |
What if the slab fails the moisture test?
When the slab fails, you have two honest options: wait for it to dry and retest, or mitigate. You do not have the option of installing over it and hoping.
Waiting works only when the moisture is the slab still drying and there is a working vapor barrier underneath holding back the ground. Then time and conditioned air get you there, and a retest confirms it. Waiting does nothing when the source is ground water with no barrier to stop it, because the slab never dries.
Mitigation is a topical moisture barrier, most commonly a two-component epoxy membrane applied to the prepared slab surface under ASTM F3010. These systems are built to knock a high emission rate down to a level the flooring adhesive can take, with some products rated to control emission across a wide range. Mitigation adds cost and time, and it has to be applied to a properly prepared slab to bond, but it saves the floor and it saves the callback. The call is straightforward once you stop fighting it: mitigate the slab that will not dry, or budget for a floor that fails. Whatever you choose, retest or verify per the mitigation manufacturer before the flooring goes down.
Slab preparation under ASTM F710
ASTM F710 is the practice for preparing a concrete floor to receive resilient flooring, and it is the umbrella the moisture and pH tests live under. It commonly calls for the slab to be clean, smooth, structurally sound, flat, permanently dry, and free of anything that blocks the adhesive bond, and it directs that all slabs be tested for moisture and pH regardless of age or grade.
Permanently dry is the operative phrase, and it is why the moisture test is not optional under F710. The standard does not let you call a slab ready on age or appearance. It has to be tested. The surface also has to be free of curing compounds, sealers, adhesives, paint, dust, and any old residue, because all of that breaks the bond and some of it skews the moisture read.
Flatness and patching matter to the finished floor but they are a separate question from moisture. Do not let a clean, flat, freshly ground slab fool you into skipping the test. A slab can be perfectly prepped on top and still be wet through, and the prep does nothing about the water coming from below.
New slabs dry slow, so test instead of guessing
A new slab dries a lot slower than people expect, and the rule of thumb everyone quotes is roughly 30 days per inch of slab thickness under good drying conditions before it reaches flooring-ready moisture. A 4 inch slab on that rule is months, not weeks, and that assumes the building is conditioned and the slab can dry.
Treat the 30-days-per-inch number as a planning figure for the schedule, not as permission to install on day 120 without a test. Real drying time swings hard with the water-cement ratio of the mix, the temperature and humidity, whether the building is closed in and conditioned, and whether there is a vapor barrier doing its job. A low water-cement ratio mix starts with less water to lose and dries faster, which is one reason the mix design is a flooring decision, not just a structural one.
So plan the schedule around the slow rule, and then test to confirm. The slab does not care about the calendar. It is ready when the number says it is ready, and the only way to know the number is to run the test under service conditions.
No vapor barrier means no passing test
If the slab on grade has no vapor barrier under it, or the barrier was punctured, torn, or lapped open, the slab has a permanent moisture supply from the ground and a moisture test will keep failing no matter how long you wait. This is the case people waste the most time on, because they treat ground-fed moisture like drying moisture and keep retesting a slab that has no path to dry.
The tell is a slab that reads wet long past any reasonable drying time, or a slab that dries on the surface and reads high again on the next RH probe. That is ground water cycling up through a slab with nothing stopping it. No amount of conditioned air fixes a missing barrier, because the source never quits.
On a slab like that the only real options are a topical moisture mitigation membrane on top or, if the slab is not yet poured, a proper under-slab vapor barrier installed first. The under-slab vapor barrier guide covers the ASTM class, thickness, placement, and sealing that actually stop ground moisture before it ever reaches the slab. Test results are how you find the missing barrier, but they cannot substitute for it.
Curing compound can skew the test
A curing compound left on the slab can throw the moisture test and wreck the flooring bond, so it has to come off before you test and before you install. Membrane-forming curing compounds work by sealing moisture into the fresh concrete, which is the opposite of what you want when you are trying to read how much moisture is leaving the slab and trying to glue a floor to the surface.
On the calcium chloride test especially, a film on the surface holds moisture back and can make the slab read drier than it is, hiding the problem until the floor goes down and fails. On the RH test the probe reads at depth and is less fooled by the surface film, but the film still has to go because it is a bond breaker. The flooring adhesive will not hold to a slab coated in cure-and-seal.
The fix is mechanical removal, usually shot blasting or grinding, down to clean sound concrete, which also doubles as the F710 surface prep. The curing methods guide covers how to pick a cure that does not become a bond breaker in the first place, because the cleanest fix is not having to remove the compound at all. If a compound was used, plan the removal and then test the bare slab.
What to document
A moisture test nobody can find later is a test that does not protect you when the floor fails. The record is the document that decides who pays, and it is cheap insurance against a six-figure floor replacement.
Record the method and standard used, every test location on a floor plan, the RH or MVER and pH reading at each one, the slab thickness and the drill depth for RH, the date and time, the equilibration period, the service conditions during the test including air temperature and humidity, the instrument and its calibration, who ran the test and their certification, and the flooring manufacturer's published limit you tested against. If the slab was mitigated, record the product, the application, and the verification afterward.
Tie that record to the specific floor and adhesive installed, with the manufacturer's limit alongside your readings, so anyone reading it later can see the slab met the number for the product that went down. A field app like FieldOS keeps the readings, the locations, the photos, and the conditions stamped together so the record is built as the test runs, not reconstructed under pressure after a claim lands.
| Field to record | Why it matters |
|---|---|
| Method and standard (F2170 / F1869 / pH) | Defines what the number means |
| Each location on a plan | Proves coverage and finds the wet zone |
| RH / MVER / pH reading per location | The result against the limit |
| Slab thickness and drill depth | Confirms the RH probe read at 40 percent |
| Equilibration time and service conditions | A short or unconditioned test is invalid |
| Instrument, calibration, tester, certification | Makes the record defensible |
| Manufacturer limit tested against | Ties the pass to the warranty |
When the floor fails, the test record settles it
A failed floor turns into a fight, and the fight is over money. The flooring contractor blames the concrete for being wet. The concrete contractor blames the flooring for installing too soon. The GC blames both and points at the schedule. The flooring manufacturer points at its published limit and voids the warranty if the slab was over it. Everybody has a reason it is not their problem.
The moisture test record is what ends that argument. A clean, independent, documented test taken under service conditions, against the manufacturer's limit, shows whether the slab was dry enough when the floor went down. If it passed and the floor still failed, the question moves off the slab. If it was never tested, or tested wrong, the party who skipped the test usually owns the failure, and on a commercial floor that is a number with a lot of zeros.
This is the whole reason the test exists as a documented procedure with named standards, location counts, and conditions. It is not paperwork for its own sake. It is the one piece of evidence that survives the finger-pointing and assigns the cost where it belongs.
Schedule pressure does not dry the slab
The test gets squeezed at the end of the job, when the slab is barely dry, the floor is the last thing before turnover, and everyone is leaning on the schedule. Do not skip the test or shortcut the conditions to make the date. The failure costs more than the delay, every time.
Run the math on it. A delay to let a slab dry, or a few days to mitigate, is measured in days and a known cost. A flooring failure is measured in tearing out a finished floor in an occupied building, the replacement material and labor, the downtime for the owner, and the dispute over who pays. The delay is the cheap outcome.
The move is to test early enough that you know where the slab stands before the floor is the only thing left. Get an RH read while there is still time to mitigate or wait, instead of finding out the slab is wet the week the floor is scheduled. Schedule pressure is real, but it does not change the number in the slab. Only drying or mitigation does that.
Common mistakes
- No test at all, assuming the slab is dry because it looks dry or is old enough.
- A surface-only test that misses the deep moisture the sealed floor will pull up.
- Testing before the building is conditioned, so the slab is not at service conditions.
- Too few test locations, so a lucky dry spot passes a floor that is wet elsewhere.
- Testing against a remembered rule of thumb instead of the flooring manufacturer's published limit.
- Skipping the pH test, then losing the bond to alkalinity even though moisture looked acceptable.
- Leaving curing compound on the slab, which skews the read and breaks the bond.
- No mitigation after a failed test, installing over a wet slab and owning the callback.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Standards and references
ASTM F2170 is the in-situ relative humidity probe method, drilling to 40 percent of slab depth on a slab drying from one side, with a probe equilibration period commonly at least 24 hours, and it is the method most flooring manufacturers now specify because it reads where the slab will equilibrate. ASTM F1869 is the older anhydrous calcium chloride test, reporting MVER in pounds per 1000 square feet per 24 hours from the surface zone of the slab.
ASTM F710 is the practice for preparing concrete floors to receive resilient flooring, and it directs that all slabs be tested for moisture and pH regardless of age and that the slab be clean, sound, flat, and permanently dry. It commonly references an MVER cap of 3 pounds and an in-slab RH cap of 75 percent unless the flooring or adhesive manufacturer specifies otherwise. ASTM F3010 is the practice for two-component resin-based membrane moisture mitigation systems used to bring a high-emission slab down to an acceptable level.
The ICRI offers certification for slab moisture testing technicians, which is the credential a spec asks for and the basis of an independent, defensible test. The exact limits, test counts, drill depths, conditioning periods, and standard section numbers shift between editions, and the flooring or adhesive manufacturer's published number controls the pass and the warranty. Test with the RH probe, condition the building first, and meet the manufacturer's limit or mitigate. Confirm every number against the current standard and the manufacturer's instructions before you put it on a submittal.
Units, terms, and conversions
Slab moisture shows up in a few unit systems and under a few names, so the same condition reads differently across a flooring spec, a test report, and a manufacturer sheet.
Relative humidity is a percentage of in-slab RH, read at depth under ASTM F2170. Moisture vapor emission rate, MVER, is pounds per 1000 square feet per 24 hours under ASTM F1869, sometimes written lb/1000 sf/24 hr. Some sources report MVER in metric as grams per square meter per 24 hours. pH is a 0 to 14 scale of alkalinity, with fresh concrete commonly at 12 to 13. Slab thickness is in inches in the US tables, with drill depth taken as a percentage of that thickness for the RH probe.
- RH
- In-slab relative humidity as a percentage, read by an in-situ probe at depth (ASTM F2170)
- MVER
- Moisture vapor emission rate in lb per 1000 sf per 24 hr from the surface (ASTM F1869)
- pH
- Surface alkalinity on a 0 to 14 scale; fresh concrete commonly reads 12 to 13
- Service conditions
- The building at its design temperature and humidity, HVAC running, before and during the test
- Mitigation
- A topical moisture barrier, often a two-component epoxy membrane under ASTM F3010
- Equilibration
- The wait while a probe or test comes into balance with the slab before reading
FAQ
How do you test a concrete slab for moisture before flooring?
The modern method is the in-situ relative humidity probe under ASTM F2170. Drill into the slab to 40 percent of its thickness on a one-sided dry slab, set a sensor, cap it, and let it equilibrate for at least 24 hours, then read the RH. Compare that number to the flooring manufacturer's limit.
What is ASTM F2170?
ASTM F2170 is the standard test method for relative humidity in a concrete slab using in-situ probes. You drill to 40 percent of slab depth on a slab drying from one side, equilibrate the probe at least 24 hours, and read the in-slab RH. It measures deep, where the sealed slab will equilibrate under flooring.
What RH is too high for flooring?
It depends on the product, but many resilient and wood floors cap in-slab RH around 75 to 85 percent under ASTM F2170, with solid wood often below 75 percent. The flooring and adhesive manufacturer's published limit is the only one that controls the pass and the warranty, so test against that number, not a rule of thumb.
What happens if you install flooring on a wet slab?
The floor fails months later, after the sealed slab equilibrates and deep moisture rises into the adhesive. Moisture debonds the glue, cups wood flooring, blisters coatings, and the high alkalinity attacks the bond and feeds mold. The floor comes back out at your cost, and the manufacturer's warranty is void from the day of installation.
Why does RH testing beat a surface moisture test?
A bare slab dries from the top, so the surface always reads driest while the slab below stays wet. Once a floor caps the surface, the deep moisture redistributes and rises. The F2170 probe reads at 40 percent depth, which predicts that equilibrium, while a surface test measures a condition that disappears the moment you cover the slab.
How many moisture tests does a slab need?
ASTM F2170 commonly calls for three test locations for the first 1000 square feet, plus one more for each additional 1000 square feet, as a minimum. Spread them across the floor and include the likely wet spots near grade and exterior walls. The wettest representative reading governs the floor, not the average.
What do you do if the slab fails the moisture test?
Either wait for it to dry and retest, which works only when the slab is still drying over a sound vapor barrier, or mitigate. Mitigation is usually a two-component epoxy moisture barrier under ASTM F3010 that brings the emission down to what the adhesive can take. Never install over a failed slab and hope.
Why does the building need to be conditioned before testing?
Moisture movement in concrete depends on the air above it, so the slab and air must be at service conditions, commonly with HVAC running at least 48 hours before testing under ASTM F2170. Test an unconditioned shell and you measure a slab unlike the one the floor will serve, and the number is invalid.
Does a curing compound affect the moisture test?
Yes. Membrane-forming curing compounds seal moisture in and can make the slab read drier than it is, especially on the calcium chloride test, hiding the problem until the floor fails. The compound is also a bond breaker. Remove it by shot blasting or grinding to clean concrete, which doubles as the ASTM F710 surface prep, then test the bare slab.
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Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.