Concrete looks solid and “done” long before it’s actually ready to accept a coating, resin system, or waterproofing membrane. Moisture is the reason. Even when a slab feels dry to the touch, it can still be holding a surprising amount of water deeper down—or pulling moisture up from the ground through capillary action. If you install a coating too early (or without the right prep), you can end up with bubbling, peeling, discoloration, soft spots, or complete bond failure.

Testing concrete moisture isn’t just a box to tick for warranty paperwork. It’s one of the best ways to avoid expensive callbacks, schedule delays, and finger-pointing between trades. The good news: you don’t need to guess. With the right testing method and a practical plan, you can make confident decisions about when to coat, when to wait, and when to choose a moisture-tolerant system.

This guide breaks down the most common moisture testing methods, how to interpret results, and how to connect those results to real-world coating and membrane decisions—especially in climates where seasonal humidity and ground moisture can change the game.

Why moisture testing matters more than people think

Moisture problems don’t always show up right away. A floor coating might look great for weeks, then suddenly develop blisters or start delaminating after a weather shift, a change in HVAC operation, or the first time the building is fully conditioned. That’s because moisture movement through concrete is dynamic—it responds to temperature, humidity, and pressure differences between the slab and the air above it.

When moisture vapor moves upward and hits a low-permeability coating, pressure builds. If the coating can’t breathe, that vapor pressure can create osmotic blistering or push the coating off the surface. Some failures are subtle at first (a few pinholes or cloudy patches), but they tend to spread once the bond is compromised.

Moisture testing helps you answer two practical questions: (1) is the slab dry enough for the product you want to install, and (2) if it isn’t, what’s the best next move—wait, mitigate, or switch systems? Getting those answers early can save you from grinding off a fresh install later.

How moisture gets into (and moves through) concrete

Concrete starts wet. A typical mix includes water for hydration, and not all of that water gets consumed in the chemical reaction. The remainder needs time to evaporate. That drying process is slow because concrete is dense and moisture has to migrate from deeper within the slab to the surface before it can leave.

Then there’s moisture from below. If there’s no vapor barrier (or it’s damaged), the slab can continuously pull moisture from the soil. Even with a vapor barrier, moisture can enter through edges, penetrations, control joints, cracks, or areas where the barrier wasn’t properly sealed.

Finally, ambient conditions matter. If the air above the slab is humid, evaporation slows down. If the building isn’t enclosed or HVAC isn’t running, you may see wildly different readings from day to day. That’s why moisture testing needs to be done thoughtfully—not randomly—and why you should document conditions when you test.

Before you test: set up conditions that make the results meaningful

Moisture testing is only as useful as the conditions under which you do it. If the building is still open to the weather, or if the HVAC hasn’t been running, your slab may not be at “service conditions.” Many product specs and industry standards assume the slab is at or near the temperature and relative humidity it will experience in normal use.

As a rule of thumb, aim to test after the building is enclosed and conditioned. If that’s not possible, at least record temperature and ambient RH at the time of testing, and understand that your results may shift once the space is stabilized. Testing too early can lead to false confidence (or unnecessary panic) depending on the day.

Surface prep also matters for some methods. Adhesives, curing compounds, sealers, or even dust can interfere with readings. If you’re planning to coat, you’ll likely be mechanically preparing the slab anyway—so consider how your prep sequence fits with your testing plan. In many cases, testing after initial surface removal gives you a clearer picture of what’s really happening in the slab.

The main moisture testing methods (and what each one actually tells you)

There isn’t one perfect test for every situation. Different tests measure different things: moisture vapor emission rate, internal relative humidity, or near-surface moisture content. The trick is choosing a method that matches your risk level, your coating type, and the decisions you need to make.

Below are the most common methods you’ll encounter on projects. Some are quick screening tools; others are considered more reliable for making go/no-go decisions.

Plastic sheet test (ASTM D4263): quick screening, not a final answer

The plastic sheet test is simple: tape down a plastic sheet (often 18 in x 18 in) to the concrete and leave it for about 16–24 hours. If you see condensation under the plastic or darkening of the concrete, moisture is present near the surface.

This test is popular because it’s cheap and fast. It can be a helpful early warning sign, especially on renovation jobs where you don’t know the slab history. If the sheet test shows obvious moisture, you know you need deeper investigation before you coat.

But it’s not quantitative. It won’t tell you whether the slab is “dry enough” for a specific product, and it can be influenced by ambient conditions. Treat it like a smoke alarm: useful for detecting a potential problem, not for diagnosing the exact cause or severity.

Calcium chloride test (ASTM F1869): measuring moisture vapor emission rate

The calcium chloride (CaCl) test measures moisture vapor emission rate (MVER) from the slab surface, typically reported as pounds of moisture per 1,000 sq ft per 24 hours. You place a dish of calcium chloride under a sealed dome on the slab for a set period, then weigh it to determine how much moisture it absorbed.

This method has been used for decades and is still specified in some projects. It’s more quantitative than the plastic sheet test, and it’s relatively straightforward to perform. For some flooring and coating systems, you’ll see limits like 3 lbs or 5 lbs MVER.

However, CaCl is a surface-based test. It reflects moisture leaving the top portion of the slab during the test period, not necessarily the internal moisture condition. If the slab has a moisture gradient (wetter at the bottom, drier at the top), CaCl can underestimate risk—especially if the slab has been drying from the top while still holding moisture deeper down.

In-situ relative humidity test (ASTM F2170): the go-to for internal moisture

In-situ RH testing is widely considered one of the most reliable ways to assess slab moisture for coatings and flooring. The method involves drilling holes to a specific depth (typically 40% of slab thickness for slabs drying from one side) and inserting RH probes. After equilibration, you read the internal relative humidity.

Why it matters: coatings and membranes don’t just interact with the surface—they interact with the moisture dynamics of the slab. Internal RH gives you a better sense of what the slab will “push” toward the surface after you install a low-permeability layer.

Most manufacturers provide RH thresholds for their systems (often 75%–85% RH, sometimes higher for moisture-tolerant products). Always follow the specific product data sheet, and make sure you’re using calibrated probes and proper procedures. Sloppy RH testing can be worse than no testing because it creates false certainty.

Moisture meters: fast mapping and troubleshooting

Handheld concrete moisture meters (pin or pinless) are great for quickly scanning large areas and identifying wet zones, but they typically don’t provide a definitive “pass/fail” number for coatings. Think of them as mapping tools: they help you decide where to place your more formal tests, and they help you find patterns.

Pinless meters are popular because they’re non-destructive and fast. They read electrical impedance or capacitance, which correlates with moisture content near the surface. Pin meters measure electrical resistance between pins. Both can be influenced by mix design, aggregate type, rebar, and surface conditions.

Used properly, meters are incredibly useful. For example, you might scan a slab and notice higher readings near exterior walls, floor drains, or a particular control joint. That’s your cue to place RH probes or CaCl kits in those areas rather than testing only in the “best-looking” spots.

Planning your test layout: where, how many, and why it matters

Moisture is rarely uniform across a slab. You can have a dry center and wet perimeter, or a wet strip where plumbing runs, or localized moisture where the vapor barrier is compromised. If you only test one or two spots, you’re basically rolling the dice.

A practical approach is to combine mapping and verification. Start by scanning the floor with a moisture meter to identify high and low zones. Then place formal tests (RH probes or CaCl kits) in representative areas: the wettest zones, typical zones, and any areas with known risk factors like penetrations or cold joints.

Project specs sometimes dictate test frequency (for example, a certain number of tests per square footage). Even if you’re not bound by a spec, it’s worth testing enough locations to build confidence. Document your test locations and results—photos, floor plans, and notes on ambient conditions can protect you later if questions come up.

Reading the numbers without getting tripped up

The biggest mistake people make is treating moisture test results as universal. They aren’t. A “good” RH number for one coating might be unacceptable for another. The same goes for MVER limits. Always tie the test method and result back to the product requirements you’re actually installing.

Also, be careful about mixing methods in a way that creates confusion. For example, a slab might show an acceptable CaCl result but a high in-situ RH result. That doesn’t mean one test is “wrong”—it may mean the slab has a moisture gradient and the internal moisture is still high. In many coating scenarios, that internal moisture is the more relevant risk.

Finally, remember that thresholds aren’t magic. If a product says “85% RH maximum,” that doesn’t mean 86% guarantees failure and 84% guarantees success. It means the manufacturer has validated performance up to that level under certain conditions. Your job is to reduce uncertainty: test well, prep well, and choose a system that matches the site reality.

Common site scenarios and how moisture testing guides your next step

Moisture testing is most valuable when it leads to a clear decision. Here are a few common scenarios you’ll run into, and how to think through them without overcomplicating the job.

New slab, tight schedule, and the coating date is locked

New construction often comes with a hard turnover date, and coatings are usually late in the schedule. If RH testing shows the slab is still too wet for the specified system, you basically have three options: accelerate drying, change the system, or push the schedule.

Accelerating drying can involve running HVAC, using dehumidifiers, increasing air movement, and keeping temperature stable. It helps, but it’s not instant—especially for thicker slabs. If you choose this route, retest and document the trend. One-time testing won’t tell you whether the slab is actually drying fast enough to hit the target by install day.

If schedule can’t move, you may need a more moisture-tolerant primer or mitigation layer—assuming the manufacturer allows it and you can still meet performance requirements. This is where early testing is gold: it gives you time to pivot before crews and materials are already on site.

Older slab with unknown vapor barrier conditions

Renovation slabs are tricky because you often don’t know what’s underneath. If there’s no intact vapor barrier, moisture can be a long-term reality, not a temporary drying issue. In-situ RH testing and mapping can help you understand whether moisture is localized (maybe from a leak) or widespread (more likely ground moisture).

If moisture is widespread and persistent, you’ll want to think in terms of mitigation and system selection rather than waiting for the slab to “dry out.” In many cases, it won’t—at least not to the thresholds some coatings require.

Also consider salts and alkalinity. Moisture movement can carry soluble salts to the surface, leading to efflorescence and high pH conditions that can interfere with adhesion. Moisture testing doesn’t measure pH, but high moisture is often a clue that you should check pH and surface condition before installing anything film-forming.

Below-grade spaces, parkades, and podium decks

Below-grade slabs and structures exposed to weather tend to see more moisture variability. You might have hydrostatic pressure events, seasonal groundwater changes, or snowmelt loading. Testing helps you avoid installing a system that can’t handle that reality.

For decks and podiums, you’re often dealing with membranes rather than decorative coatings. Moisture testing still matters because trapped moisture can cause blistering in some systems, and it can also indicate active water ingress that needs drainage or crack treatment rather than just a surface layer.

If the structure is expected to remain damp, choose products designed for that environment and detail the system properly at joints, penetrations, and transitions. Moisture testing becomes part of a bigger waterproofing strategy, not just a pre-coating checkpoint.

How moisture affects different coating and membrane types

Not all coatings react the same way to moisture. Some are more breathable, some are more sensitive, and some are designed specifically to tolerate higher internal RH. Understanding the basic categories helps you interpret test results in a practical way.

Here’s the big picture: the less permeable the system, the more careful you need to be about moisture vapor pressure—unless the system is engineered to handle it. That’s why reading the product data sheet (and not relying on “what we used last time”) matters.

Epoxies: strong, common, and sometimes moisture-sensitive

Epoxies are popular because they bond well to properly prepared concrete, build thickness, and provide excellent chemical and abrasion resistance. They’re used in everything from warehouses to commercial kitchens to industrial facilities.

But many epoxies don’t love moisture vapor pressure. If internal RH is high, you can see blistering, whitening (amine blush or moisture-related clouding), or adhesion loss. Some epoxies are formulated to be more moisture tolerant, but you need to verify the limits and the required prep.

If you’re looking at high-performance epoxy options for demanding environments, products like 5500-FF-EX-HH epoxy are often considered for their durability and application-specific performance. The key is still the same: match the system to the slab’s moisture condition and the service environment, and don’t skip the testing that tells you whether your plan is realistic.

Urethanes and traffic membranes: flexibility meets moisture reality

Urethane systems and traffic-bearing membranes are often chosen for exterior exposure, movement, and waterproofing performance. They can bridge small cracks and handle thermal cycling better than many rigid coatings.

Moisture can still cause problems if it’s trapped beneath a low-permeability layer, especially if the system isn’t designed for installation over damp concrete. Some membranes are more forgiving, but details matter: primer selection, surface dryness requirements, and cure conditions can make or break the job.

When you’re evaluating membrane options for a deck, podium, or waterproofing assembly, an elastomeric urethane membrane is a common category that balances flexibility and protection. Your moisture testing results help you decide whether you can proceed with standard primers and detailing—or whether you need additional mitigation steps first.

Grouts and specialty resin systems: moisture can impact bond and performance

Moisture isn’t only a coating issue. It can also affect resinous grout systems, especially where bond to concrete is critical or where moisture can interfere with cure and adhesion. If you’re installing machinery bases, rail systems, or precision grouting applications, moisture conditions still matter—sometimes even more because the service loads are high.

In those cases, moisture testing can be paired with surface preparation and primer strategies to ensure the substrate is stable. Don’t assume that because a product is thick or “industrial,” moisture won’t be a factor. Vapor pressure and dampness can still lead to debonding at the interface.

For projects that call for high-performance epoxy grouting, the ProGrout epoxy system is an example of a specialized approach where substrate condition, prep quality, and environmental controls all play into long-term performance. Even when the resin system is robust, you’ll get better outcomes when you treat moisture testing as part of the installation plan instead of an afterthought.

Step-by-step: a practical moisture testing workflow you can use on real jobs

If you’re looking for a repeatable process that works across many projects, this workflow keeps things organized and defensible. It’s not the only way to do it, but it’s a solid baseline you can adapt to your site conditions and product requirements.

The goal is to move from broad awareness (what’s happening across the floor) to confident decision-making (can we coat, and if not, what changes?).

1) Gather slab history and site constraints

Start with what you can learn without touching a tool. Ask: How old is the slab? Is it on grade? Is there a vapor barrier? Has the slab been exposed to rain? Was a curing compound used? Has any sealer, adhesive, or patch been applied? Are there known leaks or plumbing runs?

Also look at scheduling realities. Do you have time to wait for drying and retest? Is the building enclosed and conditioned? Are other trades about to cover the slab with materials that could trap moisture (like insulation, temporary protection, or storage)?

This context helps you interpret test results. A high RH reading on a 10-day-old slab is expected; a high RH reading on a 20-year-old interior slab might point to ground moisture, missing vapor barrier, or a leak.

2) Visually inspect and map the floor

Walk the slab and look for clues: darkened areas, efflorescence, damp smells, curling at joints, or previous coating failures. Note cracks, control joints, and penetrations. These features often correlate with moisture pathways.

Then use a moisture meter to scan and map relative readings. You’re not trying to “pass” the slab with a meter—you’re trying to understand variability. Mark areas of higher readings with tape or a floor plan so you can target formal tests.

This step is especially valuable on large footprints. It’s a lot cheaper to scan 20,000 sq ft with a meter than to over-test randomly and still miss the wet corner that later fails.

3) Choose your formal test method and place tests strategically

If you need a strong basis for coating decisions, in-situ RH testing is often the most informative. Place probes in the wettest areas you identified, plus a few “typical” spots. Follow the standard for depth, hole diameter, cleaning, and equilibration time.

If the project spec calls for CaCl tests, make sure the surface is prepared per the standard and that domes seal properly. Place tests where they reflect the risk—don’t hide them in the driest corner just to get a better number.

Whatever method you use, label each test location and document it with photos. If you ever need to justify a decision to proceed (or to delay), good documentation is your friend.

4) Record ambient conditions and watch for trends

Moisture testing isn’t isolated from the environment. Record slab temperature, air temperature, and ambient RH. If the building isn’t conditioned, note that clearly. If rain events occurred recently, note that too.

If the schedule allows, test more than once. A single snapshot can mislead you. A trend—RH dropping week over week, or staying stubbornly high—tells you what kind of problem you’re dealing with.

When you see inconsistent results, don’t panic. Investigate. Sometimes a wet area is tied to a specific cause (like a cold joint or a crack leading to a moisture source). Finding that cause can lead to a targeted fix instead of a full-floor mitigation strategy.

What to do when the slab is too wet for your spec

This is where real-world projects get interesting. You’ve tested, and the numbers don’t meet the coating or membrane requirements. Now what? The right answer depends on whether the moisture is temporary (drying) or ongoing (source-driven).

The key is to avoid “hoping it works.” Moisture-related failures are some of the most predictable failures in the coatings world—because the physics doesn’t care about deadlines.

Give the slab time (and the right conditions) to dry

If the slab is new and there’s a functioning vapor barrier, drying time may be all you need—especially if you can improve conditions. Run HVAC, keep doors closed, and consider dehumidification. Air movement helps, but only if the air can actually carry moisture away (meaning it’s not already saturated).

Be realistic about drying rates. Thick slabs, high water-cement ratios, and cool temperatures all slow drying. If you’re under time pressure, build retesting into the schedule so you’re not guessing on install day.

Also watch out for surface treatments. Some curing compounds slow evaporation significantly. If you suspect one was used, you may need mechanical removal as part of prep, and you should test after that removal to see the true condition.

Investigate and address moisture sources

If moisture is coming from below or from leaks, waiting might not help. Look for signs of water entry at perimeter walls, slab penetrations, and expansion joints. Check exterior grading and drainage. Confirm whether a vapor barrier exists and whether it’s continuous.

In some cases, the fix is outside the slab: improving drainage, sealing exterior cracks, or repairing plumbing. In others, you may need an internal mitigation strategy designed to handle ongoing vapor drive.

Don’t forget that moisture can carry alkalinity and salts. If you see efflorescence, treat it as a sign that moisture is moving through the slab—not just a cosmetic issue to sweep away.

Select a system designed for higher moisture conditions

Sometimes the smartest move is to choose a coating or primer system that can tolerate higher internal RH or MVER—provided it fits the service requirements. This isn’t “cheating”; it’s matching the system to the substrate reality.

However, be careful: “moisture tolerant” doesn’t mean “install over standing water” or “skip prep.” Most systems still require a clean, mechanically profiled surface and may require specific primers or broadcast steps to manage vapor pressure.

If you pivot systems, update your documentation and communicate clearly with stakeholders. The goal is to avoid a situation where the installer is blamed for a failure that was baked into the original specification.

Surface preparation and moisture: the relationship people overlook

Moisture testing tells you what’s happening; surface prep determines whether your coating can bond and whether it will stay bonded. These two topics are inseparable. A slab can meet moisture thresholds and still fail if the surface is contaminated, weak, or improperly profiled.

On the flip side, aggressive surface prep can change moisture behavior near the surface by removing densified laitance or sealers that were slowing evaporation. That’s why your testing plan should consider the prep plan, not treat it as a separate world.

Mechanical profiling and laitance removal

Grinding, shot blasting, or scarifying removes weak surface paste and opens the concrete pores. This improves mechanical bond and helps coatings wet out into the substrate. It can also reveal hidden issues like soft spots, curing compound residue, or patch incompatibilities.

After profiling, the slab may show different moisture behavior because you’ve removed a layer that was acting like a partial barrier. If you test before profiling and again after, don’t be surprised if results shift. The “after” result is often more relevant to how the coating will perform.

Choose the concrete surface profile (CSP) that matches your system thickness and requirements. Too smooth and you risk poor adhesion; too rough and you may trap air or require extra material to fill peaks and valleys.

Crack and joint detailing under moisture stress

Cracks and joints are moisture highways. Even if the field of the slab tests fine, moisture can concentrate at joints, especially on grade. If you coat right over them without proper detailing, you may see localized failures first—peeling lines, discoloration, or blistering that follows the joint pattern.

Detailing strategies vary: routing and filling, flexible joint treatments, reinforcing fabrics, or membrane transitions. The right choice depends on whether the joint is moving and what the system is designed to handle.

Moisture testing can help you prioritize. If your meter mapping shows consistently higher readings along a joint, treat that area as higher risk and detail it accordingly rather than assuming a uniform approach will work everywhere.

Making your moisture testing results “project-ready” for stakeholders

Even when you do everything right, you may need to explain results to a GC, owner, consultant, or another trade. Clear communication prevents misunderstandings like “the slab is dry” when what you really mean is “the slab meets the threshold for this specific system under these conditions.”

Good reporting also helps when there’s a dispute later. Moisture-related failures can turn into expensive investigations. If you have a clean paper trail, you’ll be in a much better position.

What to include in a simple moisture report

At minimum, include: test method (ASTM standard), device/probe info, calibration status if applicable, test locations (with a marked plan), date/time, slab thickness (if known), hole depth for RH probes, equilibration time, and all readings.

Add environmental conditions: air temperature, ambient RH, and slab temperature. Note whether HVAC was running and whether the building was enclosed. If there were recent weather events, note those too.

Finally, tie results to the acceptance criteria for the product being installed. Don’t just list numbers—state what the numbers mean for that system and what the recommended next step is.

How to avoid confusion when specs and manufacturer limits differ

Sometimes project specs call for one method (like CaCl) while the manufacturer prefers another (like in-situ RH). Or the spec limit might be stricter than the product requirement. This can create awkward situations on site.

If you see a mismatch early, raise it early. Ask the specifier which standard governs acceptance and whether alternate testing is acceptable. It’s much easier to resolve this before installation than after a failed floor.

If you end up performing multiple test types, present them clearly and avoid implying they’re directly interchangeable. Explain what each one measures and why you used it, especially if one method shows higher risk than another.

A few field tips that prevent expensive surprises

Moisture testing has a learning curve, but a handful of habits can dramatically improve outcomes. These are the kinds of small decisions that keep projects smooth.

They’re not replacements for standards or manufacturer instructions—just practical guardrails that help you avoid the most common pitfalls.

Don’t test only the “nice-looking” areas

It’s tempting to place tests where it’s easiest: open space, clean concrete, no equipment in the way. But moisture problems often hide at edges, near penetrations, and in shaded or cooler zones.

Use your mapping step to force yourself to test risk areas. If you’re worried about someone seeing a test hole or dome, coordinate timing so it doesn’t interfere with other work—but don’t avoid the locations that matter.

If you can only afford a limited number of tests, make them count by placing them where failure would be most likely, not where success is most likely.

Be cautious after rain events and washdowns

If the slab has been exposed to rain, snow, curing water, or washdowns, surface moisture can temporarily skew some readings. That doesn’t mean the slab is “fine” once the surface dries; it means you should understand whether you’re measuring a temporary condition or the slab’s internal moisture state.

In-situ RH testing is generally less sensitive to short-term surface wetting once properly equilibrated, but it still requires correct procedure. For surface-based methods, timing matters even more.

If the site has frequent wetting events, consider whether the final system needs to tolerate that reality. Sometimes the moisture issue isn’t just pre-install—it’s ongoing service exposure.

Retest after major changes in conditioning

If the building goes from unconditioned to conditioned, or if HVAC starts running full-time, slab moisture behavior can change. The slab may start drying faster—or you may see moisture drive increase if the interior air becomes much drier than the slab.

When schedules allow, a second round of testing after conditioning stabilizes can prevent surprises. This is especially useful when initial testing happened early and decisions were made under uncertainty.

Retesting also helps confirm that mitigation steps (like dehumidification) are actually working. If the numbers aren’t moving, you can adjust strategy before you waste time.

Moisture testing isn’t about perfection—it’s about making smart, defensible choices before you commit to materials that don’t forgive mistakes. When you combine a solid testing plan with good surface prep and a system that matches the slab’s real condition, you dramatically increase the odds that your coatings or membranes will perform the way everyone expects.