Leveling Compounds: Concrete Moisture Can Slant the Results
When embarking on a flooring project, the slightest imperfections in a concrete slab can significantly impact the outcome. While the charm of uneven stone paths in a garden may be sought after, the same cannot be said for indoor flooring.
Leveling compounds offer a solution, but as with any process, the details matter. And concrete moisture plays a big role in whether a leveling compound succeeds.
Read on to learn how concrete moisture affects leveling compounds and how to help ensure a level, long-lasting floor.
- Concrete Leveling Compound: How to Properly Prep
- Primed for Perfection
- Testing the Subfloor’s Moisture Condition
Concrete slabs are somewhat infamous for not being entirely level, even with the best care during the initial pour. Some flooring types/applications do not have the flexibility to withstand these uneven areas of a subfloor.
The standard solution is to apply a leveling compound, which can effectively level the slab surface to tolerances that are within specification. However, that leveling compound also has some requirements for successful installation and operation.
Concrete Leveling Compound: How to Properly Prep
One of the challenges of using a leveling compound is not just getting it level. It’s also working fast enough to get it in place. From the moment you start to mix, you have a limited working time to get the compound in place and smoothed out before it starts to set and loses its “self-leveling” characteristics. Check the manufacturer’s data sheet for the working time and the drying time before the next step of the project.
What does that have to do with concrete moisture?
It means that you need to be positive that the concrete slab is properly prepped and ready for the compound to go down in ideal conditions: removing all dust and dirt from the floor by vacuuming, using the appropriate cleaner for stains or spills, having all tools at the ready, and then damp mopping the slab lightly, letting it dry completely before applying the compound. Obviously, different manufacturers may have different installation/preparation requirements for their products, so consult the manufacturer directly for specific guidance.
It also means testing to be sure the slab does not have elevated relative humidity (RH) levels prior to doing any leveling. If there are elevated moisture levels in the slab, they usually need to be addressed before you proceed with the leveling portion of the project.
Side note: Although not directly related to the moisture in the slab, the amount of water that you add to the leveling powder is critical. Manufacturers have taken great pains to determine the proper amount of water to be utilized when mixing their products. If you want the products to perform “as advertised,” follow their instructions.
(The same moisture concerns are a reality if you’re using leveling compound over a wooden subfloor too, although the risks are greater because of differences in the expansion and contraction rates of the two building materials.)
Regardless of conditions at the surface of the slab or subfloor, moisture hidden within the slab can rise to the surface and cause the cured and dried compound to react by losing adherence, flaking, or breaking up.
Primed for Perfection
Manufacturers commonly recommend priming the concrete surface before applying the leveling compound. Primer improves adhesion and helps control how much the porous concrete draws water out of the compound.
In this situation, it is doubly important to know the slab’s moisture condition before proceeding. If the moisture level is high, remediation steps should be taken before the application of any primer or leveling compound, or you face the risk of a flooring failure.
Free Download – 7 Things You May Not Know about Concrete Slabs
Testing the Subfloor’s Moisture Condition
So how can you accurately determine a slab’s internal moisture condition?
Several different options are commonly used for concrete moisture testing, but only one method actually measures the moisture condition in the slab, rather than just taking a surface assessment.
RH testing has been specifically and scientifically developed to determine what the final moisture level of a concrete slab would be if the slab were sealed (with a flooring system or sealant, for example) at that point in time.
With RH testing, you can also determine the moisture conditions that the leveling compound will be exposed to over time.
RH testing, like with the Rapid RH®, works by installing a sensor in a test hole drilled in the concrete slab.
In the case of the Rapid RH, the sensor remains installed in the slab, and by means of a device called the Total Reader®, readings can be taken quickly and as often as needed.
With some other systems, the probe has to equilibrate again each time it’s inserted for a reading, which takes time.
RH testing can be used on new concrete slabs to determine whether the concrete has reached the RH level specified by the architect or the flooring manufacturer, and on older slabs to check that moisture levels haven’t changed over time.
When your floor depends on a solid, level surface, don’t let moisture conditions in the subfloor sabotage your leveling compound application. With Rapid RH testing, you can help make sure your completed floor stays on the level.
Jason has 20+ years’ experience in sales and sales management in a spectrum of industries and has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture tests. He currently works with Wagner Meters as our Rapid RH® product sales manager.
Last updated on October 14th, 2026
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We already tested our concrete subfloor with insitu probes and it showed 89% in one bedroom. According to your post you suggested remediation steps before application of self leveling… but what are those steps or choices? If we apply this “remediation” to the one room where moisture is high wouldn’t the moisture start to migrate to the rest of the house? Would we have to “remediate” the whole house? Please help we desperately want to put a floor down since we’ve been sleeping in our livingroom for over 4 years! Thanks.
Lori:
Thanks for the question. You have multiple topics you have brought up here that I can do justice with an email response. If you would like to discuss, please call 800-634-9961 X235.
Primer also keeps the concrete below from sucking the moisture out of the leveling mixture, which would make it to thick to spread itself level.
Cool post. I’ll be forwarding this to friends
Nice!
Super helpful information for DIY projects! I’m sure hiring an expert helps ensure the job is done right. Thanks for all the information.
How does the average homeowner test a new small concrete repair area (30 sq feet) for moisture content before applying leveling concrete?
And, how soon after the new concrete has been poured can leveling concrete be applied?
Thanks
Rob:
Thanks for the question. My answer to questions like this are the same whether a homeowner or contractor because there are only a few accepted methods for moisture testing. You would either need to do a relative humidity test in the concrete or a calcium chloride surface test. As far as when to apply the leveler, you need to consult the specific manufacturer’s website for their guidelines. Good luck.
I had no idea that there are tests to make sure that moisture doesn’t ruin the concrete leveling. This is a great thing to know beforehand so that the primer and other steps are not missed. My brother would love to know this as he looks into a similar service.
Really interesting! Thanks for sharing!
I hadn’t heard of that self-leveling before, but I had heard that you have to work quickly around concrete. That tip must’ve been because of the time limit based on moisture content. A good start is important but concrete moves even when settled; sometimes you’ll have to get it levelled again.
Hi Jason I recently did a test on a slab and the reading sown a H1 what does this mean
I know that the slab is wet but what does that reading stand for.ps we use mapai ultra plan
When “Hi” shows up on the LCD screen of the Easy Reader, it means that the reading is in excess of 99.4%, whether that is RH or temperature.