What Are Acceptable Concrete Moisture Levels?

Moisture in a concrete subfloor can mean the difference between a successful flooring installation and a failed flooring installation. This is because concrete can transfer its moisture to the floor covering above, ultimately damaging your floor.

Small amounts of moisture won’t do much damage. But excessive moisture could lead to total flooring failure!

So, how do you know the concrete has the right moisture level?

Join us in a crash course on:

• Recommended concrete moisture levels
• Concrete moisture measurement methods
• Why concrete moisture levels matter

Let’s get started.

Recommended Concrete Moisture Levels

According to the ASTM standards, recommended concrete moisture levels are a relative humidity (RH) of 75% or below or whatever the manufacturer of the floor covering specifies.

Many manufacturers may allow for RH levels that are above 75% as some floors may be more durable than others.

In the end, it’s best to go with the moisture recommendations of your flooring manufacturers. They understand, better than anyone, what will harm the flooring materials they produced.

You can find recommended RH levels on their websites.

Now that we’ve talked about acceptable moisture levels, let’s look at your moisture measurement options.

Concrete Moisture Measurement Methods

You can measure concrete moisture levels with a few different methods, though some provide more exact results than others.

Let’s explore three different moisture testing methods and compare them with one another:

1. Calcium chloride
2. Concrete moisture meter
3. RH testing

Calcium chloride test method

Calcium chloride testing uses calcium chloride to determine the moisture evaporation rate of concrete.

This method comes from the ASTM F1869 standard and involves observing how much moisture the calcium chloride (CaCL) has absorbed from the concrete. This method has been a commonly used moisture test for many years, though it doesn’t really have the scientific pedigree that the RH F2170 standard has. Recently, floor covering installers have been moving away from this test method.

The CaCl test does have its limitations. For one, the ASTM standard doesn’t allow its use for lightweight concrete. And second, this test, like so many methods, only measures moisture at the surface of concrete, and can be highly influenced by surface conditions, profile, and porosity.

Why does this matter?

Well, most of concrete’s moisture exists at the bottom of the slab—not the top. So, surface testing methods don’t give a complete picture of how much moisture is in the entire slab.

Another concern with the CaCL test is that it takes 72 hours to complete the test.

Concrete moisture meter method

Concrete moisture meters can take quick moisture readings.

This non-destructive method can provide a general understanding of how much moisture a concrete slab contains.

As a surface-testing method, it can’t provide a complete assessment of a concrete slab’s moisture condition. However, it can help identify high moisture levels.

Unlike other forms of testing, it only takes mere seconds to get a reading. All you have to do is press the sensor of the meter against the concrete.

This makes it an ideal method for comparing the moisture levels of different areas of the slab, making it easy to quickly locate the areas with the most moisture. In this way, concrete moisture meters, like our C555, can be used to locate trouble areas that need further relative humidity (RH) testing.

These meters are best used in conjunction with RH testing as concrete moisture meters are not approved by ASTM standards to have the final say in whether it’s safe to install floor coverings.

Concrete moisture meters should only be used on their own if it’s not practical to use RH testing or if the floor covering manufacturer says they’re sufficient to use to determine if the concrete is ready for the floor covering.

Relative humidity testing method

The best way to determine the moisture levels of a concrete slab is to use RH testing according to the ASTM F2170 standard. RH testing indicates relative humidity instead of moisture content, providing an accurate estimation of when the concrete is ready to be used as a subfloor.

Unlike other testing methods, it tests deep within a piece of concrete instead of just on the surface. Since most of concrete’s moisture is located at the bottom of the slab, the deeper measurements will provide a more accurate understanding of the concrete’s moisture levels.

All you have to do is drill several holes in the concrete, place probes inside, and wait 24 hours for the probes to equilibrate. Then, you can get an RH reading.

And if you use our Rapid RH, you’ll often be within 3-5% of the final reading only an hour after you install the sensor, so you can get a good enough idea the same day about whether you should schedule a flooring install or if you’re likely to have to push it out a few weeks or have a conversation about next steps.

With this data in hand, you’ll be able to determine whether your concrete is dry enough to install flooring above it.

Why Concrete Moisture Levels Matter

If concrete subfloors release enough moisture they can cause the wooden floorboards above them to warp and bend.

Concrete with high moisture levels can release moisture into the materials above it, causing extensive flooring damage.

It could cause:

• Wooden floors to shrink, crack, crown, or cup
• Carpet to ripple or develop mold and mildew
• Linoleum floors to warp or become discolored
• Vinyl floors to buckle, shift, or swell

Moisture could also become a vehicle for the movement of alkaline salts, which could reach your adhesive and cause it to fail.

To prevent these issues, flooring professionals and contractors need to measure concrete moisture levels beforehand. This helps them determine whether the concrete is ready to receive the floor covering or needs to release more moisture.

Protect Your Project from Moisture

Excess moisture is the invisible enemy of flooring installers and contractors everywhere. Though you might not always be able to detect moisture with your eyes, you can definitely see its after-effects in the damage it causes to floors, ceilings, and walls.

The good news is, you can prevent these problems by making sure your concrete meets manufacturer recommended RH levels by testing with concrete moisture meters and in RH situ probes.

Ready to get started?

Check out our Rapid RH probes and C555 Concrete Moisture Meter for quick and accurate testing!

And if you want to learn the ins and outs of concrete moisture testing, sign up for our free webinar, “The Why, What, and How of Concrete Moisture Testing.”

Jason Spangler

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 September 6th, 2024