[Flooring Installation] Concrete Floors and Moisture

concrete flooring installation

In this two-part series, we're examining moisture's relationship with flooring and how to prevent major issues caused by it.

Check out our other installment: "[Flooring Installation] Moisture in Wood Floors"

What's the only thing more frustrating than having to redo an expensive flooring job?

Having to redo one because of an issue that could have easily been avoided in the first place.

When it comes to installing flooring over concrete slabs,  moisture is one of those issues that can lead to costly repair or replacement later on. 

For flooring installers – and the property owners they work for – the best way to avoid a costly moisture issue from a concrete foundation causing damage to flooring is by investing the time in the appropriate testing before starting a job.

Untested Concrete Slabs: A Recipe for Disaster

Consider this scenario:

New call-to-actionYou're the owner of a large chain of big-box retailers that's growing. Across the country, you're building new stores at a steady pace. Like many department stores, your stores have large areas of flooring covered in tile – a very expensive investment for a building that has the same square footage as a few acres of land.

A few weeks after your latest store opens, you get a frantic call from its manager – a big section of tile floor has lifted and cracked.

What do you do?

Certainly, a few calls are in order to get the problem assessed and fixed – two things that are sure to make for costly bills. You'll also be calling the contractor who put in the flooring. And for safety, you're closing off at least that section of the store – something that will also cost you money.

So what got you here in the first place?

More likely than not, a failure to properly test the slab to begin with.

Moisture and Concrete Slab Floors

Concrete floors and moisture have an interesting relationship.

To most, there doesn't seem to be much of a relationship at all. Beyond the water used in making concrete,  the building material doesn't appear to be affected by it.

However, this is far from the truth. Concrete, particularly when it's new and freshly poured, can absorb a great deal of moisture – more than enough to cause major problems if you're installing flooring on top of it. If the concrete hasn't had enough time to cure in a protected environment at service temperature, any moisture inside the slab can eventually be released.

In fact, a concrete slab's ability to absorb water is one of the first things to consider when installing floors on top of it. If there's too much moisture in the concrete, there's a high probability the expensive tile floor installed over top of the slab will eventually crack or come loose from a moisture issue. 

Can moisture come up through concrete?

Yes, and you'll see it in pre-existing concrete slabs.

In older buildings, it's not uncommon for there not to be a proper moisture barrier (if one exists at all) underneath the concrete foundation. With concrete being a porous material, moisture can move through the material in the form of water vapor. That's why on a rainy day or when the weather is generally more damp, you might notice a musty smell in a basement, as moisture vapor is being released through the floor. This isn't a concern in new construction – installing a concrete floor moisture barrier to prevent direct contact with the ground is standard.


Back to our store example.

What caused the titles to buckle, tent, and become unglued from the slab?

Moisture released from the concrete slab. The concrete slab wasn't tested for moisture before the tile was put down and moisture vapor was released, forcing the floor covering upward.

The worst part?

A simple concrete Rh test could have prevented this scenario in the first place.

Testing Concrete Slabs Before Flooring Installation

New call-to-actionFor any flooring installer putting in tile or any other covering over a slab, a concrete moisture test beforehand is an absolute must.

Remember our example above? If you're found liable for the issue because there wasn't appropriate testing done, you might end up footing the repair bill and replacing it at a cost to you.

It doesn't happen with a pin-type or pinless moisture meter and measuring actual moisture content in concrete. Rather, it happens by following the ASTM F-2170 and the standards it prescribes. Under the ASTM F-2170, you'll use a hygrometer with reusable probes and the in situ testing method to check the slab's relative humidity (Rh).

Why this route?

Moisture travels through concrete as water vapor i.e. humidity. Concrete Rh testing via the in situ method happens in drilled holes in the slab, in which a probe is inserted and sealed in to create a controlled microenvironment. 

A Deeper Dive: Want to learn more about the ASTM F-2170 and how to test moisture in concrete using in situ testing?  Check out our article: "A Look at the ASTM F-2170 & Why It's Important"

Or ... see in situ testing in action:


What's an Acceptable Moisture Level in Concrete Floors and Slabs?

After at least a 24-hour testing period, your probe readings should be showing a Rh of 75 to 90% or less. It’s best to meet Rh standards set by the flooring manufacturer.  

Anything beyond that threshold is almost a guarantee that the flooring – be it tile, laminate, hardwood, or even carpet – will eventually be affected by moisture and damaged. Beyond tile tenting, excessive moisture can cause a wide variety of costly issues:

  • Mold & mildew
  • Warping (wood and laminate flooring) 
  • Cupping (wood and laminate flooring) 
  • Fading/discoloration (carpet fading due to exposure)

What happens immediately when test results show excessive Rh?

For the flooring contractor, it means work doesn't start until the issue is handled. It's up to the property owner to work toward a solution, whether it's:

  • Giving the slab more time to cure
  • Running a dehumidifier to encourage water vapor release
  • Sealing the slab's surface
  • Starting a deeper investigation into the source of the moisture


But What About Moisture Meters for Concrete Slabs?

Keep in mind, pin-type and pinless meters can be used to check a slab for moisture. However, it's important to have the right perspective on the moisture meter readings – both are used for rapid evaluation. 

A pin-type meter can be used to measure surface-level moisture content while a pinless meter helps you zero-in on areas within its depth of measurement High readings from both are a sign that an area needs more investigation.


Concrete Floors and Moisture Testing for Peace of Mind

Regardless of whether it's at a large shopping center or in a finished basement, flooring is an investment.

When installed over concrete, Rh testing completed before work starts is the #1 way to make sure that investment lasts and your business is protected from liability for a flooring failure.

Moisture Can Impact All Elements of a Building

Learn more by downloading our guide, "Measuring Moisture in Buildings"

How to Measure Moisture in Buildings - Protimeter