Is the Concrete Slab Ready for You to Install Underlayment?

Using the right underlayment and installing it correctly can make or break a flooring installation project. Doing it “right” begins with concrete slab readiness.

For tile installations, leveling of the concrete floor is critical. The “lippage” caused by height differences between adjoining tiles can cause tile damage and even safety issues.

Underlayment is a broad topic. So, to keep it simple, this article does not discuss installation over wood subfloors and provides only a few examples of underlayment and its uses. Also, the terms “underlayment” and “subfloor” are sometimes used interchangeably in the flooring industry.

In this article, “underlayment” always means a layer between the concrete subfloor and the floor covering (the surface layer).

Underlayment Examples

Underlayment can serve a variety of uses, from protecting the floor covering from moisture and movement in the concrete subfloor to satisfying sound insulation requirements in multi-floor buildings. Floor covering manufacturers will usually specify any underlayment required for their products.

Always check the floor covering manufacturer’s specifications and warranty to ensure a selected underlayment is approved for that floor covering product.

Wood Flooring

The National Wood Flooring Association (NWFA) recommends an “impermeable vapor retarder with a perm rating of less than or equal to .15” for wood flooring installation over concrete. Examples include six mils construction-grade polyethylene film, double felt, chemical retarders, or a urethane membrane.

For details, see the NWFA Installation Guidelines, which are available for purchase at www.nwfa.org.

You may need a pourable, self-leveling underlayment for glue-down and floating floors to correct minor imperfections in the concrete slab surface.

For floating wood floors, manufacturers sometimes recommend padded underlayment.

For floating wood floors, manufacturers sometimes recommend padded underlayment. The smooth surface allows the planks to move when seasonal humidity changes cause them to expand and contract. Padded underlayment can also help reduce footfall noise and provide thermal insulation. Examples are felt, foam, rubber, cork, and rubber cork-rolled goods. Some padded underlayment products for wood floors include a vapor retarding layer.

Ceramic, Porcelain, and Stone Tile

For tile installations, leveling of the concrete floor is critical. The “lippage” caused by height differences between adjoining tiles can cause tile damage and even safety issues. Mortar beds were the preferred tile leveling underlayment for many years. However, pourable, self-leveling underlayments are now more commonly used due to construction time constraints.

Carpet

As with other floor covering types, you may need a pourable, self-leveling underlayment to level and finish the slab surface. A carpet pad may also be required. Padding can provide shock absorption, noise reduction, and thermal insulation.

Noise Reduction

In multi-floor buildings, you may be required to install acoustic underlayment, sound-absorbing carpet padding, or other products to help reduce impact noise, such as footsteps, and airborne noise, such as voices and music.

Concrete Slab Preparation

Before installing underlayment, the concrete slab must be clean, free from contaminants, and dry.

Testing Concrete Moisture

To be sure the concrete slab is dry enough, first check the manufacturer’s specifications for underlayment and floor covering. Find the specified concrete moisture limit.

Then, test to ensure the moisture level does not exceed the limit. Manufacturers usually give the limit for both moisture vapor emission rate (MVER) and in situ relative humidity (RH) testing.

The MVER method (also called calcium chloride (CaCl) testing) has been used in the United States for a long time. But more and more flooring professionals now prefer situ RH testing.

MVER testing only measures the surface moisture before installing the flooring. In situ, RH testing measures moisture within the concrete slab, which travels to the surface and remains there once a non-permeable material is installed.

So, many flooring experts consider in situ RH testing to understand better whether the flooring could fail due to excess moisture in the concrete slab.

In Situ RH Testing Tools

The recognized standard for in situ RH testing is ASTM F2170. Many floor covering and underlayment manufacturers now require ASTM F2170-compliant RH testing. To purchase a copy of the ASTM F2170 standard, go to www.astm.org/Standards/F2170.htm.

In situ RH testing measures moisture within the concrete slab, which travels to the surface and remains there once a non-permeable material is installed.

When choosing a product for in situ RH testing, look for a complete reading, recording, and reporting system designed to follow the ASTM F2170 standard. For example, you will need testing probes that measure RH and temperature at each testing site and an instrument that measures the ambient temperature and RH in the room.

Per the ASTM F2170 standard, you must do three tests for the first 1000 feet and at least one additional test for each additional 1000 feet. You must also read the ambient temperature and RH during concrete RH testing. Ambient temperature and RH monitor also help you comply with the ASTM F2170 standard to maintain service conditions in the room for at least two days before testing begins.

The product should include probe dimensions and extenders that make installing testing probes at the correct depth easy. Per ASTM F2170, that is at 40% of the slab’s thickness for a slab drying from one side and at 20% for a slab drying from two sides.

The product should also include accessories that make it easy to thoroughly clean each drilled hole to remove dust that could cause false readings.

The ASTM F2170 standard requires calibration of each probe within 30 days of each use. The easiest way to meet this requirement is with single-use probes with calibration certificates from the manufacturer.

Free Download – 4 Reasons Why Your Concrete Is Taking Forever to Dry

When selecting an in situ RH testing product, think about how easy it will be to take readings and meet ASTM F2170 requirements for reporting. Look for a product that lets you record and store test probe site readings and ambient readings while you are away from the job site.

Better yet, find a product that can send the data directly to your mobile device once you return to the job site, with free mobile apps make it easy for you to create reports. An in situ RH testing system with these features makes recording and reporting faster, more accurate, and more secure.

How can you be sure the concrete slab is dry enough to install underlayment? Verify maximum moisture threshold for the flooring system and follow the ASTM F2170 standard for in situ RH testing, when applicable. What is the easiest way to do that? Select a testing system specifically designed to meet the standard.

For more information, call us at (541) 291-5123.

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 May 2nd, 2023