How to Prepare a Subfloor for a Floor Installation

Subfloor prep, particularly moisture testing, can make or break your flooring project.

All it takes is a wet subfloor, and your customers end up with warping in their wood floor, delamination in their resilient floor, or mold growth under their carpet. The list could go on.

The next thing you know, they’re calling you to deal with it.

Thankfully, you can skip the hassle with proper subfloor prep. As a result, you’ll:

• Avoid lumps, bumps, or unwanted gaps in the flooring
• Extend the life of the floor
• Maintain the floor’s beauty
• Prevent moisture damage, flooring disintegration, or other hazards
• Save the cost of repairs
• Protect your reputation

So, let’s walk through some general steps for subfloor prep and talk more specifically about our area of expertise, moisture testing. You’ll learn:

• How to prepare a concrete subfloor
• How to remove moisture from the subfloor
• Ways to test for moisture

How to Prepare a Concrete Subfloor for Various Floor Types

Whether you’re preparing a subfloor for hardwood flooring, luxury vinyl plank (LVP), laminate, or engineered hardwood, the goal is a clean, dry, and level subfloor. Start by assessing the existing subfloor and figuring out what needs to be done. Look for:

• Evidence of moisture issues (salt deposits or efflorescence)
• Unevenness
• Cracks or gaps

Use a level or a straight edge to check the slab’s flatness.

Once you have an idea of the floor’s condition, it’s time to move on to three key aspects of subfloor prep:

1. Moisture testing

First, ensure that the installation environment is at its service temperature and humidity. ASTM F2170 calls for these conditions for at least 48 hours before testing. Then, you’re ready to begin moisture testing.

In-situ relative humidity (RH) testing, the test method in ASTM F2170, is widely considered the most reliable method for concrete moisture testing. Unlike surface-based tests, it tells you about the moisture deep in the slab.

The reason? After concrete is poured, it begins to dry from the surface. Moisture will migrate to the surface and evaporate, leaving the surface drier than deep within.

With an RH test, you drill into the concrete to 40% of the slab’s depth (20% if the slab dries from both the top and bottom) and insert sensors there to get an idea of how much moisture is deep within the concrete. Once a floor covering goes down, that moisture can move toward the top. If the RH at that depth is within the floor covering manufacturer’s limit, you’re much less likely to have moisture issues.

ASTM F2170 doesn’t set a pass/fail limit. The acceptable RH comes from the floor covering manufacturer, and 75% is a common limit.

At times, floor installers may opt to use a concrete moisture meter, depending on their business model and manufacturer requirements. Later, we’ll help you determine which option you should choose.

2. Leveling

An uneven subfloor will result in an uneven floor. And having a level subfloor also has structural implications. It’ll help prevent the floor covering from cracking or warping due to the pressure of higher and lower spots.

For high spots, use a sander or grinder to even them out.

For low spots, use a self-leveling compound. Before you begin, make sure the floor is clean and all the tools and materials are prepared.

3. Cleaning

After you’re finished grinding and leveling the subfloor, use a vacuum, broom, or damp mop to clean it. Make sure you remove any debris, dust, old adhesives, paint, or coatings as these could prevent bonding if the floor is a glue-down floor.

How to Remove Moisture from the Subfloor

After it’s been poured, the concrete needs time to dry before it’s ready to have flooring installed on top of it. The reason for this is that concrete starts out with a large amount of water. In fact, a truckload of concrete can contain more than 320 gallons of water!¹

Unfortunately, the Contract Flooring Association lists, among the main causes of moisture-related flooring failures, fast-track projects that don’t allow “sufficient time for the construction moisture in the subfloors to reach acceptable levels.”²

To speed up the process, use dehumidifiers and fans. Also, ensure that the temperature and relative humidity remain at in-service levels. Otherwise, fluctuations can impact the moisture levels in the concrete.

How will you know when the concrete is dry enough? Moisture testing will help. Let’s look at which method is best for your business.

Which Method Should You Use to Test Moisture in a Concrete Slab?

In the flooring industry, there are three common ways to test the moisture of a concrete slab: RH testing, calcium chloride testing, and a concrete moisture meter.

Though calcium chloride testing is a traditional method that has been used for many years, we don’t recommend it because it only measures moisture coming off the top of the slab, not the moisture deeper in the concrete. In fact, the standard for this test, ASTM F1869, doesn’t allow its use on lightweight concrete.

For that reason, we’ll focus on RH testing and concrete moisture meters below. The one you choose will depend on the type of projects you’re working on and your business model.

Here are some things to consider in your decision:

Relative humidity testing

As we’ve already mentioned, RH testing is widely considered the most reliable option. The sensors take about 24 hours to equilibrate, but once equilibration is finished, you’ll get readings within minutes.

For large, high-stakes projects, particularly commercial ones, RH testing is ideal because it allows you to be sure of the floor condition before you put a floor covering on top. This way, you don’t have to worry about repairing flooring worth tens of thousands of dollars.

Our Rapid RH^® L6 test kit provides you with everything you need to perform RH testing per ASTM F2170. It also connects with the DataMaster™ smartphone app so that you can record and store your data and print reports.

Concrete moisture meters

Though RH testing is widely considered the most reliable, it’s a little more time-consuming and may require multiple trips to the jobsite. This may not be possible for your business. If you’re working on smaller residential projects and you want to avoid going to the jobsite more than once or waiting days for results, then you might opt for a concrete moisture meter.

These meters provide quick and easy readings of the top half-inch of the slab. All you have to do is press the meter on the concrete, and you’ll have a reading in seconds.

ASTM F2659, the guide for concrete moisture meters, covers them as a preliminary evaluation tool, not a final determination of concrete moisture condition, but some floor covering manufacturers do allow their use. Be sure to check with them first.

If you decide to go with a concrete moisture meter, check out our C555 meter. In Wagner’s testing with nine commonly used concrete mix designs, the C555 had the lowest error of the meters tested in every mix.

Concrete Subfloor Prep Takeaways

Ready to begin your subfloor prep? Here are some takeaways to keep in mind:

• Ensure that your subfloor is level, clean, and dry before putting a floor covering on top.
• Use RH testing for reliable concrete moisture results, especially if you’re working on high-stakes projects.
• Use a concrete moisture meter for smaller projects when the floor covering manufacturer permits its use.
• Allow the concrete slab sufficient time to dry. It should be within the acceptable RH range specified by the manufacturer.

Taking these steps will greatly increase the probability of a successful floor for your clients, and a worthwhile project for you.

Improve your floor installation experience by taking a look at our resources or checking out our Rapid RH products.

1. “A Look Inside the Concrete Slab,” Ise Logik, April 12, 2022.
2. Guide to Contract Flooring, Contract Flooring Association, p. 99.

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Last updated on October 22nd, 2026