The Ultimate Guide to Subfloor Prep for a Wood Floor Installation

Many people put a lot of focus on the floor finish when it comes to installing a new wood floor. The finish is what provides the shine, the aesthetics, and the ‘ooh’ and ‘aah’ factor after all. Additionally, the flooring finish can also enhance overall durability and ease of maintenance. While this step should never be ignored – there is another equally important step. That is Subfloor Prep.

Inspecting the Subfloor

As a contractor, the first step in any new flooring installation must be an inspection of the subfloor. Depending on the home or business, there could be varying moisture levels. Before any floor install

The ultimate guide to subfloor prep for hardwood floors

 begins, moisture levels must be checked, and heating and cooling units must be operational. Once this is determined, your subfloor inspection can begin. The first step is deciding whether the subfloor is on wood joists or slab. Then you can proceed with the following checks: 

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Wood Joists

  • Determine the thickness of the wood joists to ensure it can be used for wood floors.
  • Inspect the levelness and soundness.
  • Determine the relative humidity inside and measure the moisture content of the subfloor. Only certain levels of moisture are okay to begin flooring installation.

According to the National Wood Flooring Association’s Installation Guidelines, wood subfloors need to be “flat, dry, sound, clean, and free of squeaks and any protruding nails or staples.” The guidelines state that the subfloor should not have height variations exceeding 3/16 of an inch over a 10-foot span. High areas should be sanded down, and low areas should be filled in or shimmed. For dryness, the subfloor’s moisture content should be within 4 percent of the moisture level of the acclimated wood flooring.


  • Determine whether the concrete slab’s moisture level is suitable.
  • Check for a moisture barrier. For concrete slabs, there should be a 6-mil polyfilm moisture barrier or equal beneath the slab.
  • Determine the age of the concrete slab. Slabs need to be at least 30 days old before a moisture test can be performed (when dealing with new builds).

After you have inspected the construction of the subfloor, it is important to determine how much prep work will be required. Such as:

  • Removing old flooring
  • Flattening or leveling the concrete
  • Patching minor cracks
  • Sanding
  • Replacement of the subfloor

The NWFA guidelines state that concrete subfloors must be “flat, dry, sound, clean, and free of abrupt highs and lows.” The flatness requirement means there should be no more than a 3/16-inch variation in height over a 10-foot span. High spots should be ground down using 20-grit (#31/2) paper, and low spots should be filled or shimmed. For dryness, the subfloor should not emit more than 3 pounds of moisture vapor per 1,000 square feet within 24 hours. Additionally, the National Wood Flooring Association (NWFA) advises that concrete should cure for at least 60 days before installing wood flooring on top unless moisture tests show that the moisture content is within acceptable limits earlier.

Preparing the Subfloor

If the home or business was built before the 1980s, be cautious of asbestos in old vinyl tiles or adhesive. If found, there is a process for safe removal and remediation that needs to take place prior to any subfloor prep. Here is a good checklist to follow:

Step 1: Clean the Subfloor

Thoroughly sweep and vacuum the subfloor to remove all dust, dirt, and debris. A clean surface isHow to measure subfloor moisture essential for the adhesive or underlayment to bond properly. Also make sure to scrape off residue using a scraper to remove any remaining glue, paint, or other residues. This step ensures that the new flooring will lay flat and adhere properly.

Step 2: Make Necessary Repairs to the Subfloor

If you’re working with a wooden subfloor, ensure all the boards are securely nailed or screwed down. Loose boards can cause squeaking and an uneven surface. Make necessary repairs if there are cracks or major damage to the concrete slab.

Step 3: Ensure the Subfloor is Level

Check the subfloor for levelness using a long spirit level or a straight edge. The subfloor should be flat within 3/16 inch over 10 feet or 1/8 inch over 6 feet. If you find any high spots, sand them down to create a flat surface.

For low spots, use a leveling compound to fill them in. Follow the manufacturer’s instructions for mixing and applying the compound.

Step 4: Moisture Test (again)

You may have already conducted a moisture test during the inspection, but it is vital to test a second time prior to installation. Wood floors are sensitive to moisture and these levels can change rapidly. Use a moisture meter to ensure the subfloor’s moisture content is within the recommended range for your wood flooring.

If you find excessive moisture, identify and address the source before proceeding. This may involve improving ventilation, repairing leaks, or installing a moisture barrier.

Step 5: Install Underlayment & Moisture Barrier

Select an underlayment suitable for your wood flooring type. The underlayment can provide a moisture barrier, sound insulation, and cushioning. The moisture barrier is of utmost importance, as concrete will naturally emit moisture in the form of vapor. Without a barrier, this can cause damage to the subfloor or cause the floor to become warped or uneven. 

Types of Moisture Barriers

It is important to adhere to the curing time when installing a moisture barrier. Typically, the recommendation is to wait 60 days after placing the barrier to begin installing the new floor.

The ‘Rule of Thumb’ for Moisture

The moisture content of the wood subfloor should be close to that of the solid wood flooring being installed. For strip flooring, the subfloor’s moisture should be within 4 percentage points. For plankThe Ultimate Guide to Subfloor Prep flooring, it should be within 2 percentage points. For example, if the strip flooring is at 9% moisture, the subfloor should be between 5% and 13% moisture. Also, both strip and plank flooring need a layer of felt paper between the floor and the subfloor.

Here are the various types of moisture barriers used: 

  • Low-end PVC vinyl
  • Polyfilm — 6- or 8-mil polyethylene film in 36-inch or 48-inch-wide rolls
  • 15-pound roofing felt in asphalt mastic 
  • Two-part epoxy
  • Rubberized elastomeric membrane

New chemicals have been introduced to help speed up the curing process, but it is important to read the manufacturer’s instructions for specific moisture barriers. Below are three of the many types of moisture barrier options available at City Floor Supply.

Holland 15 lb HWD Underlayment Felt Wakol PU-280

Enhance Wood Flooring Longevity with Subfloor Prep

Proper subfloor preparation is crucial for the successful installation and longevity of wood floors. By taking the time to meticulously prepare the subfloor, you can enhance the durability, appearance, and overall performance of wood floors, providing a solid foundation for years of use and enjoyment.

At City Floor Supply, we provide contractors with the tools and steps they need to ensure high-quality floor installation. For additional questions, contact our team today at (800) 737-1786 or by emailing

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