Subfloor Preparation: Steps for a Successful Flooring Installation

Uneven floors. Squeaking. Premature wear and tear. Moisture problems.

You don’t want to risk any of these by failing to prepare your subfloor properly.

The subfloor is the layer onto which flooring materials are installed, supporting the weight of the flooring and anything placed on the flooring. Preparing the subfloor properly is critical to achieving a long-lasting, attractive, and functional floor.

Here, we’ll help you achieve just that by guiding you through the steps for preparing your subfloor:

1. Inspection
2. Cleaning
3. Leveling
4. Priming
5. Moisture barrier installation
6. Underlayment installation

Plus, we’ll provide you with a handy checklist of all the essential steps.

Before we get into those steps, let’s take a moment to discuss the different types of subfloor materials and the factors that can affect subfloor preparation.

Understanding Subfloor Materials

There are several types of subfloor materials commonly used in flooring installation, including:

1. Plywood: Plywood is a strong and stable subfloor material made from thin layers of wood veneers glued together with alternating grain directions.
2. Oriented strand board (OSB): OSB is a type of subfloor material made by compressing wood strands with adhesive resin. It is less expensive than plywood, but its edges are more prone to swelling when they get wet.
3. Concrete: Concrete subfloors are commonly used in commercial and industrial buildings, slab-on-grade homes, and residential basements. They are strong, durable, and fire-resistant, but they can hold a lot of moisture, so they must be tested before flooring goes down.
4. Existing flooring: Sometimes, the existing flooring serves as the subfloor upon which new flooring is installed. However, it must be in good condition, flat, and free from any defects that may affect the new flooring installation.

Factors Affecting Subfloor Preparation

Before you begin preparing your subfloor, consider the following factors:

• Type of flooring material: Different types of flooring materials require different subfloor preparation methods. Check the flooring manufacturer’s instructions for the method you need.
• Condition of the subfloor: The condition of the subfloor is critical to the success of the flooring installation. Subfloors that are damaged, rotting, or uneven require repairs or replacement before the new flooring can be installed.
• Environmental conditions: Humidity and temperature can affect the subfloor’s moisture content and the installation process. Follow the manufacturer’s recommendations for your particular environmental conditions, and monitor with a device like the Wagner Smart Logger™ (for ambient conditions) or Wagner Floor Sentry^® (for long-term monitoring inside the floor).
• Building codes: Building codes may dictate the type of subfloor material and the subfloor preparation requirements for different types of flooring materials.
• Subfloor material compatibility: Make sure the subfloor material is compatible with the flooring material to be installed.
• Subfloor thickness: The thickness of the subfloor material affects how stiff and stable the floor is. Make sure the subfloor meets the manufacturer’s recommended thickness for the flooring material to be installed.

With that, let’s get into preparing the subfloor.

Subfloor Inspection

Subfloor inspection involves checking the subfloor for defects, damage, or irregularities that may affect the flooring installation. Because the subfloor is the foundation for the flooring material, it must be strong, flat, and dry enough for the flooring you’ve chosen.

Checking for potential problems before installation is key to a successful and long-lasting floor.

Otherwise, you may be in for costly repairs or even a complete floor replacement in the near future.

During a subfloor inspection, you or your flooring professional will examine the subfloor and look for any issues that may impact the quality of the flooring installation. Check, too, for any electrical or plumbing lines in or under the subfloor that the installation could affect.

Let’s find out what kinds of defects to look for:

Detecting Subfloor Defects

Subfloor defects can affect the stability, appearance, and longevity of the flooring material, making it critical to find them and deal with them. Common subfloor defects include:

• Moisture damage: Subfloors that have been exposed to moisture for an extended period may develop water damage. This can cause the subfloor to swell, rot, and develop mold and mildew. There can also be invisible moisture problems that will affect the flooring over time, so always measure the moisture conditions of the subfloor before installation.
• Rot: Rotting subfloors are weak and unstable and can cause the flooring to sag and warp.
• Loose fasteners: Loose or missing screws or nails can cause the subfloor to creak or squeak, and the flooring may not adhere correctly.
• Unevenness: Uneven subfloors can cause the flooring to become uneven and create tripping hazards.
• Termite damage: Termites can cause significant damage to the subfloor, leading to a weakened structure and possible collapse.
• Cracks and gaps: Cracks and gaps in the subfloor can let moisture through and can telegraph through thin or resilient flooring.
• Sagging: Sagging subfloors can be caused by a variety of factors, including moisture damage, rot, and termite damage. They may cause the floor above to be uneven.
• Shifting: Wood subfloors can shift as they take on and lose moisture with changes in humidity. This can result in unevenness and create tripping hazards.

Subfloor Inspection Techniques

There are three main ways to look for defects:

• Visual inspection: Many of the defects, damage, or irregularities can be found simply by careful observation.
• Moisture testing: Use a wood moisture meter on wood subfloors and relative humidity (RH) testing on concrete to check the subfloor’s moisture and spot any moisture damage or potential for mold growth.
• Other instrument testing: Use tools such as straightedges and levels to measure the flatness and levelness of the subfloor.

Addressing Subfloor Defects

Defects in the subfloor can affect the stability, appearance, and longevity of the flooring material, so it’s important to address any that are detected.

The type and severity of the subfloor defect will determine the necessary repairs.

For example, if the subfloor has suffered from moisture damage, the affected materials may need to be replaced and the source of the moisture fixed to prevent further damage.

In cases where the subfloor has suffered from rot or termite damage, the affected materials may need to be completely replaced. This is because rot and termite damage can compromise the structural integrity of the subfloor, making it unstable and potentially dangerous.

In addition to replacing damaged materials, other repairs may include tightening loose fasteners and filling cracks and gaps in the subfloor. These repairs will help make sure the subfloor is flat and stable, providing a strong foundation for the flooring material.

After making sure your subfloor is free of defects, move forward to the next step.

Subfloor Cleaning

A clean subfloor is one that’s free of debris, contaminants, and any residue that could interfere with the flooring installation process. If any of these remains, they could cause the flooring material to not adhere properly or lead to unevenness or gaps.

Old adhesives, paint, or other materials can also interfere with the bond of the new flooring material, leading to potential issues in the future.

Subfloor cleaning is typically done with a broom, vacuum, or other cleaning tools. In some cases, a mild cleaning solution may be used to remove any stubborn contaminants or residue.

Be sure to clean the following:

• Dust and dirt: Remove by vacuuming, sweeping, or using a damp mop (and then allowing the subfloor to dry).
• Adhesive residues: Adhesive residues left behind from previous flooring installations can interfere with the adhesion of the new flooring material. Remove them through sanding, scraping, or grinding.
• Paint and coatings: Sand, grind, or use a chemical stripper to get rid of paint and coatings on the subfloor.

Different subfloor materials require different cleaning methods.

For instance, plywood subfloors can be cleaned with a vacuum or broom, while concrete subfloors may need more aggressive methods, such as grinding or shot blasting, to remove old coatings and adhesives. Check the flooring and adhesive manufacturers’ recommendations for how the surface must be prepared.

Subfloor Leveling

Subfloor leveling involves correcting any unevenness or irregularities in the subfloor so the new flooring is installed on a flat surface.

It helps keep the new flooring from cracking, warping, or buckling, which can lead to costly repairs and an unsightly appearance. Subfloor leveling also helps improve the overall durability and longevity of the new flooring.

Determining Subfloor Levelness

Determining the levelness and flatness of a subfloor is the first step in the subfloor leveling process. Most flooring manufacturers specify how flat the subfloor must be, so check their requirements.

Here are some tools and techniques that work well:

1. Straightedge: A straightedge is a long, straight piece of metal or other rigid material that can be placed on the subfloor to check for any dips or high spots. If the straightedge rocks, or if there are gaps under it, the area is uneven.
2. Laser level: A laser level projects a laser beam across the surface of the subfloor to check for any dips, high spots, or uneven areas. The position of the laser beam will indicate if there are any deviations.
3. Feeler gauges: Feeler gauges are thin metal blades of varying thicknesses. By sliding them into the gap between the subfloor and a straightedge, you can measure how deep a dip is.
4. Measuring tape: Used with a laser level or a string line, a measuring tape can help you find high spots and dips that need to be leveled.

Once you have determined the levelness of the subfloor, you can then determine the best method for leveling the surface. Subfloor leveling is often best done by a professional flooring installer.

Types of Subfloor Leveling

There are several methods used to level a subfloor, including:

1. Using a leveling compound. A popular and effective method, these compounds are self-leveling, meaning that they will flow and settle on their own, filling in low spots and creating a flat, even surface. They are typically cement-based blends with polymers and are available in different formulations for different types of subfloors. They are easy to work with and many set quickly, making them a good choice for many projects. Patching compounds can be troweled into smaller low spots.
2. Shimming. On wood-framed floors, thin strips of wood can be fastened to the tops of low joists to bring the subfloor up level.
3. Sanding or grinding down high spots in the subfloor. This can be time-consuming and physically demanding, but it is an effective solution for removing high spots in a subfloor.

A professional flooring contractor will be able to assess the subfloor and recommend the best method for leveling it.

Cost of Subfloor Leveling

The cost of subfloor leveling will vary depending on the size of the area being leveled and the method used. The cost can also be influenced by the complexity of the job, the type of subfloor material, and the location of the project.

Before You Level

Before starting the subfloor leveling process, clean the surface, repair any cracks or damage, and remove any existing flooring material. Also confirm that the subfloor’s moisture is within the limits set by the leveling compound and flooring manufacturers.

Subfloor Priming

Subfloor priming involves applying a primer to the subfloor before leveling compound or flooring adhesive goes down. Primers help the compound or adhesive bond to the subfloor and control how much moisture a porous surface pulls out of it. Most standard primers are not moisture barriers.

Types of Primers

You have several options for a subfloor primer. Always use the primer the leveling compound or adhesive manufacturer specifies:

• Water-based primers: Water-based acrylic and latex primers are the most common choice. They are easy to apply, dry quickly, and clean up with water.
• Epoxy primers: Epoxy primers are two-part products made from an epoxy resin and a hardener. They provide a very strong bond, and some are formulated as moisture mitigation systems for concrete slabs with high moisture. They are harder to work with than water-based primers.
• Solvent-based primers: Solvent-based primers are less common today but are still specified for some products. They require good ventilation during application.

Proper Priming Techniques

The appropriate priming technique depends on the type of primer being used. Check the manufacturer’s instructions for the one you have.

Here are some general guidelines for priming:

1. Before priming the subfloor, clean and prepare the surface by removing any dirt, dust, or debris and repairing any cracks or damage. Confirm the subfloor’s moisture is within the primer manufacturer’s limits.
2. If you’re using an epoxy primer, mix the resin and hardener in the correct proportions according to the manufacturer’s instructions.
3. Apply the primer according to the manufacturer’s instructions, using the recommended application method, such as a brush, roller, or sprayer. It’s important to apply the primer evenly and to avoid over-applying, which can cause the primer to become too thick and less effective.
4. Allow the primer to dry completely before installing the flooring material. The drying time will vary depending on the type of primer and the conditions in the room, so follow the manufacturer’s recommendations for drying time.
5. Ventilate the work area during and after applying the primer, especially if it’s a solvent-based primer. The fumes can be harmful if inhaled.

Moisture Barrier Installation

Moisture barriers, installed between the flooring and the subfloor, protect the flooring material from moisture that may come up from the subfloor. They play an important role in preventing moisture damage and mold, especially over concrete slabs and crawl spaces.

So, how do you know for sure you’ll need one?

Determining the Need for a Moisture Barrier

You may need a moisture barrier in the following instances:

1. The subfloor is concrete. Concrete subfloors can be a source of moisture. Depending on the RH test results and the flooring manufacturer’s requirements, a moisture barrier or moisture mitigation system may be needed to protect the flooring from moisture damage, mold, and mildew.
2. The installation site is a damp environment, such as a basement, over a crawl space, or in a coastal area.
3. There are other factors, such as certain types of flooring materials, subfloor conditions, or local building codes and regulations. A professional flooring contractor will be able to assess the installation site and determine the need for a moisture barrier.

Types of Moisture Barriers

Several types of moisture barriers can be used in flooring projects, including:

1. Polyethylene sheeting: Polyethylene sheeting is a plastic film that is laid over the subfloor, most often under floating floors on concrete. It’s cost-effective and easy to install.
2. Liquid-applied moisture mitigation systems: These coatings, often epoxy-based, are rolled or troweled onto concrete slabs to limit the moisture that reaches the flooring and adhesive. Always verify the system complies with your flooring and adhesive manufacturers’ requirements.
3. Rolled membranes and papers: Rolled membranes are sheets of material that are rolled out over the subfloor. They’re available in different thicknesses and materials, including rubber, polyethylene, and asphalt-saturated paper.

Installing Moisture Barriers

The moisture barrier should be installed on a clean, dry subfloor and sealed at the seams and around any penetrations, such as pipes and ducts, to ensure a continuous barrier. Follow any manufacturer’s instructions and local building codes or regulations.

Underlayment Installation

Underlayment is a layer of material installed on top of the subfloor and under the actual flooring material. It provides additional support, cushioning, and insulation to the floor.

You may want to consider underlayment for the following benefits:

• Increased comfort: Underlayment provides cushioning, making the floor more comfortable to walk on.
• Noise reduction: Underlayment reduces the amount of noise that travels through the floor, making it ideal for multi-story buildings.
• Moisture protection: Some underlayments include a built-in vapor barrier film that protects the flooring from moisture coming up from the subfloor.
• Improved durability: Underlayment can help protect the flooring material from wear and tear, increasing its durability.

Some of the most common types of underlayment are cork, rubber, and foam. Follow the manufacturer’s instructions for each type to ensure that the underlayment does its job.

Keep This Subfloor Checklist with You

If you want a floor that lasts for years to come, properly preparing your subfloor is essential. In this article, we’ve given you an overview of how to prepare the subfloor for your flooring installation so that you can save money and time in the long run.

To summarize all that we’ve covered, here’s a simple checklist you can refer to:

1. Inspect the subfloor for defects, damage, or irregularities.
2. Test the subfloor’s moisture: use a wood moisture meter on wood subfloors and RH testing per ASTM F2170 on concrete.
3. Address any subfloor defects or issues.
4. Clean the subfloor to remove debris, contaminants, and any residue that could interfere with the flooring installation process.
5. Level the subfloor so that it is flat and stable.
6. Prime the subfloor, if the leveling compound or adhesive manufacturer calls for it.
7. Install a moisture barrier, if necessary, to protect the flooring material from moisture that may come up from the subfloor.
8. Install underlayment to provide additional support, cushioning, and insulation to the flooring system.

With these simple steps, you’ll set yourself up for flooring success. And always remember to check the manufacturer’s installation guidelines before you install the finished floor. Good luck!

For more on how subfloors affect hardwood flooring in particular, read our article “How Subfloors Affect Hardwood Floor Installation.”

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Last updated on October 13th, 2026