Subfloor Preparation: Essential 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 less stable and more prone to water damage.
  3. Concrete: Concrete subfloors are commonly used in commercial and industrial buildings, slab-on-grade homes, and residential basements. They are strong, durable, and resistant to moisture and fire.
  4. Existing flooring: Sometimes, the existing flooring serves as the subfloor upon which new flooring is installed. However, it must be in good condition, level, 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. Verify the methodology needed from the manufacturer’s instructions.
  • 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).
  • 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: Ensure that the subfloor material is compatible with the flooring material to be installed.
  • Subfloor thickness: The thickness of the subfloor material can affect the stability and levelness of the subfloor. It is essential to ensure that 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 level to ensure a proper installation.

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, a professional inspector will examine the subfloor and look for any issues that may impact the quality of the flooring installation. They may also check for any electrical or plumbing systems that may be present in the subfloor and assess their impact on the flooring installation.

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 allow air and moisture to penetrate the subfloor, leading to mold growth and structural damage.
  • 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: Shifting subfloors can be caused by changes in temperature, humidity, and other environmental factors. They can result in unevenness and create tripping hazards.

Subfloor Inspection Techniques

Subfloor inspectors have three different ways to look for defects:

  • Visual inspection: Many of the defects, damage, or irregularities can be found simply by careful observation.
  • Moisture testing: Moisture testing involves using a moisture meter or a concrete relative humidity test to check the subfloor and determine whether there is any moisture damage or potential for mold growth.
  • Other instrument testing: Instrument testing involves using specialized tools and equipment to measure the flatness, levelness, and structural stability 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 area may need to be treated to prevent further moisture 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 to ensure that the subfloor is flat, level, and stable, providing a strong foundation for the flooring material.

After ensuring your subfloor is defect free, 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 or scraping.
  • Paint and coatings: Sand 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 using a vacuum, broom, or damp mop, while concrete subfloors may require a more robust cleaning method, such as a floor buffer or a pressure washer. Check the manufacturer’s recommendations for a cleaning method that is safe and effective for the subfloor material you’re using.

Subfloor Leveling

Subfloor leveling involves correcting any unevenness or irregularities in the subfloor to ensure that the new flooring material is installed on a level surface.

It’s important to prevent the new flooring from cracking, warping, or buckling, which can then 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 of a subfloor is the first step in the subfloor leveling process.

Here are some tools and techniques that work well:

  1. Straight edge: A straight edge is a long, straight piece of metal or plastic that can be placed on the subfloor to check for any dips or high spots. If the straight edge rocks or wiggles, it indicates an uneven area.
  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 that can be used to measure the height of different areas of the subfloor. By placing the feeler gauges between the subfloor and the straight edge, you can determine the levelness of the surface.
  4. Measuring tape: A simple measuring tape can also be used to determine the levelness of a subfloor. By measuring the height of different areas of the subfloor, you can identify any high spots or 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. Note that subfloor leveling should be done by a professional flooring installer to ensure a successful outcome.

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 made from a mixture of cement, sand, and polymers and are available in different formulations for different types of subfloors. They are easy to work with and dry quickly, making them a good choice for many projects.
  2. Shimming. Shimming involves installing thin pieces of material, such as plywood or metal, between the subfloor and the new flooring to level it.
  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. Self-leveling compounds are typically the most expensive option, while shimming is often the least expensive. The cost can also be influenced by the complexity of the job, the type of subfloor material, and the location of the project.

Subfloor Preparation

Before starting the subfloor leveling process, clean the surface, repair any cracks or damage, and remove any existing flooring material. The subfloor should also be dry and free of any moisture or humidity.

Subfloor Priming

Subfloor priming involves applying a primer to the subfloor to improve adhesion and create a barrier between the subfloor and the flooring material. This will prevent the subfloor from separating or lifting over time. It’ll also keep moisture or other contaminants from seeping into the flooring.

Types of Primers

You have several options for a subfloor primer:

  • Water-based primers: Water-based primers are a popular choice for subfloor priming because they are easy to apply, dry quickly, and are environmentally friendly.
  • Latex-based primers: Latex-based primers are water-based primers that are easy to apply and provide good adhesion. They are a popular choice for subfloor priming because they are easy to clean up, dry quickly, and are environmentally friendly. Latex-based primers are typically compatible with a wide range of flooring materials, including carpet, vinyl, and hardwood. They are also a good choice for subfloors that are sensitive to moisture, as they do not contain solvents that can contribute to moisture problems.
  • Oil-based primers: Oil-based primers are a good choice for subfloors that are porous or have been previously treated with an oil-based product. They are more durable than water-based primers and provide better protection against moisture.
  • Epoxy-based primers: Epoxy-based primers are solvent-based primers that provide excellent adhesion and durability for high-traffic areas. They are made from a mixture of epoxy resin and a hardener. Epoxy-based primers are a good choice for subfloors that are prone to moisture, as they provide excellent protection against moisture and other contaminants. However, epoxy-based primers can be more difficult to work with than latex-based primers and require proper 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. It should be dry and free of any moisture or humidity.
  2. Mix the primer according to the manufacturer’s instructions, if using an epoxy primer. This typically involves mixing the epoxy resin and hardener together in the correct proportions.
  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. Ensure proper ventilation in your workshop during and after applying the primer, especially if it’s a solvent-based primer. The solvents can produce fumes that are harmful if inhaled.

Moisture Barrier Installation

Moisture barriers, installed between the floor and subfloor, protect the flooring material from moisture that may seep up from the subfloor. They play an important role in preventing moisture damage and mold and are especially important in areas with high humidity or frequent exposure to water, such as bathrooms or kitchens.

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, and installing a moisture barrier is necessary to protect the flooring material from moisture damage, mold, and mildew growth.
  2. The installation site is a damp environment, such as a basement, crawl space, or 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 to create a moisture barrier. It’s cost-effective and easy to install.
  2. Vapor retarder paint: Vapor retarder paint is a type of paint that’s applied to the subfloor to create a moisture barrier. It may be a good choice for concrete subfloors and can provide additional protection against mold and mildew. Always verify this complies with your flooring and adhesive manufacturer’s requirements.
  3. Rolled membranes: Rolled membranes are sheets of material that are rolled out over the subfloor to create a moisture barrier. They’re available in different thicknesses and materials, including rubber, polyethylene, and asphalt.

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: Underlayment can act as a moisture barrier, protecting the flooring material from moisture that may seep up from the subfloor.
  • Improved durability: Underlayment can help prevent 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 installing your subfloor is essential to that process. 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. Address any subfloor defects or issues.
  3. Clean the subfloor to remove debris, contaminants, and any residue that could interfere with the flooring installation process.
  4. Level the subfloor to ensure that it is even and stable.
  5. Prime the subfloor to improve adhesion and create a barrier between the subfloor and the flooring material.
  6. Install a moisture barrier, if necessary, to protect the flooring material from moisture that may seep up from the subfloor.
  7. 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!

Last updated on October 17th, 2023

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