Skim Coating Subfloors: A Comprehensive Guide

how to skim coat subfloor

Skim coating is the process of applying a thin layer of compound to a subfloor to create a smooth and even surface for new floor coverings. It is often used to eliminate issues with the existing floor, such as bumps or uneven surfaces, and can be applied to a variety of materials, including concrete, wood, and tile. The thickness of the skim coat can vary depending on the needs of the project, ranging from a thin layer of 1/16 to 1/32 to thicker coats of up to 2 inches. The key to a successful skim coat is proper surface preparation and applying the compound at the right thickness.

Characteristics Values
Purpose Repair damaged walls, smooth down intentionally applied wall texture, cover a popcorn ceiling, prepare subfloor for new floor coverings
Application Methods Trowel, roller, brush, or sprayer
Application Time 2-3 hours
Cost $20-$50
Thickness 1/16” to 1/32"
Coverage 150-200 sq. ft. per bag
Walkable Time 30 minutes
Flooring Install Time 30 minutes for most floor coverings
Drying Time 1-2 hours for thicknesses greater than 1/16"

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The correct tools for the job

Skim coating a subfloor is a process in which a compound is applied to the subfloor to create a smooth and even surface for new floor coverings to be installed. The correct tools for the job will ensure the process is efficient and effective.

The specific tools required will depend on the type of subfloor being worked on and the desired finish. For example, a concrete subfloor may require different tools than a wooden one. Additionally, if a textured finish is desired, different tools will be needed than if a smooth finish is preferred.

  • Trowel: A trowel is used to apply the skim coat compound to the subfloor. It is a flat, rectangular tool with a straight edge that spreads and smooths the compound.
  • Squeegee: A squeegee can be used instead of a trowel, especially if a textured finish is desired. Squeegees can be used to create unique designs and patterns in the skim coat.
  • Roller: A roller is another application tool that can be used to apply the skim coat compound, especially for larger areas. Rollers can also be used to roll on a primer or sealer before the skim coat is applied.
  • Brush: Brushes can be used to apply the skim coat compound for smaller areas or detailed work. They can also be used to add stains or dyes to the skim coat for a decorative finish.
  • Hawk: A hawk is a flat, square tray that is used to hold the compound while skim coating. It is held in one hand, while the trowel is used with the other.
  • Mud Pan: A mud pan is used to hold the compound and can also be used to wipe off excess compound from tools.
  • Damp Rag: A damp rag is useful to have on hand to wipe off tools and clean up any messes during the skim coating process.
  • Space Heater and Fan: These can be used to speed up the drying time of the skim coat, especially in humid environments.
  • Sanding Tools: Depending on the desired finish, sanding tools such as sandpaper or a pole sander may be needed to smooth out the skim coat after it has dried.

It is important to have the correct tools for the job to ensure a successful skim coating project. The specific tools required will depend on the unique circumstances of the project, so it is essential to plan and prepare accordingly.

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Preparing the subfloor

Check Moisture Conditions:

Before beginning any work, ensure that the house is roofed, walled, and windowed, and that the heating and air conditioning units are operational. Allow enough time for the interior atmospheric conditions to stabilize at normal living conditions. Check the moisture content of the subfloor and record it. Ensure that the moisture content of the wood subfloor is within the acceptable range, typically within 4 percentage points of properly acclimated solid strip flooring.

Assess the Subfloor Construction:

Determine whether the construction is on wood joists or a slab. If it's on wood joists, check the thickness of the subfloor and ensure it is approved for flooring application. For concrete slabs, verify that the slab is at least 30 days old before considering moisture testing.

Clean, Flatten, and Repair the Subfloor:

The subfloor must be clean, dry, and flat. Remove any old flooring, debris, or residue. Sand or grind down any high spots and use patching compounds and shims to level low spots. Ensure the subfloor is structurally sound, re-nailing or reinforcing joists if necessary.

Add a Moisture Barrier:

To protect against moisture rising through the subfloor, install a proper moisture-vapor retarder. If a wood subfloor is laid over a concrete slab, cement a membrane to a clean and smooth concrete surface. For concrete subfloors, apply a "skim" coat of asphalt mastic and then roll out a 6-mil or 8-mil polyethylene film, embedding it in the mastic.

Test for Moisture:

Conduct moisture testing to ensure the subfloor and concrete slab are dry within specifications before proceeding with the skim coat application. Use a hand-held moisture meter to measure moisture content in the subfloor. For concrete slabs, follow the manufacturer's recommendations for acceptable moisture content and preferred testing methods.

Prepare the Subfloor Surface:

Once the subfloor is clean, dry, and flat, you may need to add an underlayment, especially if you're installing over a concrete slab. Install plywood over the slab to provide a smooth and even surface for the skim coat application. Ensure the subfloor is free of protruding nails or staples, and spot-sand any high spots to ensure a smooth finish.

By following these steps, you will have a properly prepared subfloor that is ready for the skim coat application, ensuring a successful and long-lasting flooring installation.

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Choosing the right compound

Type of Compound

The most common types of compounds used for skim coating are joint compound, cement-based compound, and veneer plaster products. Joint compound is versatile and can be used for both walls and subfloors. It is available in lightweight and all-purpose options and can be thinned with water for easier application. Cement-based compounds, such as Sika® Level SkimCoat, are ideal for repairing and levelling concrete subfloors. They offer good adhesion and rapid setting times. Veneer plaster products are another option, but they may be harder to find in local stores.

Workability and Finish

Consider the workability and finish you desire for your project. Choose a top-tier compound that offers better workability and an exceptional finish. Look for a compound that is easy to spread, smooth out, and apply in thin, even layers. The consistency of the compound should be creamy and pudding-like, allowing it to hold on to your application tool without dripping.

Environmental Considerations

If you're concerned about reducing your carbon footprint and environmental impact, consider using compounds with lower CO2 emissions during production. For example, the production of Rapid Set cement emits significantly less CO2 than Portland cement. Additionally, look for compounds that are safe to use and dispose of, minimizing any potential harm to your health and the environment.

Application Method

The application method you plan to use will also influence your compound choice. For large areas, paint or texture rollers are ideal for applying a consistent skim coat. For precision work around edges and corners, a drywall or taping knife offers more control. Skimming knives are specialized tools designed for skim coating, providing a smoother application over a broader area. Texture sprayers are suitable for larger projects or achieving specific textures but require specialized equipment and technique.

Project Requirements

Consider the specific requirements of your subfloor skim coating project. If you need to fill holes, cracks, or joints, opt for a compound suitable for patching and levelling. If your subfloor has previous treatments, such as wallpaper or paint, ensure the compound can adhere well to those surfaces. Additionally, consider the desired thickness of your skim coat, as some compounds are better suited for featheredge applications, while others can handle thicker layers.

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Application methods

Skim coating a subfloor is a process in which a compound is applied to the subfloor as an underlayment to reduce variations in the floor and provide a smooth and adherable surface for new floor coverings. The application methods for skim coating a subfloor depend on the tools used and the desired look. Here are some common methods:

  • Trowel: A trowel can be used to apply a skim coat to the subfloor. This method is suitable for patching and skim coating concrete, wood, tile, stone, and non-water-soluble adhesive residues on concrete. It can be used to apply the compound from a featheredge up to a certain thickness, typically around 1/2" (12 mm).
  • Roller: A paint roller cover and frame can be used to roll out the skim coating compound onto the subfloor. This method is popular as it allows for quick application and easy cleanup. However, it is important to work in small sections to prevent the compound from drying up.
  • Spray: Using a texture sprayer, the compound can be sprayed onto the subfloor. This is the fastest and most effortless way to apply the compound. However, it requires renting a special sprayer, and the cleanup process may be more involved.
  • Knife: A drywall knife can be used to apply and smooth the compound during skim coating. While this method keeps tool purchases to a minimum, it is labour-intensive and slow.

It is important to note that proper surface preparation and applying the compound at the right thickness are key to the success of skim coating a subfloor. Additionally, priming the subfloor before skim coating can help prepare the surface and seal any loose paper or residue.

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Finishing touches

Once you have skim-coated your subfloor, there are a few finishing touches you can consider to ensure a smooth and aesthetically pleasing surface.

Firstly, it is important to allow the skim coat to dry completely. The drying time can vary depending on the product used and the thickness of the application. For example, ARDEX SKM™ Skim Finish, a trowelable patch and skim coat product, has a walk-on and installation time of approximately 30 minutes for a skim coat layer. However, for thicknesses greater than 1/16", the drying time can be extended to 1-2 hours.

After the skim coat has dried, you can proceed with sanding or feathering the edges to ensure a smooth transition between the coated and uncoated areas. This step is particularly important if you had to grind down any edges or patches to achieve an even surface.

Additionally, you can consider adding a decorative touch to your skim-coated subfloor. Cement-based microtoppings and skim coats can be used to create a clean canvas for decorative treatments such as stains, dyes, or stencilled patterns. You can apply layers of different hues to achieve interesting colour variations. Alternatively, you can use a product like Ardex Feather Finish, which acts as a drying agent for the adhesive and helps the glue grip the finish of the vinyl.

Finally, you can proceed with installing your desired floor covering. This could be sheet vinyl, luxury vinyl plank, or any other flooring of your choice. Just ensure that you follow the manufacturer's instructions and recommendations for the best results.

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Frequently asked questions

Skim coating is a process in which a compound is applied to the subfloor to create a smooth and adherable surface for new floor coverings.

Skim coating can be applied to concrete, wood, tile, stone, and non-water-soluble adhesive residues on concrete.

You will need a trowel, a putty knife, a scraper, a mixing container, and a roller or brush for application.

Depending on the product used, skim coats can take 30 minutes to 2 hours to dry.

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