Gypsum board floor underlayments are an excellent choice for smoothing out uneven or rough floors before installing a final flooring surface. These underlayments consist of a gypsum-based material that is poured or spread over the existing floor surface and then smoothed out to create a flat, level surface. Here is the ultimate guide to installing gypsum board floor underlayments.

Prepare the Surface

Before installing the underlayment, it’s essential to prepare the surface to ensure a smooth and stable base. Remove any debris, dirt, or old flooring material from the surface using a broom, vacuum, or scraper. Make sure the surface is flat, level, and free of any bumps or cracks that could affect the installation.

Install a Moisture Barrier

If you are installing the underlayment over a concrete subfloor, it’s important to install a moisture barrier to prevent moisture from seeping up into the underlayment. Use a high-quality moisture barrier that is specifically designed for use with gypsum board underlayments. Apply the moisture barrier according to the manufacturer’s instructions and allow it to dry completely before installing the underlayment.

Mix the Gypsum Board Underlayment

Mix the gypsum board underlayment according to the manufacturer’s instructions. Most underlayments come in powder form and are mixed with water to create a smooth, pourable consistency. Use a mixing paddle and a drill to mix the underlayment thoroughly, making sure there are no lumps or clumps in the mixture.

Spread the Underlayment

Spread the underlayment onto the prepared surface using a notched trowel or a gauge rake. Spread the underlayment evenly, making sure to fill in any low spots or depressions in the floor surface. The thickness of the underlayment will depend on the manufacturer’s instructions and the level of unevenness in the floor. Typically, the underlayment should be at least 1/4 inch thick but can be up to 1 inch thick for severe leveling.

Smooth Out the Surface

Once the underlayment is spread over the surface, use a smoothing tool, such as a straightedge or a screed, to smooth out the surface and create a level finish. Use long, sweeping motions to create a smooth, even surface. Work quickly to ensure that the underlayment does not start to set before it is smooth out.

Allow the Underlayment to Dry

Allow the underlayment to dry completely according to the manufacturer’s instructions. This can take anywhere from a few hours to a few days, depending on the thickness of the underlayment and the humidity and temperature of the environment. Do not walk or work on the underlayment until it is completely dry.

Sand the Surface

Once the underlayment is dry, use a power sander or a sanding block to sand the surface and create a smooth, even finish. Sand the surface gently to avoid creating any scratches or gouges in the underlayment. Vacuum the surface thoroughly after sanding to remove any dust or debris.

Apply a Primer

Before installing the final flooring surface, apply a primer to the surface of the underlayment. Use a primer that is specifically designe for use with gypsum board underlayments. Apply the primer according to the manufacturer’s instructions and allow it to dry completely before installing the final flooring surface.

Install the Final Flooring Surface

Once the primer is dry, you can install the final flooring surface over the gypsum board underlayment. The underlayment should provide a smooth, level surface that is ready for the installation of any type of flooring, including tile, hardwood, or carpet.


Installing a gypsum board floor underlayment is a relatively straightforward process that can provide a smooth and level surface for any type of final flooring material. By following these guidelines, you can ensure a successful installation that will create a durable and long-lasting flooring surface.