How To Level a Concrete Floor

Uneven concrete floors can be a major headache for homeowners, whether they’re in their basement, garage, or kitchen. Fortunately, leveling them is a project you can do yourself with the right tools and techniques.

In the accompanying video, This Old House general contractor, Tom Silva, demonstrates how to use a self-leveling compound and a rail system to create a smooth, even surface.

What is Self-Leveling Compound?

Self-leveling compound is also known as liquid floor underlayment or floor resurfacer. It’s a versatile product that flows like thick syrup and hardens into a smooth, level surface, often within an hour. Manufacturers formulate the compound so there’s minimal headroom loss in low spots on your floor. 

You can find a few different types of self-leveling compound at the store. For basements, we recommend using a Portland cement-based product instead of a gypsum one. Portland cement is more resistant to moisture and, therefore, ideal for areas prone to humidity.

Preparing the Concrete Floor

Before applying the self-leveling compound, use a degreaser to remove any surface contaminants that could weaken the bond. If the floor has significant cracks or damage, you should also repair these with an appropriate patching compound before you continue with the leveling process.

Setting Up the Rail System for Leveling

The rail system is a guide for the self-leveling compound and helps you spread the compound evenly across the entire area. 

Materials Needed

You’ll need these materials from your home improvement store to make the rails:

• 2×4 wooden rails of the appropriate length
• Circular saw
• Construction adhesive
• Hammer drill with carbide-tipped masonry bit
• Level
• Masonry nails
• Screed made from a 6-inch-wide straight-edged board
• Scribing tool
• Shims

Installing the First Rail

The first rail lays the groundwork for subsequent ones, so take extra care to ensure it’s leveled and placed properly.

1. Position a straight 2×4 rail diagonally across the floor, with one end on the room’s high point.
2. Use shims to level the rail as necessary.
3. Match the bottom of your scribe tool to the surface of the floor. Then, trace the uneven contour of the slab onto the side of the board.
4. Cut the rail along the scribed line with your circular saw. Check the fit of the board in the same position as where you did the scribing.
5. Secure the cut rail to the floor using construction adhesive and masonry nails. Silva recommends pre-drilling pilot holes so he doesn’t accidentally split the wood.

Adding Subsequent Rails

Once the first rail is in place, you’ll use a similar process to add the others.

Place the second rail lengthwise across the floor, resting one end on the first rail to form a triangle with the rails and the walls. Shim up, scribe, and cut the second rail like you did with the first. Repeat the process for any other needed rails.

As you prepare and install each rail, check for levelness multiple times. Any deviation will affect the level of your floor at the end of the project.

Applying the Self-Leveling Compound

Once the rail system is in place, you can apply the self-leveling compound.

This step requires careful mixing and application to achieve the best results. Work quickly, and don’t try to apply the compound to your whole floor at once since it doesn’t take long to start setting.

1. Mix the compound. Silva uses a ½-inch drill fitted with a mixing paddle, and he mixes until the consistency resembles a thick batter.
2. If you’re working around cabinets or appliances, place padding between them and your worksite for protection.
3. Slowly pour the compound inside the perimeter of the rails, slightly overfilling them.
4. Run your screed across the rails to level off the compound and make it even with the top of the rails. Work quickly in this step, as the compound doesn’t take long to start setting.

Curing and Finishing the Floor

After applying the self-leveling compound, let it cure overnight. The exact curing time for your product may vary depending on what it’s made from and the surrounding environmental conditions.

Before you install new flooring over the resurfaced slab, be sure the compound is sufficiently dry and cured. Perform a final inspection to ensure the surface is smooth and even. You may be able to gently sand down any remaining imperfections.