How to Level a Concave Basement Floor

Sweep away any dirt or debris from the basement floor with a stiff bristle push broom. Wash the floor after sweeping it; use a mop and a pH-neutral cleanser. Rinse the cleanser clear with clean water and then allow the floor to dry for about six hours.

Remove the cover of any floor drains and fill the drain entrance with an expanding foam, leveled off at the surface of the floor.

Locate the center of the concave surface with a leveling bar. Drag the bar over the surface of the floor, along the length of the room, while shining the light from a flashlight beneath the bar’s edge. Watch for the changing level of the floor beneath the bar, looking for a point at which the floor stops sinking beneath the bar and begins to rise. Mark the point of the floor where this occurs with a piece of chalk.

Repeat the process of locating the slope by moving in a line perpendicular to the first line over which you dragged the bar. Mark the spot where you note the changing levels with the chalk. Draw a line parallel to the wall at the chalk marks to create two crossing lines on the floor. Where the two marks meet one another is the center of the concave area.

Remove any baseboard along the edge of the floor with a pry bar by applying pressure to the rear of the boards until they pop free of the walls.

Brush a layer of bonding adhesive over the floor with a low-nap paint roller attached to an extension pole. Wait for the adhesive to dry until it no longer appears wet. The adhesive should still feel sticky.

Mix the self-leveling compound in a large bucket with a wooden stir stick, following the manufacturer’s instructions.

Pour the compound onto the floor at the point where the chalk lines meet. Spread the compound over the surface of the floor, extending it to the edges with a squeegee. Build up a layer of compound at the edges of the floor that's 1/8 inch thick by continuing to pour the mix in the center and spreading it outward. Spread the mix toward the wall that's farthest from the door to the basement, then spread toward the door so you can avoid stepping on the compound.

Wait 48 hours for the compound to dry and check the floor with the leveling bar again. If the concavity still exists, add more of the compound to the floor, spreading it as before and allowing it to dry. Repeat until the floor is level.