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- 1). Clean the concrete subfloor. Use a garden hoe to remove any drywall mud left on the concrete for new construction. Use a scrub brush on any stubborn debris. A mop is useful for removing surface dirt and dust that will affect the thin-set's ability to bond with the concrete.
- 2). Move a straight 2-by-4 around the concrete surface to find low spots. Look for gaps between the concrete and the bottom of the wood. Mark an "L" on the concrete with a marker wherever you find low spots. A permanent black marker works well for this.
- 3). Apply a self-leveling compound to all the low areas in the concrete. The leveling compound will flow out and fill the low areas. Install the leveling compound per the label's direction. Let the leveling compound set and dry.
- 4). Cut a strip of isolation membrane the length of the crack in the concrete. The membrane comes in 24-inch widths. Apply a layer of thin-set over the crack with a trowel. Place the isolation membrane over the crack and work it into the thin-set. Apply a layer of thin-set over the top of the isolation membrane. Use the flat edge of the trowel to apply the thin-set. Feather out the membrane's edges with the surrounding concrete. Repeat for all cracks in the concrete, and let the thin-set mortar dry thoroughly.
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