How to Prepare a Slab Floor

Better Homes & Gardens
·5 min read

No matter what kind of finished flooring you plan to install, the substrate must be properly prepared. In the case of a concrete slab, it should be solid, smooth, and level. Inspect the slab's condition before you build new framing and install any flooring materials.

First look for large cracks and sagging sections. If they're present the base is not adequate. You'll need to remove sagging sections and pour new concrete. Cover isolated, inactive cracks with an isolation membrane. Completely cover new concrete and any other floors you suspect might develop cracks. Fix active cracks—those that are growing longer or wider—instead of covering them up. Active cracks can indicate serious structural problems and can pull the floor apart.

In addition to cracks, look for water damage. Concrete floors, particularly those at or below grade, are especially vulnerable to water, so don't install any kind of tile on a slab until you fix water problems. Condensation on water pipes and walls occurs in hot weather and is not technically a water problem. Increase ventilation to relieve condensation. Install or fix gutters and slope soil away from the foundation so water runs away from it. If this doesn't fix the problem, consult a drainage specialist.

Except to spend about 30-45 minutes prepping each square yard of floor. You'll need to be comfortable using a level, trowel, and power grinder.

What You Need

To Repair or Degloss the Surface:

  • 4-foot level

  • Hammer

  • Cold chisel

  • Margin and mason's trowels

  • Grinder

  • Sanding block

  • Vacuum

  • Brush

  • Mop

  • Hydraulic cement or thinset

  • Muriatic acid

  • Rubber gloves

To Repair a Structural Defect:

  • Sledge

  • Crowbar

  • Wheelbarrow

  • Gravel

  • Reinforcing wire

  • Concrete mix

  • 2x4 screed

To Install a Membrane:

  • Roller

  • Mason's trowel

  • Rented floor sander

  • Membrane

  • Adhesive

How to Repair the Surface or Structure

Step 1: Check and Level

Divide the slab into imaginary 6-foot sections and check each section with a 4-foot level. Mark cracks, high spots, and other defects with a carpenter's pencil. Cracks may be a sign of a structural defect that will require a professional to correct. Others may be DIY repairable.

Step 2: Open Cracks

Use a small sledgehammer and a cold chisel to open small cracks so you can fill them. If possible angle the chisel into each side of the crack to create a recess wider at the bottom of the crack than on the top. This will help hold the patching cement securely.

Step 3: Wash and Fill Crack

Wash out the crack with water and fill it with quick-setting hydraulic cement or thinset. Use a margin trowel or mason's trowel and feather out the edges until the patch is level with the surrounding surface.

Step 4: Fill Depressions

To fill depressions in the slab, pour a small amount of thinset or self-leveling compound into the depression and trowel level. Add thinset or compound until the surface is level and feather the edges of thinset even with the floor.

Step 5: Grind Down

Grind down high spots using a grinder equipped with a masonry-grit abrasive wheel. A right-angle grinder makes this job go quickly. Hold a vacuum hose near the grinder to remove the dust as you work. Vacuum and damp-mop the surface thoroughly.

How to Apply an Isolation Membrane

Step 1: Apply Membrane Adhesive

Apply the membrane adhesive equally on both sides of a crack or expansion joint. Use a roller to apply the adhesive and spread on a light but even coat.

Step 2: Apply Membrane

Follow the manufacturer's instructions to apply the membrane to cured or wet adhesive. Apply the membrane over the adhesive, following the contour of the crack across the surface.

How to Prep a Slab with a Waxed, Sealed, or Smooth Finish

Thinset and other adhesives will not bind to surfaces that are waxed, sealed, painted, or finished to a gloss with a steel trowel. You can tell if your slab is waxed or sealed if spilled water beads up on it. If water soaks in and the surface is otherwise slick and smooth, it is likely to have been finished with a steel trowel.

Roughen slick or painted finishes with a sanding block or rented floor sander. Make your own sanding block by tacking a sheet of coarse abrasive on an 8-inch 2x4. Using light pressure, scuff the entire floor. Vacuum the floor when finished.

Remove wax and sealers with a solution of 4 parts water to 1 part muriatic acid. Using a scrub brush, scour the floor with the acid wash. Rinse the slab with clear water and let it dry thoroughly.

Muriatic acid is highly caustic. Follow the manufacturer's directions and wear eye protection, rubber gloves, and old clothing. Ventilate the area.

How to Prep with Self-Leveling Compounds

Self-leveling compounds, technically not a substrate, are used to level depressions in slabs and subfloors. Most call for only light mixing with water and level themselves when poured. Quick-setting brands allow tiling within hours.

Self-leveling compounds work best when applied in thicknesses of less than 1 inch. If using a compound to fill a deeper depression, make more than one pour, following the manufacturer's directions.

Pour the compound after completing the repair work. This sequence ensures the compound stays clean and ready for tiling.

Commercially applied gypsum-based compounds are excellent for leveling floors on which surface radiant heating systems have been installed.