Prepping a Slab Floor

This story shows how to prevent tile floors from cracking by properly preparing a slab floor.


Water is the enemy of all building materials. Concrete floors, particularly those at or below grade, are especially vulnerable. 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.

Cover isolated, inactive cracks with an isolation membrane "bandage." Completely cover new concrete and any floors suspected of developing cracks. Fix active cracks before tiling -- don't cover them up.


From 30 to 45 minutes per square yard

Repair or degloss surface: 4-foot level, hammer, cold chisel, margin and mason's trowels, grinder, sanding block, vacuum, brush, mop Repair structural defect: sledge, crowbar, wheelbarrow Install membrane: roller, mason's trowel, rented floor sander.

Using a level, trowel, and power grinder

Remove or repair existing flooring

Repair/degloss surface: hydraulic cement or thinset, muriatic acid, rubber gloves Repair structural defect: gravel, reinforcing wire, concrete mix, 2x4 screed Install membrane: membrane, adhesive

Step 1

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 structural defect. Some may be repairable. Others may require professional help.

Step 2

Use a high-quality hammer or small sledge 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 at the top. This will help hold the patching cement more securely.

Step 3

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

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

Step 5

Grind down any high spots you have marked 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.

What If... A Slab has 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.

Comments (1)
bobdijanni wrote:

Shouldn't isolation memrane be installed AFTER step 3? Option is to fill crack and screed flush with a hydraulic cement, the prime and install membrane.

5/16/2016 12:59:16 PM Report Abuse
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