Preparing a Slab Floor

Intro

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 its condition before you build new framing (if needed) 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.

Prestart Checklist

Tools
Repair surface: 4-foot level, sledgehammer, cold chisel, carpenter's pencil, margin and mason's trowels, grinder, masonry-grit abrasive wheel, vacuum, brush, mop Repair structural defect: sledgehammer, crowbar, wheelbarrow Install membrane: roller, mason?s trowel

Skills
Using a level, troweling, grinding with a power grinder

Prep
Remove finished flooring

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

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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 a structural defect that will require a professional to correct. Others may be D-I-Y repairable.

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Step 2

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.

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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.

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Step 4

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.

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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.

Step 5

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.

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Apply an isolation membrane (slip sheet) over cracks: Step 1

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.

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Apply an isolation membrane (slip sheet) over cracks: Step 2

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.

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