Preparing an Existing Slab For Mortar

Take these steps to make sure your concrete slab is solid, smooth, and level before you start the job.


It's easy to mortar a concrete patio or walkway that is solid, smooth, and level. If you plan to mortar your present slab, check its condition before you start the job.

First check for large cracks and sagging sections. If you find such conditions, the base is not adequate -- you'll need to remove the slab and pour a new one.

Even if your present slab appears to be good, you need to make sure it will stay that way. Dig along the perimeter of the slab and look for a 4-inch gravel base and 3 to 4 inches of concrete. If an adequate subbase is present, check the surface for drainage. A slab must slope at least 1 inch for every 4 feet and should not contain spots higher than 1/8 inch in 10 feet.

A surface crowned in the center for drainage will work as long as the crowning is gradual. You can repair minor holes or flaking with the techniques shown.


30 to 45 minutes per square yard

Repair surface: level, hammer, cold chisel, trowels, grinder, vacuum cleaner, sanding block, brush
Repair structural defects: sledgehammer, crowbar, wheelbarrow, rented concrete saw or jackhammer

Leveling, troweling, grinding

Repair surface: hydraulic cement, thinset, self-leveling compound
Repair structural defects: gravel, reinforcing wire, epoxy bonding agent, concrete mix, 2x4 lumber

Step 1

Working in 6-foot-square sections, check the surface with a 4-foot level. Mark any cracks, high spots, and other defects with a carpenter's pencil. Cracks may be a sign of structural problems. Some may be repairable. Others may require professional help.

Step 2

Use a small sledge and a cold chisel to open small cracks so you can fill them. Angle the chisel into each side of the crack to create a recess wider at the bottom than the top. Doing so helps 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 to feather out the edges until the patch is level with the surrounding surface. When the patch has cured, install an isolation membrane.

Step 4

To fill depressions in the slab, pour a small amount of self-leveling compound into the depression or trowel on a skim coat of thinset. Add thinset or compound until the surface is level. If you're using thinset, feather the edges even with the floor. Self-leveling compound will do this on its own.

Step 5

Grind down high spots with an angle grinder fitted with a masonry-grit abrasive wheel. Hold a vacuum cleaner hose near the grinder to remove the dust as you work. Vacuum and damp-mop the surface thoroughly.

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