Installing tile over concrete takes a little more work than applying it over a concrete backer board. Where there is uneven or cracked concrete, you need to prepare the surface for the tile application. Otherwise, your new tile floor is liable to crack at the same location as the existing concrete floor. The tile will also eventually crack at any expansion joints in the concrete slab. With a little work you can install the tile over uneven or cracked concrete.
Scrape the concrete floor with a hoe removing any debris sitting on top of the concrete. Sweep the floor.
Mark any high or low spots in the concrete floor. Use a clear 5 cm by 10 cm (2 by 4 inch) piece of lumber to slide all around the floor on its narrow edge. Look for any gaps between the floor and the bottom of the board. This will indicate a low area in the concrete. Place an "L" where ever the problem exists. Use a permanent marker for this step.
Pour on a self levelling concrete patch compound over the low spots. Follow the detailed instructions on the package label for application. This will bring up the low spots so that the floor will become level. Levelling compound is available at your local timber merchants. Let the levelling compound dry.
Place an isolation membrane over any cracks in the concrete floor. Begin by spreading a 75 cm (30 inch) wide layer of tile adhesive (thin set) over the centre of any cracks. Use an 3 mm (1/8 inch) notched trowel.
Work a 60 cm (24 inch) strip of the isolation membrane into the tile adhesive. Spread out the membrane with a flat trowel, working the excess tile adhesive out from under the isolation membrane. Feather out the edges of the thin set from the membrane. Let the tile adhesive dry over night before proceeding.
Lay out a row of tile in both directions to establish the centre of the tile design. Mark the concrete with a chalk line snapped from opposite walls to indicate the centre line of the room. Lay the first two rows along those lines. Leave room for the grout lines widths when laying out the tile design.
Install the tile, moving by 60 cm (2 foot) square areas. Wipe up any dirt from the concrete before you use the thin-set mortar. Hold the notched trowel at 45 degrees and apply the thin-set until it covers the entire 60 cm (2 foot) square. Do not cover the reference chalk line.
Place the tile so that it sets with one edge precisely on the reference line and gently push down on it. This ensures that there is a strong bond between the tile and concrete. Place the next tile along the side of the last tile. Place two tile spacers between the tiles to ensure there remains an even grout line between the tiles.
Repeat for the remaining tiles. Placing tile spacers between the tiles will ensure that the tiles spacing remains even. The tile spacers also help to keep straight lines between adjacent rows of tile. The tile spacers come in different sizes. Let the tiles dry over night before you grout the floor.
Wipe the surface of the tiles with a damp sponge to removes the haze left behind by the grout. Let the grout dry thoroughly before applying a grout sealer.
Mix the thin-set and grout to the consistency of peanut butter. Cut any tiles using a tile saw outside to cut down on the mess. Use thick knee pads when working on concrete.
Wear safety glasses when using power tools.