Tiling over tile
If your old ceramic tile is worn or dated, you can lay new tile right over the old, and avoid the huge job of tearing out the old tile. In fact, pros do it all the time. But this assumes that the floor underneath is solid (concrete) and that there are no cracks in the existing tile (indicating underlying problems in the concrete).
80-grit sanding belts
Thin-set mortar with latex fortifier
Step 1: Check the surface
Make sure all the tiles are solidly bonded to the floor by tapping them lightly with a wood mallet or a chunk of 2×4.
Note: A hollow sound is an indication that a tile is loose.
Remove and reset any loose tiles with thin-set.
Step 2: Grind off the high spots and clean the tiles
Lay a 1.2m level on the old tile and mark any high spots.
Grind them down using a right-angle grinder with a masonry wheel.
Sand all the tiles with a belt or orbital sander (80-grit) to scratch any surface glazes.
Remove any mouldy or loose grout with a rotary tool or carbide scraper.
Vacuum the tiles and clean with detergent and water to remove dirt, sealers and wax.
Rinse the surface with clear water and let it dry.