It’s not uncommon to apply ceramic tiles over old tiles. New ceramic tile can be laid over existing tile if the old tile is in good condition and level. Old tiles may require some cleanup and preparation before new tile can be laid.
Cracked or fractured tiles are unsuitable for using as a base for new tiles, unless they are repaired. Loose tiles need to be tightened with filler or a leveling agent. New tiles should not be laid over more than one old layer of tile. Old tiles must be well bonded to a subfloor, solid and flat.
New ceramic tile cannot be laid until the existing tile is cleaned to remove sealers, grease, soap scum and wax. Old tiles with glossy surfaces must be sanded using 80 grit sandpaper. This roughens the surface, enabling new tiles to stick firmly.
Use a wide putty knife to apply thinset mortar to the old tile. Fill in grout joints and any dips in the old tile. Install the new ceramic tile over the mortar. Allow it to dry, and apply grout.