Tiling over a fireplace surround that already has tile attached is easier than you might think. Years ago, most people would tear out the old tile and replace it with the new. Nowadays, it is quite possible to tile over the old tile and avoid the time and money of cleanup and backerboard installation. The average do-it-yourself homeowner can cover their old tile fireplace surround with new tile in one to two days, depending on the size and scope of the project.
Remove any loose tiles from the fireplace by gently prying them away with a putty knife. Use a wire brush to clean the excess loose materials from the void. Fill the damaged tile slot using multipurpose thinset mortar and a putty knife. Make sure that the thinset is even with the surface of the existing tile. Allow the thinset to dry for 24 hours before proceeding.
Rough the surface of the old tile with 60-grit sandpaper. This will remove any slick surface for better adhesion. Wipe the surface down using TSP (trisodium phosphate). This is a powdered cleaner that will also etch the surface of the tile and help with adhesion. It should be mixed with water. Follow the box's mixing instructions.
Apply a bonding agent to the surface of the existing tile. Use a roller and brush to thoroughly cover the surface of the tile. Follow the drying instructions on the can before you proceed to install your new tile.
Place level and square layout lines on the fireplace surround and check to see how your new tile will fit. If there will be a piece on the end of the installation, make sure the piece ends where the surround meets the wall. Never begin your installation on one side, working your way around. This could make the appearance of the tile uneven. Make a center mark on the fireplace in order to work your tile to the right and left of the surround, and always begin your first row at the bottom of the surround and work up.
Mix your multipurpose thinset in a 5-gallon bucket using a drill and paddle bit. Add one gallon of water, and begin adding the thinset powder while you are mixing. Add enough to get a pancake batter consistency. You may also add a latex additive to the mixture in order to increase adhesion of your tile.
Lay your first row of tile at the bottom of the fireplace, making sure that the first row is level. Add spacers beneath the first row if necessary to get application straight and level. Continue up the fireplace, adding spacers between each row. After the tiles have dried for 24 hours, remove the spacers and grout.
Mix the grout in a 5-gallon bucket in the same way as the multipurpose thinset. Using a grout trowel, sweep the grout into the tile joints and wipe away the excess using a wet sponge, leaving no excess grout on the surface of the tile.