Some homeowners can save time and money by tiling over existing tilework. You can safely do it by following the steps below once you have mad sure that the existing tile is stable and does not show any signs of cracking and that the increased dimensions of the second layer of tile will not interfere with aesthetics or dimensions around doorways or fixtures.
Carefully inspect every inch of the existing tile and grouting for cracks. Use a butter knife or other small metal object to tap on each of the tiles to make sure there are no hollow areas within the tiled wall. The integrity of your new wall literally hangs upon the soundness of the old one.
Wash the existing tile thoroughly to remove oils, wax, grime and other coatings that will interfere with the bonding process.
Sand or rough up the surface of the old tile with a disk or belt sander in order for the new tiles to bond properly. Clean with water and dry thoroughly to remove particles left from sanding. Attach a support board beneath your base to help support the new tiles while the adhesive sets.
Start from the bottom row of tiles. Apply the adhesive with a notched trowel, spreading it out to the edges of the old tile. Apply replacement tiles carefully, tapping in to set with a rubber mallet. Clean off any excessive adhesive from the tile before it dries. Tiles should set in about thirty minutes.
Seal the tiles with grout, wiping off excess. Finish with caulking when dry.
Some adhesives emit toxic fumes; make sure you have proper ventilation when performing the job.