Installing tile in a bathroom seems like the most logical choice for walls, since tile easily stands up to changes in pressure and humidity commonly found in a bathroom. Even if tile already exists in your bathroom, you may need to install new ones to change the style or design of your room. You can lay new tiles over old ones as long as you take the time to properly prepare the old tile wall to hold up the new tiles.
Clean the entire surface of the old tile wall with a rag dampened in white spirit or a similar solvent cleaner. This cleaner will remove dirt and dust as well as oil, wax and grease. These contaminants must be removed to ensure a strong bond.
Sand the surface of the tile with fine-grit sandpaper to lightly roughen up the surface. The rougher surface will more readily accept the bond with the new tile adhesive.
Fill in grout joints and any other dips in the old tile surface with Portland cement filler or leveller, applied with a putty knife. Apply the product to the dips and use a screed to scrape off any excess, creating a completely flat and level surface on which you can install your new tiles. If you allow the grout joints to simply sit empty, the new tile adhesive could bubble with air, creating uneven tiles or causing the tiles to fall off. Allow the filler to dry completely (for at least 24 hours, unless otherwise instructed by specific product directions) before continuing.
Mix tile thinset with acrylic latex additive per product instructions. Using the latex additive in place of plain water creates a better, thicker mixture for adhering to the mostly smooth surface of the tiles.
Spread a ¼-inch layer of thinset over the existing tile with the flat end of a trowel. Scrape the notched side of the trowel back through the thinset after applying an even layer to create ridges that will help with new tile adhesion.
Set the new tiles in place in the thinset. If necessary, use a rubber mallet to gently tap the tiles into place, ensuring that they stick completely in the thinset. Maintain equal spacing between each tile for grout joints; use spacers if necessary to keep these joints even.
Continue to apply thinset and set the tiles until you have covered the entire wall surface. Allow the thinset to dry overnight before continuing.
Remove any spacers from the wall. Mix grout and water per product instructions; spread the grout diagonally across the tile wall with a rubber grout float, filling in all joints between tiles. Allow the grout to set for 15 minutes before continuing.
Wipe all excess grout from the tile wall with a damp sponge. When the grout dries completely, buff away any haze with a dry cloth to restore shine to the tiles.
When spreading thinset, only cover an area on which you know you can install tile within 15 to 20 minutes. Otherwise the thinset will dry too much to use, and you will need to scrape it off and start over. If necessary, cut tiles with a sharp utility knife or tile cutter to fit around obstructions or on the edges of your installation area.
Do not install new tiles over old tiles that are broken or loose. Tear out or repair damaged or loose tiles and fill in gaps with the cement filler. Similarly, glazed ceramic tiles will not bond well with the cement and likely will not support your new tile.