In the case of older homes, or those with wallpaper too difficult to remove, it is sometimes necessary to tile directly over the material. To do so, you must first prepare the paper to accept the adhesive that holds the tile to the wall. It takes only a few hours to ready the wall in most cases, and once the wallpaper is ready for the tile the process is just as with any other tiling project.
Roughen the surface of the wallpaper using sandpaper rated between 220 and 320 grit. The thinset has difficulty bonding well with smooth or coated wallpapers. Use a clean cloth to wipe the wall down to remove dust or particles of wallpaper stuck to the wall.
Spread thinset over the wall using a notched trowel. Apply a layer approximately 1/8-inch thick, and spread smooth across the wall.
Place the first tile at the base of the wall. Wiggle the tile slightly to settle it into the substance and help bond it to the wall. Use a level to check the tile and make sure it is square. Place spacers between the tiles to guarantee uniformity and continue to lay more tiles.
Cut any tiles with tile snips to fit around obstructions or other fittings. Once cut, place the tiles just as any other tile, using spacers and securing into the thinset. Remove all spacers, and let all tiles set and harden on the wall for 24 hours.
Mix grout according to the package instructions and press the grout evenly into the spaces between tiles with a trowel held at a 45-degree angle. Smooth the grout across the seams between tiles in multiple directions to ensure filling.
Fill a bucket half full with warm water and moisten a clean sponge. Wipe the excess grout from the surface of the tiles with the sponge before it hardens. Leave the grout to dry and harden for 48 hours.
Use a wallpaper scoring tool to roughen the paper without the mess of sandpaper. The scoring is less thorough, and may cause the thinset to not bond as well to the wall.
When wiping the grout from the tiles, avoid pulling the grout from between the tiles. Be sure the thinset is thick enough to hold the tile securely to the wall or the tiles may loosen and fall over time.