Though it may not be as common as bare wood, tile can be laid on a window sill if you want to change the appearance. Since window sills have less space than walls or floors, in general, you may want to stick with smaller tiles, which fit more easily in the available space and provide you with more design options than their larger counterparts. If you already have interior tile installed on a window sill, you must first remove the old tile before laying the new.
Break up the grout between the tiles by placing the tip of a flat-edge screwdriver small enough to fit between the tiles without touching the edges of the tiles against the grout, and striking the bottom of the screwdriver with a hammer to chip out a piece of grout. Then, move the tip of the flat-edge screwdriver into the chipped area in the grout, pointing the screwdriver tip in a slightly upward angle and strike the screwdriver again. Chip away no more than 1/4 inch area of the grout at a time.
Slide a putty knife between the edge tiles, beneath the bottom of the tile and the window sill. If you cannot push the putty knife by hand, tap the bottom with the hammer to force the putty knife beneath the tile. Force the putty knife under the tile only 1/8 to 1/4 inch at a time, and then slide the knife tip under the edge of the tile and tap with a hammer again.
Pry the tiles up from the window sill once approximately half of the tile has been separated from the sill. Place the putty knife under the tile and pull upward. If the tile doesn't come free with just a little pressure, continue tapping the putty knife beneath it to remove more of the adhesive before trying again.
Sand the window sill with medium-grit sandpaper to remove the grout and tile mastic left behind on the window sill. Brush the sill with a cleaning brush to remove the dust.
Lay the tiles out on the window sill in the pattern that you want. When laying out tiles, use an appropriately sized tile spacer for the size tiles you are using which will be listed in the paperwork provided with the tiles. Plan the design for the sill so that you are cutting the back and edge tiles.
Cut down the tiles at the edges to complete your tile window sill design. As you cut the tiles to size, put them into their places in the design to ensure a proper fit.
Remove the tiles from the window sill and lay them nearby in the same design that you have on the sill. Spread tile mastic on the sill with a small trowel.
Place the tiles back onto the sill, beginning at the front edge and working backward. Use the tile spacers between tiles to maintain the proper spacing. Leave the spacers in the tile for at least 30 minutes to allow the tiles to have enough time to set properly in the mastic. Wipe up any adhesive on the tops of the tiles.
Spread grout over the tiled sill with a trowel, filling in all the spaces. Move the edge of the trowel across the face of the tiles to remove as much of the excess grout as possible and then carefully wipe any grout left behind up with a dry cloth. After 30 minutes or so, run a wet sponge over the tiles to clean grout residue.