Tile is installed on shower walls because of its superior ability to repel water when properly installed. Cement backer board is a common base for tile walls. Concrete is less susceptible to moisture than drywall. It is also sturdy and can be cut by scoring it with a razor knife. Unfortunately, the one thing cement backer board can't withstand is an impact, especially one that can damage the tiles covering it. Damaged backer board must be repaired before the shower is used again to prevent water damage to the interior of the wall.
Remove a 3-by-3-foot section of tile around the damaged area, using your hammer and chisel. Chip away at the grout between the tiles, then pry each one free. If you have replacement tiles that match the original tiles, the old tiles don't have to come off the wall intact.
Scrape away any leftover grout and adhesive from the existing backer board with your putty knife. Focus your removal efforts on the outer area of the board so there will be less material to deal with during the sawing phase of the demolition and a smooth surface for attaching the new tile.
Slip the point of your keyhole saw into the damaged area of the backer board. Push and pull the saw, while putting pressure on one side, to cut through the backer board. Cut a square that goes from the center of the damage to the center of the wall studs on each side, and at least 8 inches above and below the damaged area. Remove the backer board from this area.
Measure and cut a piece of new backer board to fit the empty spot. Attach the replacement piece to the wall studs by driving 1-inch screws through the board and into the studs. Place a screw every three inches up each side of the backer board to hold it firmly in place and make a solid foundation for your new tiles.
Seal the seams around the backer board with concrete patch, using your putty knife. Smooth the patch as much as possible. Allow it to dry for 24 hours.
Spread tile adhesive over the entire area with your putty knife. Hold your small-toothed tiling trowel at a 30-degree angle to the wall and slide it across the wall to texture the adhesive. Make a cross-hatched pattern on the wall to provide superior hold and coverage for the tiles.
Press new tiles into the adhesive, leaving a 1/8-inch gap around each tile. Make sure all the tiles are securely fastened to the wall and that the face of each new tile is recessed just far enough to be flush with the existing wall tile. Cut tiles with your wet saw as necessary to match the existing pattern on the wall.
Fill the grout bag with grout. Twist the end of the bag shut. Hold the tip of the grout bag against the wall. Squeeze the bag and drag it along to fill each of the gaps between the tiles. Take your time, and be sure each grout line is smooth and even.
Let the grout set about 15 minutes and then wipe with a damp sponge. Let dry and then buff the haze off.
Let the grout cure for 24 hours. Squirt grout sealer onto each grout line. Wait 12 hours for the sealer to cure completely.