Bathrooms are constantly exposed to moisture, and some areas suffer it worse than others. The small piece of wallboard above the shower, for example, is constantly bombarded with steam, heat and direct hits from splashing water. If you can see visible crumbles of wallboard, it means that the integrity of the wallboard has been compromised. Once the wallboard has reached this point, the damage is too deep to repair cosmetically, and it will only get worse if you try to cover it up.
Cut around the perimeter of the damaged section to a depth of 1/2-inch using a utility knife. Cut vertically from the top of the shower to the ceiling on both sides, and horizontally from wall to wall above the enclosure.
Hammer the straight end of a curved prybar into the wallboard 2 inches up from the top of the shower enclosure, one inch from the wall. Pull out the pry bar and insert the curved end into the hole. Use the curved end to pull or pry the wallboard away from the wall. If it breaks and creates a hole, that’s OK. Insert your hand into the hole.
Pull off the wallboard in pieces if necessary, inserting the prybar where needed behind studs to pry it off. If it resists in corners, use the utility knife to cut it loose and remove it. Pull out any nails using the hammer or diagonal pliers. Note the locations of the studs and nails for reference.
Measure the distance between the studs directly above the shower enclosure. Using a miter saw, cut a 2-by-4 to the measurement. Place a 3/16-inch drill bit in a drill/driver. Stand the 2-by-4 on edge. Place the tip of the bit one inch from the end of the 2-by-4. Tip the bit up at 30 degrees. Drill down through the corner of the 2-by-4 to penetrate out through the end. Do both ends. These are toenail holes for screws.
Tap the stud flush between the wall studs directly above the shower enclosure. The flat side of the stud should be facing your body. Insert 3-inch screws in the holes. Drive them in tight to secure the stud between the two wall studs. This is a backer for the new piece of drywall.
Measure the area where you removed the old wallboard. Cut a new piece of wallboard to the measurement using the utility knife. Place the new piece of wallboard in place. The bottom of the wallboard fits flat against the backer, directly on top of the old piece of drywall where you cut it off. Screw the wallboard to the studs and the backer with 1 1/2-inch drywall screws and drill/driver.
Fill all the nail holes and joints with drywall compound using a 4-inch, flat trowel. Allow the compound to dry overnight.
Cut a piece of waterproof panel board to fit the new drywall using a table saw. Apply construction adhesive to the panel board. Fit the panel board onto the drywall. Press it firmly with your hands to bond it to the drywall.
Apply a 1/4-inch bead of silicone around the perimeter of the waterproof paneling. Use a wet fingertip to smooth the bead. Allow 24 hours before using the shower.