Things You'll Need
Fiberglass mesh tape
Setting-type joint compound
Cutting cement backer board produces dangerous dust. Wear a dust mask approved by the National Institute for Occupational Safety and Health.
Remodeling a bathroom often requires cement backer board be installed as a substrate for tile or another wall covering in a tub surround or other area that is exposed to large amounts of moisture. At some point along the wall, the cement backer board will meet with the drywall used in the rest of the room. This joint needs to be flush and smooth in order for the tile or other wall covering to be installed correctly and to create a professional, finished appearance.
Create a straight end to the drywall by either making a straight cut with a drywall saw or installing an extra strip of drywall to close the space between the drywall and where the backer board will end.
Install additional two-by-four backing in the wall where the joint between backer board and drywall will be. This can be done prior to installing the extra drywall strip to close the space, or screw a two-by-four to the back of the last drywall sheet. Place it so that half of its wider side is behind the drywall and half is exposed so that the backer board can be attached to it.
Nail shims to the two-by-four so that the front surface of the backer board is even with the front surface of the drywall. Keep in mind that ½-inch backer board is frequently less than ½ inch thick.
Screw the last piece of backer board into place, leaving a 1/8-inch gap between the backer board and the drywall.
Tape the joint between backer board and drywall with fiberglass mesh tape.
Mix thin-set mortar according to the manufacturer's instructions and apply a thin coat over the fiberglass tape with a drywall knife. Push the mortar through the mesh tape and into the 1/8-inch space between the backer board and the drywall. Run the drywall knife over the mortar to create a smooth finish and remove any excess material.
Install your tile as planned. If the tile does not completely cover the joint, apply a coat of setting-type joint compound, which is more water-resistant than typical joint compound, to the joint. Allow the joint compound to cure, prime it and paint it the same color as the rest of the non-tiled area.
Matt Brown has been writing professionally for more than 15 years. He shares his experience in home remodeling and do-it-yourself projects with his readers. Brown earned his bachelor of arts in communications from Marist College in Poughkeepsie, N.Y.