How to Add a Bathroom to a Bonus Room Above a Garage with a Vaulted Ceiling

Define the square footage for the new bathroom. Use a 5-by-8-foot space at minimum to install a tub-shower combo, toilet and sink space. Enclose the bath walls using 2-by-4-inch studs. Nail a top plate on ceiling joists to define the bathroom ceiling. Count on having a vaulted ceiling in the bathroom as well. Use rented scaffolding to install the plates at the ceiling’s highest levels, which can be 12 feet or more.

Drop a plumb bob to the floor to line up floor plates. Mark with carpenter’s chalk so you can install a 1-by-4 board sole plate around the bath outline. Nail the sole plate around enclosure walls and mark the space where you will cut the bathroom door opening.

Cut studs to form an 8-foot tall wall space with a top plate. Build this wall framing on the floor, but plan to build a second tier of studs that will reach into the vaulted space. Use scaffolding to measure studs that will reach from the 8-foot high top plate to the plates at the highest part of the ceiling.

Nail the unit’s higher studs into place at the floor level, capping those studs with a top plate that will fit under the ceiling's top plate -- resulting in a double top plate. Construct higher walls this way so the framing will not wobble or shift when you tilt it into place. Build all wall framework needed for each bath wall at floor level and then tilt it into place.

Run plumbing lines and electrical wiring while stud walls are open. Plan to hang a chandelier high in the vaulted ceiling space, so install the electrical box unit for the chandelier by standing on a ladder or scaffolding.

Install a vent space for the toilet, shower-tub and sink space by creating a wet wall, preferably using an outer wall of the garage. Frame this wet wall, which will carry all plumbing with 6-inch wide studs. Use this wider framing so the vents and plumbing pipes can fit inside the wall much easier.

Add drywall over studs on the bathroom enclosure walls. Nail up sheets of drywall over studs on both the bonus room side and the bathroom side. Staple insulation on outer house walls between studs before nailing up drywall. Finish drywall seams with joint tape and drywall joint compound. Apply three coats of joint compounding, sanding the seams between applications.

Install a bath-shower combo unit over sub-flooring. Position the unit over the drain. Use a tub drain wrench to install the top half of the drain. Attach the tub unit to the wall using drywall screws. Align the spout and drain as you install the unit, using shims to level the unit as you go. Cut a space for the shower valve using a rotosaw. Ensure that shower wall panels interlock properly. Screw them into wall studs and install all shower trim before finishing up plumbing details.

Buy a vanity cabinet for the sink space. Use wood shims to level the cabinet and connect all water lines and drains under the cabinetry. Seal the space between the vanity and wall with a clear sealant. Attach cabinet doors last.

Finish the floor with tile. Buy 12-inch squares of mosaic tiles mounted on mesh. Snap a chalk line to divide the floor into quadrants, so you can create a tile floor pattern more easily. Seal the tiles before applying grout and mortar. Comb the mortar into place, set the tiles with a piece of board and rubber hammer, and apply grout.

Cut and install baseboard materials over the finished flooring to hide any gaps. Caulk seams around the floor edges as need. Seat the toilet over the floor drain with a wax seal.

Complete room details. Nail door jamb materials into place and hang the bathroom door. Secure towel bars and any bathroom shelving into place. Use 4-inch screws driven into wall studs with an electrical screwdriver to hold any wall materials tightly. Install a chandelier in the high ceiling space to accent the vaulted bathroom ceiling.