How to Put a Bathroom in an Attic

Draw the house floor plan to include the attic and the floor beneath it. Sketch a basic layout for new bath space, taking the staircase accessibility into consideration. Plan to route water lines and drains from a bathroom directly under the attic bath, if possible.

Reinforce the floor joists, if needed. Check local building codes, which typically require the joists to support 30 pounds of live weight per square foot. Hire an expert to address these issues, nailing or bolting larger joists to old ones -- which is a carpentry technique referred to as “sistering.” Ask a pro to bolt metal flitch plates to joists as another alternative.

Enlarge the attic staircase footprint, if necessary. Measure the stair dimensions and hammer long nails from the attic side downward into the ceiling below to create the outline. Use a carpenter's level to draw the shape on the attic floor.

Ask an expert carpenter to cut the rafters with a circular saw. Ensure that there are no double rafters cut, because these are support beams for the house structure, literally holding the house intact. Construct the stair framing box by cutting 2-by-12-inch boards that you will miter at all corners. Secure the stair opening framework pieces with 4-inch flathead stainless steel screws and nail them to the attic's floor joists. Construct a wider staircase around this opening.

Build framing for the bathroom. Draw the bath outline on the subflooring. Allow 4½ inches for the thickness of the walls, which includes ½-inch-thick drywall and wall studs. Cut 2-by-4-inch boards to form the bottom plate for the bathroom walls. Secure the plate to the floor joists using 3-inch screws.

Use a plumb bob to figure out the location of the frame's top plate. Hang the plumb bob from the ceiling so its point touches the bottom plate. Mark the ceiling at that point so you'll know where to nail the top plate.

Mark the top plate and bottom plate every 16 inches to show the center of each wall stud. Cut each stud to fit individually if the attic has sloping walls. Drive 3-inch screws diagonally through the studs and into the plates to attach the studs.

Run wiring and plumbing to the bath while the stud walls are open. Use a hole saw to cut openings in the attic subflooring for wiring or pipes.

Nail drywall to cover the bath area. Use a staple gun to install fiberglass insulation between studs. Add drywall to the interior walls and ceiling of the bathroom. Cover drywall seams with drywall tape, smooth on joint compound, and allow it to dry. Sand the seams, then repeat the process two more times.

Install a pre-made tub-shower unit. Assemble the shower pieces and floor pan to fit a framing enclosure. Measure for the plumbing to fit the faucet and knob unit from the side wall and floor, marking the locations on the back of the shower unit. Drill pilot holes from the back of the tub, using them as guidelines. Use a hole saw to cut openings from the front of the tub. Connect the shower unit to the wall framing by nailing the flange running around the unit. Pre-drill the nail holes before driving the nails.

Install a pedestal sink. Cut off the home's water supply and route copper water pipes and drain lines into the bath wall space. Prepare the copper lines and use a hacksaw to cut off waste lines. Assemble the sink with its pedestal and tighten all the lines. On the underside of the basin, slide a gasket and washer onto the drain. Level the sink and use lag screws to hold it to the wall.

Seat the toilet over the main drain. Twist and press the closet flange into the soil pipe. Rotate the collar until it's positioned correctly over the hole. Use screws to tighten the collar into the subfloor. Hook up water supply lines and seat the toilet over a wax ring with the flat side of the ring to the floor.