How to Build a Soffit Against Finished Walls

Using a stud finder, locate the ceiling joists and wall studs in the area where you're installing the soffit. If you're building the soffit around ductwork, follow the path of the duct. If it's decorative, place it in a direction that's perpendicular to the ceiling joists.

Snap a chalk line across the wall to mark the soffit. If it's around ductwork, place it 2 inches beneath the bottom of the duct.

Hold a 2-by-4 board against the wall so the the top of the board is along the bottom of the chalk line. Have someone help you with this if you cannot keep the board steady. Nail the 2-by-4 to the studs in the wall.

Cut sheets of half-inch plywood to fit the width of the soffit.

Cut 2-by-2 boards into strips long enough to fit the soffit.

Place one of the plywood strips on top of two of the 2-by-2 boards, aligning the outer edges of the plywood board and the outer edges of the 2-by2s. Screw the plywood to the 2-by-2s every 6 inches with 1 5/8-inch screws.

Mark the location of the soffit onto the ceiling with a chalk line or laser level.

Lift the soffit assembly into position against the ceiling, again having an assistant help you. Place the soffit assembly so that one of the 2-by-2 edges is against the ceiling and the other edge hangs down toward the floor. Position the assembly so that the plywood side faces in toward the wall that the soffit will extend from. Screw the assembly to the ceiling joists by driving 3-inch screws through the 2-by-2 boards along the ceiling.

Measure the distance between the plywood board of the soffit assembly and the wall that the soffit will extend from. Cut 2-by2 boards that fit this dimension to be the lookouts. These boards span the distance between the soffit assembly and the 2-by-4 nailed to the wall, and provide a surface to attach the drywall to.

Nail the lookouts into place, using one lookout every 16 inches along the length of the soffit. Drive the nails straight through the 2-by-2 boards on the outside of the soffit assembly and into the lookouts. Attach the nails on the other side by toenailing them to the 2-by-4. Toenailing is driving the nails into the lumber at a 45-degree angle.

Cut drywall sheets to fit the sides and bottom edges of the soffit.

Screw the drywall to the sides and bottom of the soffit with drywall screws into the 2-by-2 boards, the footings and the 2-by-4 board nailed to the wall.

Nail a metal corner bead to the outside corner where the bottom and side edges of the soffit meet.

Cover the metal corner bead and any joints between drywall sheets with joint compound. Press drywall tape into the compound with a putty knife, and allow it to dry overnight. Sand the joints smooth, then add a second layer of joint compound and drywall tape. Allow it to dry, then add a third layer.

Apply primer to the drywall and allow it to dry, then paint the soffit with two coats of paint. Allow the first coat of paint to dry before applying the second.