How to Build a Drywall Soffit

Measure for the size and placement of the soffit against the wall. Place your measuring tape at the top of the ceiling and measure down. Remember to take into consideration the thickness of the plywood box and the drywall as you measure. Draw pencil marks across the wall to mark the bottom edge of the soffit frame. For example, if the height of the finished soffit box will drop 10 feet down from the ceiling and you intend to use 1/2-inch plywood and 1/2-inch drywall, measure 9 inches down the wall.

Snap a chalk line at the pencil marks. The bottom board of wood, or the "bottom ledger board" of the soffit, will sit on this line.

Mark the location of the studs on the wall using a stud finder.

Measure the length of the bottom ledger board and cut a portion of 2-by-4 according to the measurement.

Nail the lumber to the wall to create the bottom ledger board portion of the soffit. Nail into the studs that you had previously located with the stud finder.

Measure for the size and placement of the soffit upon the ceiling. Place your measuring tape in the corner of the ceiling at the wall, and measure for the depth of the soffit box. For example, if your finished soffit box will extend 12 inches away from the wall and you intend to use 1/2-thick plywood and 1/2-thick drywall, measure 11 inches out from where the ceiling meets the wall. Snap a chalk line to connect your measurement marks.

Mark the location of the joists in the ceiling using a stud finder.

Measure the length of the top ledger board for the ceiling, and cut a 2-by-4 according to the measurement.

Nail the lumber to the ceiling to create the top ledger board portion of the soffit. Nail into the joists that you had previously located with the stud finder.

Place the level against the bottom of the bottom ledger board out toward the room. The level mimics the missing bottom of the soffit box frame and allows you to measure for the facing boards that will be installed here. Measure the height from the top ledger board at the ceiling down to where the level meets with the bottom ledger board at the wall.

Cut a section of OSB or plywood according to the measurement you just made from the top ledger board to the level. This board will be the front facing board for the soffit box. For best results, cut the board so that the straight-edged, factory edge of the board faces down toward the floor and your cut edge abuts the ceiling.

Cut a section of 2-by-2 lumber the same length as the facing board. Screw the 2-by-2 to the factory edge bottom of the facing board. This 2-by-2 section of wood gives the facing board some stiffness and provides a platform upon which the drywall will be secured.

Cut another section of OSB or plywood for the bottom side of the soffit box. Place the board against the underside of the bottom ledger board and the bottom of the 2-by-2 facing board. Screw the pieces together.

Cut two sections of drywall to cover the soffit box. Hammer the corner bead where the two portions of drywall meet at the corner of the soffit box.

Apply several layers of joint compound to the corners, allowing the compound to completely dry between coats. Sand the compound smooth. Prime and paint the soffit.