How to Build a Soffit Under Duct Work

Measure the sides and bottom of the duct and add 2 inches to each measurement, then measure the length of the duct. If the duct is close to the wall, you may want to just build a soffit that covers the bottom and the outer edge of the duct, with the bottom panel running to the wall. If the duct is in the center of the room, build a soffit that covers the bottom and both sides of the duct.

Snap a chalk line on the wall 2 inches beneath the duct. Place a 2-by-4 board against the wall underneath the chalk line, and attach it to the wall. Drive galvanized screws into the studs if the wall is finished, and masonry screws directly into the wall is it's concrete. If the duct is in the middle of the room, skip this step.

Cut half-inch plywood boards for the sides of the soffit.

Cut 2-by-2 boards to fit the length of the duct. You need two of these boards for each side panel.

Lay a plywood board on top of two of the 2-by-2 boards, lining up the plywood so that the outer edges are lined up with the outer edges of the 2-by-2s. Drive 1 5/8-inch screws through the plywood and into the 2-by-2s every 6 inches. Make a second side panel if applicable.

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

Raise the side of the soffit into place against the ceiling joists with the help of an assistant, positioning it so that the 2-by-2 boards face out away from the duct. Attach the side panel to the ceiling joists with 3-inch screws. Install the other side panel, if applicable.

Measure the distance between the inner edges of the plywood on the side panels, or between the side panel and the wall, and cut 2-by-2 boards to fit this length. These boards are called the lookouts, and they are used to strengthen the soffit and give you an object to screw the drywall to.

Install the lookouts along the bottom of the soffit every 16 inches by hammering nails through the 2-by-2 on the side of the soffit and and into the lookouts. If the soffit is attached to the wall, drive the nails at a 45-degree angle through the 2-by-2s into the 2-by-4 you attached to the wall. This is called toenailing.

Cut 1/2-inch drywall sheets to fit along the bottom and sides of the soffit.

Attach the drywall to the 2-by-2 boards (and the 2-by-4 board if applicable) with drywall screws. Use one screw every 6 inches.

Attach metal corner bead to the bottom corners of the soffit by nailing it to the 2-by-2 boards through the drywall.

Apply joint compound to the joints between the drywall sheets and to the metal corner bead. Place drywall tape over the joint compound, and run the putty knife over it to press it into the joint compound. Allow the joint compound to dry overnight.

Apply two more layers of joint compound, allowing each application to dry overnight. Sand the joints smooth, and remove the dust with a clean rag.

Cover the soffit with a coat of primer, and allow it to dry. Paint the soffit with two coats of paint, allowing the first coat to dry before you paint the second coat.