How to Make Soffits to Hide Ductwork

Screw 2-by-3 boards across the floor joists on each side of the run of exposed ductwork to serve as support studs, leaving a ¼-inch gap between each side of the duct and the studs. Align the boards so they touch the joists on their narrower edge.

Create forms -- essentially ribs in a shallow U-shape -- to create a surface that allows you to later attach drywall to complete the soffit. Measure the distance from a floor joist above the duct to the bottom of a ductwork section at its middle, where it has more depth than at a crimped seam where it meets another length of duct. Add ½ inch to this measurement. Cut two 2-by-3 boards to this depth.

Cut a third board to the width of the duct plus the width of the of the support studs, in this case, 3 inches: two times the actual width of each 1 ½-inch support stud. Create a U-shaped form by drilling pilot holes and then attaching the two short pieces to the long piece with two wood screws creating each butt joint.

Mark a spot 16 inches from the beginning of the duct run. Slide the first form up under the duct with the help of an assistant, so the form touches the outsides of the support studs and is centered over the mark.

Screw one side in place. Check that the bottom of the U-shaped form is level and screw in the second side of the form. If you can’t enlist an assistant, hold one side of the form in with a bungee cord and the other with a single temporary screw to level the form.

Continue building forms and installing them on 16-inch centers.

Fasten drywall to the soffits with four or five drywall screws on the bottom of each form and two to three on the sides. Align the end of each 4-foot length of drywall to fall in the center of a form, where it abuts the next drywall section. Finish the drywall with joint compound applied to the screw dimples and a combination of joint compound and tape on the seams. Paint the completed soffit to match your walls.