How to Frame a Ceiling for Drywall Under Ductwork in a Basement

Take measurements of the duct that is being covered by the soffit. Measure the length of the duct, the width of the duct and the distance between the bottom of the ceiling joists and the bottom edge of the duct. Add 2 inches to the width of the duct and the distance between the duct and ceiling joists so that the soffit frame isn't built directly next to the duct.

Snap a chalk line against the wall 2 inches beneath the duct, if the duct is located close to the wall. Screw a 2-by-4 board to the wall along the bottom edge of the chalk line. Use masonry screws if the 2-by-4 is being attached to concrete, and galvanized screws if it's being attached to drywall. Skip this step if the duct is in the center of the room.

Measure and cut 2-by-2 boards that run the length of the duct. If the soffit is extending out from the wall, cut two boards; if it's in the center of the room, cut four boards.

Measure and cut sheets of half-inch plywood that match the distance between the ceiling joists and bottom of the duct, plus 2 inches.

Construct the side panels for the soffit by laying two 2-by-2 boards on the ground parallel to each other, and laying the plywood across them. Align the edges of the plywood with the edges of the 2-by-2s, and connect them with 1 5/8-inch screws. Use one screw every 6 inches for the length of the side panels.

Mark the location of the side panels onto the bottom edge of the ceiling joists, keeping it 2 inches away from the side of the duct. You can do this by snapping a chalk line or by using a laser level.

Screw the side panels to the ceiling joists with the help of an assistant. Position the side panel with the plywood side facing the duct, and use 3-inch screws to attach it. Install the second side panel, if there is one.

Cut lookouts out of 2-by-2 boards to fit along the bottom of the soffit. The drywall will be screwed to the lookouts along the bottom. If the soffit extends out from the side wall, measure the distance between the side panel and the 2-by-4 screwed to the wall. If there are two side panels, measure the distance between them.

Attach the lookouts to the bottom of the soffit framing, using one lookout every 16 inches. If the soffit extends from the wall, attach the outer edge by driving nails through the side panel into the lookout, and the inner edge by driving the nails through the lookout and 2-by-4 board at a 45-degree angle. If the soffit has two side panels, drive the nails through the side panels into the lookouts on both sides.