How to Frame a Basement Soffit Around an HVAC

Mark a line on the wall near the duct that is 2 inches beneath the duct, using a chalk line or laser level. Screw a 2-by-4 board to the wall along the bottom edge of the line. Use galvanized screws if the wall is finished, and masonry screws if you're attaching it to concrete. If the duct is in the center of the room, skip this step, as you'll construct a three-sided soffit to cover the duct.

Cut 2-by-2 boards that run the length of the HVAC duct. These boards will be used to frame the side panels of the soffit, so if the soffit extends from the wall, cut two boards. If the duct is in the center of the room and you're constructing two side panels, cut four 2-by-2 boards to fit your needs.

Cut half-inch plywood boards that fit the length of the duct, and the height of the duct plus 2 inches. You add the extra 2 inches so the bottom of the soffit doesn't come into contact with the duct.

Lay two of the 2-by-2 boards on the ground parallel to each other. Lay the plywood boards for the side panels on top of the boards, aligning the outer edges. Screw 1 5/8-inch screws through the plywood and into the 2-by-2 boards every 6 inches. This is one of the side panels. Build the second side panel using the same process, if applicable.

Using a chalk line or laser level, mark a line on the bottom of the ceiling joists 2 inches away from the duct.

Lift the first side panel into place with an assistant's help, then screw the 2-by-2 boards to the ceiling joists with 3-inch screws. Install the second side panel, if applicable.

Measure the distance between the side panel and the 2-by-4 board on the wall, or between the two side panels. Cut 2-by-2 boards to fit that distance. These boards are called lookouts, and are used to attach the drywall after the frame has been completed.

Install the lookouts along the bottom of the soffit, using one every 16 inches. Connect them to the side panels by driving nails through the side panels and into the end of the lookouts. If you're installing the soffit against the wall, nail the end of the lookout to the 2-by-4 board at a 45-degree angle.