How to Build a Box Around Air Ducts

Measure the length of the exposed duct and record the exact figure. Find the width and depth and add at least 4 inches before writing down the figures. Ductwork can expand and contract slightly, so the extra room allows for safe clearance.

Cut four boards to the length of the air duct to form plates for the soffit sides, much like the top and bottom boards of stud walls.

Lay all four boards on your work surface with the edges flush. Measure across the length of the boards, marking every 24 inches. Use a square or straightedge to mark all the boards at once.

Pair the boards together, laying them parallel to each other. Count the number of 24-inch marks on one board of each pair. Use the saw to cut enough boards, measuring the depth of the ductwork with the spare allowance figured in, to match each pair.

Line the short boards up with the marks on one of the plate boards. Nail through the plate into the short boards, using 8d nails or driving screws. Begin and end with a board, flush with the plate end.

Flip the plate over, with the short studs attached. Attach the opposite plate similarly. Repeat with the other two plates and short studs.

Hold one side of the soffit frame in place, beside the duct. Adjust the location to allow for the required clearance. Sink nails or screws through the top plate into the wood joists above every 8 inches or so and through the end stud into the wall adjacent to the duct. Repeat with the opposite side of the duct and the other side of the frame.

Cut more boards, the same number as used for the studs, to the width of the ductwork, including the spare allowance. Measure the width again, with the soffit frame in place, to verify the measurements.

Place a width-long board against the bottom of the duct box side, lined up with the end of both sides of the frame and flush with the boards. Use a nail or screw to drive through the side wall into the width board on either side of the duct. Continue across the length of the frame, attaching similarly. These boards box in the bottom of the air duct frame.

Cover the entire soffit with your choice of material. Drywall, cut to size with a utility knife, secured with screws or nails and finished with tape and mud works well. You can also choose paneling or even plywood with carpet or wallpaper on top.