How to Box in a Heater Duct

Measure the length of the duct and record the dimension. Repeat with the width and depth. Typically ductwork runs along one surface (such as the underside of the ceiling, or the upper side), is exposed on the opposite and enters into or continues through a partition or wall. The length is a duct's run across the room, the width is a duct's horizontal measurement -- from side-to-side across the duct -- and the depth is akin to the height, or the vertical measurement. Treat round ductwork similarly, using the diameter for depth and width.

Add 3 or 4 inches to the depth and width measurements -- or, more precisely, to any exposed edge. This allows the ductwork to expand and contract freely without affecting the soffit framing.

Cut four boards to measure the length of the duct. These will form the plates for the soffit's walls. Use either 2-by-4-inch boards or a smaller board, such as a 1-by-1-inch, for the entire wood framework.

Align all four boards evenly. Running the tape measure up the length of one of the boards, mark every 16 inches for standard stud spacing. Lay a carpenter's square across the boards, ensuring that the ends are flush, and mark through all the boards at once.

Count the number of marks on one plate. Cut additional boards, twice the number counted, to run as studs between the plates. Use the duct depth calculated, including the clearance. A 6-inch-deep duct, for example, would need a soffit about 9 inches deep with clearance. Subtract the thickness of both plates and cut the studs to that length if space is a consideration. Alternatively, simply cut the studs to the depth measurement and allow the soffit to be slightly larger on the outside.

Lay a plate on the work surface, resting on its thinnest edge if applicable. Line a stud up with the first mark, turning the stud to rest on its thin edge as well. Thus, the wide face of each runs vertically. Nail through the plate, into the end of the stud, with 8d nails or screws. Stagger the nails slightly to avoid splitting the wood.

Continue across the length, adding one board flush at the end of the plate and another at the beginning. Flip the plate and studs over and attach the opposite plate, aligning the marks with the stud ends, similarly. Repeat with the remaining two plates and studs.

Cut additional frame members, using the duct width measurement you calculated, including the clearance allowance. Subtract the width of the plates, on either side, to ensure a proper fit. Thus, a 9-inch-wide duct -- 12 inches with clearance -- framed with 2-by-4s, for example, requires 5-inch-long boards running widthwise. Framed with 1-by-1s, this same duct needs 10-inch-long boards.

Lift one side of the soffit into place, pressing the plate against the wall or ceiling, depending on the way the duct runs. Adjust the location to allow for the clearance between the duct and the soffit wall. Drive nails or screws through the soffit plate into the joists or studs framing the ceiling or wall. Repeat for the opposite side of the duct, running a tape measure to verify its exact placement before attaching. When complete, two miniature walls run on either side of the duct.

Insert a widthlong board between the soffit walls, starting at the beginning and continuing the length of the soffit. Nail or screw into place, ensuring each is flush with the bottom edge of the plate involved. When complete, these boards provide the bottom of the duct frame, when running along the ceiling, or the facing if running vertically up a wall.

Sheath your soffit with the material of your choice. Cut drywall to size, using a utility knife and straightedge to score the surface and snapping to size. Attach with drywall screws through the frame and finish with tape and mud. Paneling, plywood or other choices are also available, although drywall is considered a fire-resistant material and provides the best option.