How to Make a Ceiling Duct

Go into the attic or crawlspace above the ceiling. Find the air-supply ductwork, and mark it. There are a couple of ways to find the right line. If you are in a single-story home, follow the lines coming from the furnace, as they usually go straight into the attic. For multistory homes, turn the heat or air conditioning on, and feel the lines. If it is warm or cold, you have the right line.

Clear away insulation from the area where the vent will be. Measure from the air-supply duct to the desired vent location. Try to have as direct a line as possible so that airflow isn't restricted. The key here is to have as few turns as possible. If the heat or cooling is on, turn it off now.

Hold a duct-starter collar up to the air-supply line. This piece will tie the new line to the existing one. Trace around the smaller end of the collar with the marker. Cut out this area with metal shears. Insert the starter collar into the hole you just cut. There will be a lip on the collar that keeps it from sliding all the way in. Use sheet metal screws to attach the collar to the duct line. Seal the seam with HVAC-rated aluminum tape.

Determine the area for the vent while you are in the attic. This is on the floor of the attic or the ceiling of the room below, depending on how you look at it. Trace around the register box to make a cutting line for the vent hole. Do this while you are still in the attic. Cut out the area with a drywall saw. If there are no joists nearby for the register box to attach to, use 1-by-2-inch lumber to make a simple frame. Attach the 1-by-2-inch lumber to the closest joists with nails so that the lumber is resting against the drywall of the ceiling and is around the hole you just cut. This gives you something to attach the register box to.

Add prefabricated duct pieces to the starter collar. Attach with sheet-metal screws, and seal seams with HVAC-rated aluminum tape. Use as many pieces as necessary to reach the vent location. Attach the register box to the framing with nails so that it is lined up with the new ductwork. Attach the duct to the register box with sheet-metal screws, and seal the seam.

Add duct strapping to the new duct line every 3 to 5 feet, beginning at the main line. Wrap the strapping around the duct, and attach the ends to ceiling joists. This will hold the ductwork in place.

Install the register cover. Cover the hole in the ceiling with the register vent cover. Screw into place with the supplied hardware. Turn the heating or cooling system back on, and check for leaks in the attic, as well as airflow through the vent.