How to Make a Duct Flange From Sheet Metal

Measure the circumference of your duct. You need to know how many inches it is around so that you can custom-make your flange and attach it as a single piece. This will prevent air leakage.

Measure a length of sheet metal that matches the measurement of your duct circumference. Use a black marker to mark the place where you will need to cut the metal. Make it approximately 1 inch longer than the measurement you got for the duct circumference.

Draw a line parallel to the edge of the sheet metal, about two inches away from the edge. Draw this line with a straight edge and a black marker. With tin snips, cut along this line. When you get to the end of the length you want, make a 90-degree cut. You now have a strip of sheet metal long enough to wrap around the duct. It will be 2 inches wide.

Make your sheet metal strip into a loop and place it around the duct. It should overlap the duct by about 1 inch. Tape it into place with aluminum tape. Apply the tape all the way around the duct where the metal loop overlaps it. Do not skip any places where the flange touches the duct, because failure to tape these spots will leave air leaks.

Connect a second piece of ductwork by pushing up against the first piece and taping your flange to the second piece with aluminum tape. Test the joint to make sure the connection is firm.