How to Make a Square Ductwork

Measure the inside dimensions of the duct you are joining to with the new piece of ductwork. Because you are working with square ducts, each side should be the same length. The maximum size square you can create with a 96-inch-long piece of 1-inch duct board is 22 inches, while a 120-inch board can yield a 28-inch square. For 1 1/2-inch duct board, the maximum size for a side of 96-inch-long board is 21 inches, while a 120-inch board can accommodate a 27-inch size.

Find the center of the length of the appropriate size piece of duct board with your measuring tape and mark it. For instance, the center of a 96-inch-long board would be at 48 inches.

Measure over -- either to the right or left of the mark, the length of the inside measurement taken in Step 1 plus 1 3/4 inches for a 1-inch-thick board, or 2 3/4 inches for a 1 1/2-inch-thick board, and make a mark. Repeat one more time in the same direction. Cut off any excess on that side with a utility knife.

Measure and mark on the other side of the center marked in Step 2, moving over your interior measurement plus 1 3/4 or 2 3/4 inches as appropriate. From that mark, measure over your interior measurement plus 2 3/4 or 3 3/4 inches as appropriate. Cut off any material past the last mark with a utility knife.

Center your V-groove tool over the first mark from the end (either end) and draw it straight down. The tool carves out a V-shaped channel that does not cut through the paper on the back side of the board. Repeat on the next two marks.

Measure 1 3/4 or 2 3/4 inches from the short edge of the first end of the board and make a mark. Run the rabbet tool down that edge.It removes a shallow rectangle of material that the other end of the board fits into, called a rabbet joint.

Measure 1 3/4 or 2 3/4 inches in from the opposite end and make a mark. Using a utility knife, strip all the material in that 1 3/4- or 2 3/4-strip off, down to the backing paper on the bottom face of the panel. This creates a stapling flap.

Fold the panel along the three V-grooves; each should form a mitered corner. Fit the end with the stapling flap into the rabbeted edge, folding the flap over to conceal the joint. You should now have a perfect square of ductwork. Staple along the flap with a staple gun every 2 inches.

Cover the staples with pressure-sensitive foil tape, wide enough to extend at least 1 inch on each side of the joint.