How to Make AC Ducts

Determine the required duct's length, height and width. Add 1 3/4 inches to both the height and width requirements to compensate for the duct board's thickness. For example, consider a duct that needs to be 14 inches wide by 6 inches high. to compensate for the material's thickness, these values will become 15 3/4 inches by 7 3/4 inches.

Place the measuring tape's end along one edge of the duct board. Spread the measuring tape across the duct board. Mark the duct board at four places corresponding to alternating height and width measurements. Position the T-square at each mark and draw a line perpendicular to the measuring tape. This divides the duct board into four segments. Use the cut all tool to remove any extra duct board material. Continuing the example from Step 1, mark the duct board 15 3/4 inches, 23 1/2 inches, 39 1/4 inches and 47 inches from the edge. If the duct board is 60 inches wide, remove the 13 inches of material past the final mark.

Use the modified shiplap cutting tools to cut along one long side and one short side of each segment. Use the female shiplap cutting tool to cut along the remaining long side and short side of each segment. Do not cut so deep as to separate the pieces from one another. Repeat this process for the remaining three segments. These cuts should leave a raised rectangle in the middle of each segment. In the example begun in Step 1, once the shiplap cuts have been made, each section should feature a raised rectangle. The first rectangle should be 14 inches wide. The second rectangle should be six inches wide. The third rectangle should be 14 inches wide. The fourth rectangle should be six inches wide.

Fold the duct board segments so the modified shiplap cuts lock into the female shiplap cuts. This results in a cube with one unlocked seam and two open ends. Seal the unlocked seam with a strip of pressure sensitive aluminum foil tape.

Repeat the above steps with additional sheets of duct board until the required length has been created. Insert modified shiplap cuts into female shiplap cuts for a snug fit. Seal multiple ducts together with strips of pressure-sensitive aluminum foil tape. For example, if a project's length requirement is twelve feet, the steps will need to be repeated twice, resulting in three cubes. Connect and secure the open ends of these cubes to produce a single duct.

Measure the dimensions of the final fabricated duct's open end. Measure, mark and remove a single corresponding rectangle in a fresh piece of duct board. This will be an end cap. Cut out the end cap with the cut-all tool. Use the female shiplap cutting tool along each of the end cap's edges. Fit this end cap into the modified shiplap edge along the end of the final length of duct. Fix the end cap in place with pressure-sensitive aluminum foil tape.