For duct over 18 inches, what should be used?

• A. 1-1/2-inch strips of 16-gauge band iron
• B. 1-inch strips of 22-gauge galvanized iron
• C. 3-inch strips of 14-gauge steel
• D. 2-inch strips of 18-gauge aluminum

Answer: (A)

When a duct section is longer than 18 inches, you reinforce the seam with external bands to keep it from leaking or pulling apart under air pressure. The standard, reliable way is using 1-1/2 inch wide strips of 16-gauge band iron. That width distributes the fastening load well across the seam, and 16-gauge thickness provides enough stiffness and strength for a durable joint when riveted or screwed into place.

The other options are less suitable: a thinner 22-gauge strip wouldn’t hold the seam securely; a thicker 14-gauge strip is heavier and more than what’s needed for this reinforcement; and an 18-gauge aluminum strip isn’t as stiff or durable as steel band iron for duct joints, making it a poorer choice for this purpose.