In standard ductwork fabrication, which seam is generally accepted?

• A. Spiral seam
• B. Pittsburgh seam
• C. Lap seam
• D. Butt seam

Answer: (B)

In standard ductwork fabrication, you want a long joint that is strong, airtight, and efficient to make. The Pittsburgh seam is a lock seam formed by interlocking the sheet edges with a male and female profile and then locking them in place, often by crimping or welding. This creates a continuous, tight joint along the length that resists separation under pressure and vibration and suits mass production on sheet metal lines. Spiral seams are typical for spiral or circular ducts, not the common rectangular ductwork, while lap and butt seams either overlap the edges or require precise alignment and welding, making them slower or less reliable for standard fabrication. So the Pittsburgh seam is the best fit for general ductwork.