A coffin lock is used to attach duct components by which method?

• A. the same as a Pittsburgh lock.
• B. soldered.
• C. for round pipe only.
• D. always riveted.

Answer: (B)

Coffin lock joints are intended to be sealed with solder. The interlocking lips of the coffin lock fit together, and the seam is then filled with solder to create a tight, leak-proof connection between duct sections. This makes soldering the preferred method for this type of joint, rather than relying on rivets or welding alone. It’s used in rectangular ductwork and isn’t limited to round piping, and it differs from Pittsburgh-style seams that rely on mechanical fasteners.