What is the purpose of the root pass in a plate weld?

• A. The root pass cosmetic finish.
• B. The root pass sets the filler size.
• C. The root pass provides penetration.
• D. The root pass seals the joint.

Answer: (C)

The root pass is the initial weld layer whose main job is to penetrate into the joint and fuse both base metals at the root. It creates through-thickness fusion so the two plates are welded together along the root, forming a solid foundation for the later passes. Without adequate root penetration, you can get lack of fusion and a weak weld, even if the subsequent passes add filler. The other options don’t fit because the root pass is not primarily for cosmetic finish, not about setting the filler size, and while a good root helps the overall seal, sealing comes from the completed weld rather than the root pass alone.