Butt joint: meaning and how to check it
A butt joint is two members lined up edge to edge in roughly the same plane, like two plates laid flat with their ends touching or nearly touching. It names how the parts meet, not how they're welded. The same butt joint can take a square, V, bevel, U or J groove weld, or another weld type entirely.
Use this diagram
How to recognise a butt joint
- Look at the planes. If the two pieces would make one flat sheet once welded, it's a butt joint. Pipe welded end to end is a butt joint too: the walls line up, they just happen to be curved.
- Look at what touches. In a butt joint an edge meets an edge. If an edge meets a face, you're looking at a T-joint or a corner joint instead.
- Then read the prep. Square edges, a V, a single bevel, a U or a J are all groove shapes for this one joint type. The edges might also have a root opening between them, a root face at the bottom of the bevel, or a strip of backing underneath.
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Definitions and Terminology
A fatigue crack in a cyclically loaded fillet-welded attachment started along the line where the surface of the weld blends into the flange plate. At which weld feature did the crack start?
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Common mix-ups
- Joint type versus weld type. "Butt weld" is shop slang. On a drawing and in the standard terms, the joint is a butt joint and the weld is a groove weld (most of the time). Mixing the two up costs points on a terminology domain that grades exact words.
- Butt versus lap. Two plates that overlap, even by a little, form a lap joint. A butt joint has no overlap at all.
- Butt versus branch connection on pipe. A branch pipe set onto the side of a main pipe is a T-, Y- or K-connection, the tubular family of the T-joint, not a butt joint or a corner joint, even though both members are pipe. Two pipes joined end to end are the butt joint.
- Thick plate, same joint. Heavy plate often gets a double-V or double-bevel prep welded from both sides. It's still a butt joint.
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Butt joints on the CWI exam
Joint types sit in Definitions and Terminology, at least 12% of Part A, which is closed book. Expect to name a joint from a sketch or a short description, and to keep joint words apart from weld words.
On Part B the butt joint comes back in plastic: the replica set includes a groove plate and a pipe specimen, and both are butt joints. Welding Inspection and Discontinuities is at least 34% of the practical, and the groove-weld measuring you do there (reinforcement height, misalignment, undercut) happens on those butt joints.
On a D1.1 Part C, butt joints drive the prequalified joint details in Clause 5 and the fit-up work in Clause 7. You read those clauses in your own copy of the code; this page doesn't reproduce any detail or tolerance from them.
Practice it
Joint and weld names are quick points once they're automatic. The CWI Part A flashcards drill the terms around them, such as where a groove's root opening and angle go on a symbol. Then work through the related terms in the welding terms glossary. If you want to see how a groove on a butt joint is called out on a drawing, the V-groove weld symbol page walks through one.
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Sources
Show the list
- AWS bookstore, terminology and symbols listing, American Welding Society. Retrieved 20 Oct 2026.
- CWI Part A Fundamentals Examination Information, American Welding Society, Rev. 03.01.2024. Retrieved 20 Oct 2026.
- AWS B5.1:2025, Specification for the Qualification of Welding Inspectors, American Welding Society, Tables 5.1 and 8.1. Retrieved 20 Oct 2026.
Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.
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