Lap joint: meaning and how to check it
A lap joint is two members that overlap, lying in parallel planes with one sitting on top of the other. The weld usually runs along the exposed edge of each plate where it rests on the other, as a fillet. Sheet metal work, splice plates, gusset connections and patch plates are common places to find lap joints.
Use this diagram
How to recognise a lap joint
- Check for overlap. If one member covers part of the other, it's a lap joint. If the members meet edge to edge with no overlap, it's a butt joint.
- Check the planes. The two members are parallel. An overlap at an angle is something else, usually a T or corner.
- Find the welds. A fillet at the top plate's edge is the classic one. A second fillet at the bottom plate's edge makes a double-welded lap. Plug and slot welds go through holes in the top plate, and spot and seam welds fuse the plates together inside the overlap.
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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
- Fillet size and plate edge. A lap fillet on the top plate's edge can't have a leg larger than the edge it sits on without melting the corner away. That's why a lap fillet on thin sheet is often smaller than the plate thickness, and why the drawing gives the size.
- Lap versus T. In a lap joint, the fillet's root sits where a plate edge meets a plate face, just like a T, which makes them easy to blur on a sketch. Check whether the plates are parallel (lap) or square to each other (T).
- Overlap as a discontinuity. "Overlap" is also the name for weld metal rolled over the plate surface without fusing. Same word, different meaning: one describes the joint, the other a defect.
- One weld or two. A lap fillet at only one edge leaves the other edge open to the weather. Whether that's allowed is a design call made on the drawing, not by the inspector.
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Lap joints on the CWI exam
Joint types fall under Definitions and Terminology, at least 12% of Part A, which is closed book. A lap joint is one of the five basic types, so name it on sight and keep it apart from the overlap defect.
On Part B, the fillet replica is a T-joint, but the gauge work carries straight over to a lap fillet: legs, convexity and undercut, each read with the kit gauges and judged against the Book of Specifications.
On a D1.1 Part C, lap joints are covered in design (Clause 4) and in the prequalified details (Clause 5). Use your own copy for what those clauses require.
Practice it
Put a lap fillet size through the fillet weld calculator to see its throat and weld metal per foot. The other joint types are side by side in the welding terms glossary, and the T-joint page shows the joint a lap gets confused with most.
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Sources
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- 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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