Weld joint type
Lap Joint
A lap joint overlaps two members and welds along the exposed edges — no edge preparation, forgiving fit-up, and the cheapest joint to fabricate.
Geometry and preparation
| Configuration | Two members overlapping face to face, with fillet welds along one or both exposed edges. |
|---|---|
| Edge preparation | None required beyond cleaning. AWS D1.1 requires an overlap of at least five times the thinner member's thickness, and never less than 1 in (25 mm). |
| Strength | Loaded in shear rather than tension. A double-fillet lap joint carries substantially more load than a single fillet and avoids the bending that a single weld introduces. |
Characteristics
- Requires no edge preparation, which makes it the fastest joint to fabricate.
- Very tolerant of poor fit-up and cutting inaccuracy.
- A single-fillet lap joint loads the weld eccentrically and tends to peel; two fillets are preferred where load matters.
- Creates a crevice between the members that traps moisture — a corrosion concern in wet or marine service.
- Poor fatigue performance relative to a butt joint because of the sharp stress concentration at the weld toe.
Weld types used on a lap joint
Where it is used
- Sheet metal fabrication and ductwork
- Automotive body panels, usually spot welded
- Structural gussets and doubler plates
- General repair where fit-up is rough
Lap Joint — common questions
How much overlap does a lap joint need?
AWS D1.1 requires at least five times the thickness of the thinner member, and never less than 1 in (25 mm). Less than that and the joint rotates under load, prying on the fillet welds.
Should a lap joint be welded on both sides?
For anything carrying load, yes. A single fillet loads the joint eccentrically, which bends it and encourages peeling. Two fillets balance the load path.
Are lap joints as strong as butt joints?
No. A full-penetration butt joint develops the full base metal strength; a lap joint's capacity is limited by the shear area of its fillet welds and it performs poorly in fatigue.
Can you use plug welds instead of fillets on a lap joint?
Yes. Plug or slot welds through the top member are used where the edges are not accessible or where a flush outer surface is wanted, and they are common in automotive work.
Other joint types
Butt Joint
A butt joint places two members edge to edge in the same plane — the simplest joint to draw and the one that most…
T-Joint
A T-joint sets one member perpendicular to the face of another, and it is normally filleted on both sides — the most…
Corner Joint
A corner joint brings two members together at their edges to form an L, and unlike a T-joint the upright lands on the…
Edge Joint
An edge joint lays two members face to face and welds along their common edge — a light-duty joint used to join…