Molten filler that rolled over the base metal toe without ever melting it. The weld looks attached — but a cold, unbonded interface hides under the rounded lip.
Overlap is a lack-of-fusion defect at the weld toe: the filler metal flowed over the base metal surface and froze there without the base ever melting beneath it. It is a shape defect with a hidden cost — the rolled edge is an unbonded interface and a sharp notch.
It rarely fails a weld by itself, but it is common on the shop floor and easy to miss: a bead can look full and strong while sitting on a cold lip. Codes reject it because the notch concentrates stress and the interface can open under fatigue.
Cold lap, cold shuts, rolled edges, toe roll-over.
Overlap hides in plain sight — look at the toe with good light, and from the side.
Overlap is a balance failure: too much fill for the heat, or heat aimed at the wrong place.
A weld wets when the arc heats the base metal to its melting point at the toe — then the filler spreads onto a molten surface and bonds. Overlap happens when the balance tips: the puddle carries more filler than the arc can melt into, or the heat is aimed at the puddle instead of the base. The filler flows over a cold, solid toe and freezes without bonding.
The result is a rolled lip with a sharp notch beneath it — a geometric stress riser sitting on an unbonded surface. In groove welds the same failure happens at the root, where the pool never wets the root face. The mechanism is identical to lack of fusion; only the location differs.
Undercut is a groove melted INTO the base metal at the toe — too much heat. Overlap is filler rolled OVER the base, unfused — too little heat at the toe. One removes metal, the other fails to attach it.
The weld should look wetted, not piled — the toe blends into the plate when the balance is right.
Clean the toe — mill scale and paint are wetting killers — and set the heat for the base thickness, not just for the fill rate.
Keep the arc on the leading toe so the base melts before the filler arrives; hold the correct work angle — MIG about 10–15 degrees drag.
Add less filler and let the pool wet out — a small, flat puddle bonds; a big, round one rolls. Out of position, keep the puddle smaller still.
More wire is the wrong answer. Overlap is almost always too much fill for the heat — raise the heat or cut the filler, then watch the toe wet in.
Grind first and look — overlap repairs are decided by what the joint line shows.
| Step | Action | Why |
|---|---|---|
| 1 | Grind the rolled lip off to expose the joint line. | You cannot judge the bond until the toe is open. |
| 2 | Inspect the exposed surface — fused, smooth metal means the weld is sound. | Some overlap is cosmetic; the unbonded interface is not. |
| 3 | If unfused material is revealed, excavate to sound metal along the whole toe. | The cold interface runs the length of the rolled edge. |
| 4 | Re-weld with corrected heat, angle, and fill — verify the toe wets in. | Same technique re-creates the same lip. |
| 5 | Inspect the repair per the code — VT at minimum. | The repair toe is now the acceptance surface. |
Do not cap over an overlap to smooth it — the notch and the cold interface stay buried. Grind first, look, and only then decide whether to fill or excavate.
The same balance problem, wearing each process's uniform.
The classic: voltage too low and wire feed too high — a cold puddle that piles instead of wetting. Work angle too steep, or pushing instead of dragging, leaves the same lip. Balance voltage and wire feed until the toe wets.
Amperage too low for the filler diameter, or filler added before the pool wets the base. Keep the arc ahead of the filler and let each dab wet the toe before adding more.
Low amps with a heavy rod — the metal drops off the electrode and rolls. Wrong rod angle and a long arc make it worse. Raise amps a little, shorten the arc, and point the rod at the toe.
Same family as MIG: voltage low for the wire feed, or the wrong drag angle. Self-shielded wire at high feed especially — it deposits fast and wets slow.
Heat first, fill second, angle third — in that order.
| Parameter | Check | Typical Fix |
|---|---|---|
| Amperage / heat | Too low to melt the base at the toe | Raise heat; preheat heavy sections so the toe melts |
| Wire feed | Fill faster than the arc can melt into | Lower wire feed or raise voltage/heat with it |
| Travel speed | Too fast fills without wetting; too slow piles the puddle | Find the speed where the toe wets before the next dab |
| Gun / rod angle | Arc pointing at the puddle, not the base | MIG: 10–15 degrees drag; keep the arc on the leading toe |
| Position | Gravity pulls the puddle off the toe | Smaller puddle, tighter weave, short steps uphill |
| Puddle control | Puddle too big for the joint | Less filler, pause at the toes so the heat catches them |
The Welding Calculator gives you a heat-matched voltage and wire-feed starting point, and the Heat Input Calculator tells you whether the pass is hot enough to wet the toe on the thickness you are welding — overlap and undercut are the two sides of that balance.
Overlap is a heat and puddle problem — the tooling that controls those two is the list.
The tools that matter are the ones that control heat and puddle: a power source with stable arc control — pulse helps out of position — and a torch or gun that lets you see the toe clearly. The rest of the fix is technique.
Overlap is found with the eyes and a gauge — the notch under the lip is the tell.
Acceptance depends on the code and project: AWS D1.1, ISO 5817, API 1104 and project specifications each set their own limits for this discontinuity. Check the governing document before judging a weld acceptable — there is no universal pass/fail rule.
The questions welders actually ask about overlap — answered straight.
Undercut is a groove melted into the base metal at the toe — the weld is below the plate surface and metal has been removed. Overlap is filler rolled over the base, unfused — the weld is above the plate and metal was never attached. One is too much heat at the toe, the other too little.
No — overlap is a lack-of-fusion defect, an unfused interface at the toe, not a crack. But the sharp notch under the rolled lip concentrates stress, and fatigue cracks commonly start there, which is why codes treat it as a rejectable discontinuity.
Gravity is pulling the puddle off the toe. Keep the puddle small, tighten the weave, and pause briefly at each edge so the heat catches the toe — while keeping the amperage high enough to actually melt it. If the lip is already there, grind it off and check the joint line.
A small rolled lip with a sound, fused joint line under it can be blended smooth and may be acceptable per the code. The moment grinding reveals an unbonded interface, it must be excavated and re-welded — covering it with filler just buries the cold joint.