Weld underfill: what it is and how to spot it

Use this diagram

Copy this HTML to show the diagram with a credit link<img src="https://filletgauge.com/charts/weld-discontinuities/weld-underfill.svg" alt="Two groove weld cross-sections with a dashed plate-surface line. A: underfill, where the weld face dips below the line across the joint. B: undercut for contrast, with a full face but a small notch at each toe" width="100%"><p><a href="https://filletgauge.com/guides/weld-underfill?utm_source=embed&amp;utm_medium=referral&amp;utm_campaign=figure_embed&amp;utm_content=weld-underfill.svg">Underfill on top, undercut below: the missing metal is in a different place. Original drawing, not to scale. by Fillet Gauge</a></p>

How to recognize underfill

1. Lay a straightedge across the joint. If the weld face sits below a straightedge resting on both plates, the joint is underfilled. On a sound groove weld, the face is at or above the plate.
2. Look along the weld. Underfill often comes and goes: low at a stop, a restart or where the welder sped up, then full again.
3. Check the back. On a single-sided weld the root surface can be below the back of the plate too. That is often called root concavity or suck-back; read which name your document uses.
4. Don't confuse it with design. A partial penetration groove or a flare groove may be designed to sit below flush. Read the welding symbol before calling it underfill.

How to measure underfill

Measure how far the lowest point of the face sits below the plate surface. A gauge with a depth pointer, zeroed on the plate, or a dial caliper's depth rod resting on the plate both work. Where the members are different thicknesses, measure from the surface of the thinner member, the one the face should reach. Also record where and how long the low area runs. The governing document tells you whether any underfill is allowed and how it is judged.

Common mix-ups

• Underfill or undercut. Undercut is a notch melted into the plate at the toe. Underfill is a low weld face. One straightedge settles it: if the face is below it, it is underfill.
• Underfill or an undersize fillet. Fillets are judged by leg and throat size, not by a plate surface across the joint, so a small fillet is undersize, not underfilled.
• Underfill or burn-through. Burn-through melts a hole through the root, often leaving a sunken face above it. Underfill is just not enough metal, with no hole.

Why it forms

Too few passes, or a cap pass that is too small for the groove. Travel speed too fast for the deposit rate. A welder who misjudges the last layer and stops short of flush. On pipe roots, too much heat or the wrong backing gas pressure can pull the root surface up into the joint.

Underfill on the CWI exam

On Part B it fits Welding Inspection and Discontinuities, at least 34% of the practical, where you measure weld profiles on replicas and tell underfill from undercut. On Part A, it belongs to Visual Examination. On Part C, the weld profile and visual acceptance rules are the places to look, D1.1 Clauses 7 and 8 for the D1.1 endorsement.

Practice it

Practice taking depth readings on the weld gauge trainer. Then read the toe look-alike, undercut, and the opposite face condition, excessive reinforcement. The weld discontinuities guide lists every type.

Check the rest of Part B

Take the free 10-question check. It's weighted to the Part B outline, scored by domain and needs no signup, so you see where measuring ranks among your weak spots.

Sources

Show the list

• A general review of geometric shape imperfections, Part 1 (Job Knowledge 67), TWI. Retrieved 25 Oct 2026.
• CWI Part B Practical Examination Information, American Welding Society, Rev. 03.01.2024. Retrieved 20 Oct 2026.
• Discontinuity names follow AWS A3.0, Standard Welding Terms and Definitions. Every description here is our own wording.