Fillet Gauge
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Root crack: what it is and how to spot it

Fillet Gauge editorial teamSources checked 4 min read

A root crack starts at the root of a weld, the deepest point where the members meet, and runs into the weld metal or the heat-affected zone. In a single-sided groove weld it opens at the back surface of the root bead. In a fillet weld it starts at the corner between the members and heads across the throat.

Two cross-sections of a root crack. A: a multi-pass groove weld with a jagged crack starting at the underside of the root bead. B: a T-joint fillet weld with a crack from the corner where the members meet running diagonally across the weld
In a fillet the crack takes the shortest path through the weld: the throat. Original drawing, not to scale.
Use this diagram

How to recognize a root crack

  1. Look at the back if you can. On open-root groove welds, inspect the root side. A root crack shows as a tight, jagged line along or across the root bead.
  2. Expect it to be hidden in fillets. A fillet root crack starts inside the joint. You only see it once it has grown through the throat to the face, as a line down the middle of the fillet.
  3. Consider the first pass. The root pass is small and has to carry the full shrinkage of the joint while it is still thin. Cracks found there often formed before the next pass went in.
  4. Check the fit-up. Wide root openings and gaps between fillet members raise the strain on the root. If fit-up was poor, look harder.

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Welding Inspection and Discontinuities

A convex fillet weld replica is checked with a leaf-type fillet gauge. The drawing calls for a single fillet size. The vertical leg fits the 3/8 in blade and the horizontal leg the 5/16 in blade. What size is recorded?

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How it is found

Where the root is accessible, visual inspection and a surface method, penetrant or magnetic particle testing, confirm it. On pipe and closed joints a mirror or borescope helps. Buried root cracks in groove welds are found by radiography, as a dark, jagged line, or by ultrasonic testing. Fillet root cracks are hard to find by any method until they surface, which is one reason inspectors watch the root pass while the weld is being made. Record the location, direction and length, and judge it under the governing document's crack criteria.

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Common mix-ups

  • Root crack or incomplete joint penetration. On film, unmelted root faces make a straight, even dark line. A crack is jagged and irregular with pointed ends.
  • Root crack or suck-back. Root concavity is a smooth, shallow dish in the back of the root. A crack is a sharp line.
  • Fillet root crack or a centerline crack. Both can show down the middle of a fillet face. A root crack started at the joint corner and grew out; a solidification crack formed in the face bead as it froze.

Why it forms

The root bead is thin and highly restrained, so it carries the shrinkage of everything around it. A concave root bead, a small root pass on thick material, wide root openings, poor fit-up, hydrogen and high restraint all add up. Too much heat input on the root can also cause hot cracking in the bead itself. A bigger root bead, good fit-up, preheat where needed and low-hydrogen practice help.

Root cracks on the CWI exam

On Part B, root cracks fall in Welding Inspection and Discontinuities, at least 34% of the practical, and in NDE Application when you read a radiograph or a report. On Part A they tie Visual Examination, NDE Fundamentals and Welding Application and Control together. On Part C, look up crack criteria and fit-up tolerances in your own copy of the code.

Practice it

Drill which method finds a buried crack in the NDE methods quiz. Then read the crack at the other edge of the weld, toe cracks, and the root flaw with the straight line, incomplete joint penetration. Every type is in the weld discontinuities guide.

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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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