Weld root: meaning and how to check it
On a cut and etched weld, the weld root is where you'd put a pencil tip at the bottom of the weld metal: on a groove weld, the far-side spots where the underside bead stops on each plate; on a fillet, the inside corner the weld grows from. The joint root differs: where members come closest before welding.
Use this diagram
How to find the root
- Before welding, find the joint root. On a groove joint it's the gap or contact line at the narrow end of the groove. On a fillet joint it's the line where the two members touch.
- After welding, find the weld root. On a groove weld, look at the far side of the joint. The weld root is the edge of the underside bead on each plate, where weld metal gives way to bare plate. On a fillet, it's the inside corner where the two legs start.
- Check what reached it. Full penetration on a groove weld means weld metal goes all the way through the joint root. Incomplete penetration means it didn't, and that shows on the root side.
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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
- Joint root versus weld root. One is part of the fit-up, the other part of the finished weld. Inspectors check the joint root before welding (gap, alignment, backing) and the weld root after.
- Root versus root face. The root face is a flat strip machined or ground at the bottom of a bevel. It's part of the joint root area, not the weld root.
- Root versus root pass. The root pass is the first bead laid in the joint. It forms the weld root, but the words mean different things.
- Fillet root and throat. Every throat measurement on a fillet starts at the root. If the fusion goes past the root, the effective throat can count that extra depth.
- Opposite the face. The weld root is on the side away from the weld face on a single-sided groove weld.
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The weld root on the CWI exam
Root terms are a staple of Definitions and Terminology, at least 12% of Part A. Expect to separate the joint root, the weld root, the root face, the root opening and the root pass from each other without a book.
On Part B, root-side conditions such as incomplete penetration, concave root and excess root reinforcement are part of Welding Inspection and Discontinuities, at least 34% of the practical.
On a D1.1 Part C, root conditions come up in fabrication (Clause 7) and in inspection (Clause 8). Read the requirements in your own copy.
Practice it
The CWI Part A flashcards include incomplete penetration at the root of a groove weld and its usual causes. Then see how gap and land are set before welding on the root opening and root face pages. The welding terms glossary keeps every root term on one page.
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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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