Back weld and backing weld symbol: what it means on a drawing

Use this diagram

Copy this HTML to show the diagram with a credit link<img src="https://filletgauge.com/charts/filletgauge/back-and-backing-weld-symbol.svg" alt="A V-groove butt joint drawn end on, welded from the arrow side, with an extra bead on the root side. The symbol has a V below the reference line and an open half-circle above it" width="100%"><p><a href="https://filletgauge.com/guides/back-and-backing-weld-symbol?utm_source=embed&amp;utm_medium=referral&amp;utm_campaign=figure_embed&amp;utm_content=back-and-backing-weld-symbol.svg">Same mark for both. A note or the drawing tells you the order. by Fillet Gauge</a></p>

How to read it

1. Find the groove. The half-circle always comes with a groove weld symbol, such as a V or bevel, on the other side of the line. It is never drawn alone.
2. Find the half-circle. It sits opposite the groove. If the groove is below the line (arrow side), the root-side bead is on the other side, and the half-circle is above the line.
3. Find the order. The symbol itself doesn't say whether the bead goes in before or after the groove weld. A note, often in the tail, or the drawing says which. Back weld: after. Backing weld: before.
4. Read the extras. A contour mark on the half-circle sets the shape of the root-side bead. Any preparation of the root before back welding, such as gouging to sound metal, is a separate instruction.

Back weld or backing weld

A backing weld is laid first, on what will become the root side, and gives the groove weld something to be welded against. A back weld is laid last, on the root side of a finished groove weld, often after the root has been cleaned out. The bead looks similar once the joint is done, which is why the mark is shared and the order has to be stated.

Common mix-ups

• Open or filled. An open half-circle is a back or backing weld: a separate bead. A filled half-circle in the same place is melt-through: no separate weld, just visible root reinforcement from welding one side.
• Backing weld or backing material. A backing weld is weld metal. Backing material, such as a steel strip under the root, uses a different mark, a rectangle opposite the groove symbol.
• Which side. The half-circle is opposite the groove, so it reads as the other side when the groove is on the arrow side. Don't read it as a second groove.
• Back weld or surfacing. The surfacing mark is two arcs side by side on the arrow side with no groove symbol, because it builds up a surface instead of joining parts.

On the CWI exam

Back, backing and melt-through marks are part of Symbols, Welding and NDE, at least 10% of CWI Part A, which is closed book. Because two of them share a shape and the third differs only by being filled, they make a good test of careful reading. Part B drawings use them under Utilization of Specifications and Drawings, at least 8% of the practical exam.

Practice it

The welding symbols quiz drills the groove shapes these marks sit opposite. Read how the groove side works on the V-groove weld symbol and bevel groove weld symbol pages, and see melt-through and the other supplementary marks on the welding symbols chart.

Read the root side right? Check the rest of Part A

Take the free 10-question check. It's weighted to the Part A outline, scored by domain and needs no signup, so you see whether symbols or another domain costs you the most points. The Part A course gives you a lesson on reading welding and NDE symbols and on every other exam subject, two full-length timed practice exams, and a worked explanation with every answer.

Sources

Show the list

• AWS A2.4:2020, Standard Symbols for Welding, Brazing, and Nondestructive Examination, free sample, American Welding Society. Retrieved 20 Oct 2026.
• Groove Welding Symbols (including back/backing and surfacing), Interpretation of Metal Fab Drawings, Open Oregon Educational Resources. Retrieved 21 Oct 2026.