Weld symbol
Groove weld with backing bar symbol
A rectangle or semicircle opposite the groove, with a letter noting whether the backing is removed, says a separate piece of material sits behind the root while the groove is filled.
Reads asArrow side: V-groove weld, 1/2 size, 45° groove angle, 1/4 root opening. Other side: backing weld.
How to read it
Backing here means a physical strip, ring, or shoe placed behind the joint before welding. The groove is filled from the front into that piece, which supports the molten pool at the root and prevents burn-through even with a generous root opening. The symbol sits opposite the groove symbol so it is clear the backing belongs to the far side of the joint.
The critical piece of information is whether the backing stays. A letter accompanying the backing indication tells you: removed after welding, or left permanently in the joint. Removable backing gets stripped and the root is usually dressed and inspected afterward, while permanent backing becomes part of the finished assembly and has to be accounted for in the joint's fatigue and corrosion behaviour.
Backing material also has to suit the job. It is normally the same or a compatible alloy so the root fuses into it cleanly, and it must sit tight against the back of the joint. A gap behind the backing bar produces exactly the defect the bar was meant to avoid, because the pool drops into the void instead of fusing across.
On the shop floor a backing bar transforms the difficulty of a joint. It lets a welder open the root generously, run higher current on the first pass, and stop worrying about blowing through, which is why single-sided full-thickness joints on structural plate so often show one. Read the callout and the removal letter together before setting up the fit.
Where each dimension goes
- Backing indication opposite the groove
- Marks that a separate piece of material sits behind the root while the groove is welded.
- Removal letter
- Says whether the backing is stripped off after welding or stays permanently in the finished joint.
- Root opening (inside the groove symbol)
- Usually generous with backing in place, since the bar supports the pool instead of the joint bridging itself.
- Groove dimensions
- Angle, depth, and size are read exactly as on any groove weld; the backing changes how the root is achieved, not how the groove is dimensioned.
Worked examples
Each one is a drawing, the sentence it comes out as, and then the mark-by-mark account of why. Read the sheet first and say the sentence yourself before you look down.
Example 1: Removable backing on a plate butt
TitleRemovable backing on a plate buttRefBacking barEx1 / 2Reads as
Fill the vee onto a backing bar, then strip the bar and dress the root.
Which mark says so
- V-groove symbolBelow the reference lineA V-groove weld on the arrow side of the joint
- 3/4Left of the arrow-side symbolGroove weld size - how deep the weld itself goes
- 45°Beyond the outer face of the arrow-side symbolIncluded angle the joint is prepared to
- 3/8Inside the arrow-side symbolRoot opening - the gap left between the members before welding
- backing symbolAbove the reference lineA backing weld on the far side of the joint
- Backing removedIn the tailA reference the drawing carries - a process, procedure or specification
The backing indication opposite the groove says a bar sits behind the root, and the letter with it marks the bar as removable rather than a permanent part of the joint.
And what it does not say
- No length is given, so each weld runs the full length of the joint.
- There is no circle at the elbow, so the weld does not continue around the joint.
Example 2: Permanent backing ring on pipe
TitlePermanent backing ring on pipeRefBacking barEx2 / 2Reads as
Weld the vee onto a backing ring that stays in the finished pipe joint.
Which mark says so
- V-groove symbolBelow the reference lineA V-groove weld on the arrow side of the joint
- 1/2Left of the arrow-side symbolGroove weld size - how deep the weld itself goes
- 37.5°Beyond the outer face of the arrow-side symbolIncluded angle the joint is prepared to
- 1/8Inside the arrow-side symbolRoot opening - the gap left between the members before welding
- backing symbolAbove the reference lineA backing weld on the far side of the joint
- Backing left in placeIn the tailA reference the drawing carries - a process, procedure or specification
Backing on the far side plus a tail noting that it stays means the ring is part of the completed assembly, so it has to be a compatible material and accounted for in service.
And what it does not say
- No length is given, so each weld runs the full length of the joint.
- There is no circle at the elbow, so the weld does not continue around the joint.
Common mistakes
- Missing the removal letter and leaving a backing bar in a joint that was meant to be stripped.
- Confusing backing material with a backing weld, which is deposited metal rather than a separate piece.
- Fitting the backing bar with a gap behind the root, which defeats its whole purpose.
- Using a backing material that will not fuse cleanly with the base metal and the filler.