Weld symbol
Back weld symbol
A semicircle opposite a groove symbol calls for a weld deposited on the back of the joint after the groove side is welded, sealing and reinforcing the root from the far face.
Reads as: Arrow side: V-groove weld, 1/2 size, 60° groove angle, 1/8 root opening. Other side: back weld.
How to read it
The back weld symbol is a plain semicircle with its flat side on the reference line. It never appears alone: it always sits opposite a groove symbol, because a back weld is by definition a weld put on the far side of a groove that has already been made. Reading it means reading the pair, groove first and semicircle second.
Sequence is the whole distinction between a back weld and a backing weld, and it is not visible in the symbol shape. In the back-weld sense, the groove is welded first, then the joint is turned, the root is gouged or ground back to sound metal, and a weld is laid in the cleaned-out root from the far face. The order is groove, then back.
That order matters because it changes the inspection story. A back weld deposited into a gouged root removes whatever incomplete fusion, slag, or oxide the first pass left, which is why the arrangement is common where a joint has to develop the full thickness but can only be prepared from one side. Drawings that need the back weld to follow a gouging step usually say so in a note or in the tail.
The semicircle is typically drawn without dimensions. Where the design does need the back weld's size controlled, that number appears to the left of the semicircle like any other size. A contour symbol beyond the semicircle's outer face calls for the back weld to be left flat, convex, or concave, most often flush where the far face has to sit against something.
Where each dimension goes
- Semicircle position
- It sits on the opposite side of the reference line from the groove symbol, marking the far face of the joint.
- Size (left of the semicircle)
- Optional. Where the back weld's dimension is controlled, it is written to the left like any other weld size.
- Contour and finish
- A bar or arc beyond the semicircle calls for the back weld face to be left flat, convex, or concave, with a letter for method.
- Tail or note
- Where the gouge-then-weld sequence is spelled out, since the symbol shape alone does not distinguish back from backing.
Worked examples
Back weld on a bevel groove
“Weld the bevel groove first, then gouge the root and back weld the far side.”
The half-vee below the line is the main preparation and the semicircle above it is the weld put in after the root has been cleaned out from the opposite face.
Back weld ground flush
“Back weld the far side of the vee and grind that face flush with the plate.”
The straight bar beyond the semicircle calls for a flush contour and the G beside it names grinding as the method, which is common where the back face has to seat against another part.
Common mistakes
- Mistaking the semicircle for a backing bar, which is a separate piece of material rather than a weld.
- Depositing the back weld without gouging the root first, so the original root defects stay buried.
- Reading the semicircle as a standalone instruction when it only ever qualifies the groove opposite it.
- Assuming the back weld has no size requirement when a number appears to its left.
Typical joints
Related symbols
Practise reading welds, not just symbols
The drawing tells you what the weld should be. The gauge trainer is where you find out whether you can tell what it actually is.