Back Weld and Backing Weld Symbol
The back weld and backing weld share one symbol: an open semicircle with its flat side on the reference line, placed on the side opposite the groove weld symbol. The only difference is order. A back weld is made after the groove weld, a backing weld before it, and the drawing says which in the tail or with a second reference line.
How to read the back and backing weld symbol
Both welds go on the root side of a groove weld. A backing weld is laid first, on the back of the joint, so the groove weld has something to weld against. A back weld is laid afterwards, usually after back-gouging the root of the groove weld to sound metal.
Because the symbol is the same, the order has to be stated. With a single reference line, the tail says BACK WELD or BACKING WELD. With multiple reference lines, the welding sequence is read from the line nearest the arrow outward, so the semicircle on a later line is a back weld and on an earlier line is a backing weld. Do not confuse it with the melt-through symbol, a filled semicircle that only asks for visible root penetration.
What each dimension position means
| Position | Meaning |
|---|---|
| Semicircle side | Always opposite the groove weld symbol it supports. |
| Dimensions on the semicircle | Normally none. Groove dimensions belong to the groove symbol; if the back or backing weld needs a size or contour, give it in a note or detail. |
| Tail (single reference line) | Says BACK WELD or BACKING WELD, which sets the order. |
| Multiple reference lines | Sequence runs from the line nearest the arrow outward. |
Example callout, read aloud
“Weld a 60-degree V-groove from the arrow side with a 1/8-inch root opening, then make a back weld on the other side after the groove weld is complete.”
Example: reading the callout piece by piece
- V below the line: groove weld on the arrow side, 60° included angle, 1/8 root opening, no size so complete joint penetration is implied.
- Semicircle above the line: a weld on the root side.
- BACK WELD in the tail: it is made after the groove weld, typically after back-gouging the root.
Result: Weld a 60-degree V-groove from the arrow side with a 1/8-inch root opening, then make a back weld on the other side after the groove weld is complete.
When it is used
- Single-V and single-bevel butt joints welded from one side, where the root is gouged and back welded for full penetration.
- Backing welds on joints with no backing bar, to give the groove weld a sound base.
- Structural and pressure work where the code requires a sound root.
Common mistakes
- Not stating back or backing. The symbol alone does not say which. Use the tail or multiple reference lines.
- Confusing it with melt-through. Melt-through is a filled semicircle and needs no extra weld; back/backing is open and is a separate weld.
- Drawing it on the same side as the groove. It always goes opposite the groove weld symbol.
- Skipping back-gouging. If the procedure calls for gouging to sound metal before the back weld, a back weld over an unsound root will not fix it.
Related weld symbols
- V-groove weld: The V-groove is the most common partner for a back weld.
- Bevel groove weld: Single bevels are often back welded too.
- Square groove weld: A back weld can complete a square butt.
- U-groove weld: Single U-grooves on thick plate may get a back weld.
- Surfacing weld: Surfacing is also a non-joining semicircle-based symbol, but for build-up.
Every weld symbol page: welding symbols chart
Calculators and further reading
Frequently asked questions
Are the back weld and backing weld symbols the same?
Yes. The order of welding decides which it is: back welds after the groove weld, backing welds before.
How does the drawing show back weld or backing weld?
In the tail of a single reference line, or by which reference line the semicircle is on when multiple lines show the sequence.
What is the difference from melt-through?
Melt-through is a filled semicircle that asks for visible root reinforcement from the groove weld itself; back and backing welds are separate welds.
Where is the back weld symbol placed?
On the side of the reference line opposite the groove weld symbol.
Sources
- AWS A2.4 Standard Symbols for Welding, Brazing, and Nondestructive Examination, American Welding Society
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