AWS A2.4 weld symbol
Back or Backing Weld Symbol
A back or backing weld symbol is a semicircle drawn opposite a groove weld symbol, calling for a bead on the far side of the joint — the sequence decides which name applies.
How to read the back or backing weld symbol
- The semicircle is always drawn on the opposite side of the reference line from the groove weld symbol.
- The shape is identical for a back weld and a backing weld — only the sequence differs, and the drawing states it in a note or in the tail.
- A back weld is deposited after the groove weld, usually onto a back-gouged root.
- A backing weld is deposited before the groove weld, giving the root pass something to fuse into.
- The semicircle is not dimensioned for size; a contour and finish symbol may be added to it.
Where the dimensions go
| Position | What it specifies |
|---|---|
| On the symbol | Contour and finish symbols only — the back weld symbol carries no size dimension |
| In the tail or a note | Whether the weld is a back weld or a backing weld, and the sequence |
Joint types
- Butt joint
- Corner joint
Practical notes
- Do not confuse a backing weld with a backing bar: a backing bar is shown by a rectangle, not a semicircle.
- Back welding after back-gouging is the standard way to achieve complete joint penetration on a single-V joint welded from both sides.
- The melt-through symbol, a filled semicircle, is different again — it calls for visible root reinforcement from a weld made entirely from one side.
Back or Backing Weld symbol — common questions
What is the difference between a back weld and a backing weld?
Only the sequence. A backing weld is deposited first, before the groove weld, so the root pass has sound metal to fuse into. A back weld is deposited last, after the groove weld and usually after back-gouging the root. The symbol is the same semicircle for both, so the drawing has to state which.
What is the difference between a backing weld and a backing bar?
A backing weld is deposited metal, shown by a semicircle. A backing bar is a separate piece of material left in or removed after welding, shown by a rectangle on the opposite side of the reference line.
Where is the back weld symbol drawn?
Always on the opposite side of the reference line from the groove weld symbol, because it describes a weld on the far side of the joint.
Is the back weld symbol the same as melt-through?
No. The back weld symbol is an open semicircle. The melt-through symbol is a filled semicircle and calls for visible root reinforcement on a joint welded entirely from one side.
Welding symbol anatomy
Every weld symbol attaches to the same framework. If any part of the diagram above is unfamiliar, the full guide to reading welding symbols walks through the whole system.
| Reference line | The horizontal backbone of every welding symbol. Everything else attaches to it, and it is always drawn horizontal on the drawing. |
|---|---|
| Arrow | The leader and arrowhead that connect the reference line to the joint. The side of the joint the arrow touches is the arrow side. |
| Weld symbol | The shape that says which kind of weld is required — triangle for a fillet, V for a V-groove, circle for a spot, and so on. |
| Arrow side (below the line) | A weld symbol drawn below the reference line calls for a weld on the side of the joint the arrow points to. This is the convention that catches people out most often. |
| Other side (above the line) | A weld symbol drawn above the reference line calls for a weld on the far side of the joint. |
| Both sides | Symbols drawn above and below the reference line call for welds on both sides of the joint. |
| Tail | An optional fork at the far end of the reference line, used to reference a specification, process or procedure such as GMAW or a WPS number. |
| Dimensions | Size is placed to the left of the weld symbol; length and pitch are placed to the right. |