Weld symbol
Backing weld symbol
The same semicircle as a back weld, but deposited first. A backing weld is laid in the root before the groove is filled, giving the groove side something solid to fuse into.
Reads as: Arrow side: V-groove weld, 1/2 size, 60° groove angle, 1/8 root opening. Other side: backing weld.
How to read it
A backing weld uses the identical semicircle glyph as a back weld and sits opposite the groove symbol in the same way. The difference is entirely one of sequence: a backing weld goes down before the groove is filled, while a back weld goes down after. The shape cannot tell you which, so the drawing has to, usually through a note or the tail.
The purpose is to close the root. Rather than leaving an open gap for the first groove pass to bridge, the fabricator deposits a weld across the back of the joint, then fills the groove from the front into that solid backing. It removes the difficulty of running an open root and makes the front-side passes far more forgiving of fit-up variation.
Because the backing weld becomes part of the joint, it is not removed afterward the way a temporary backing bar might be. That means its own quality matters: porosity or lack of fusion in the backing weld stays in the finished joint. Where the design cares about the size of that deposit, a dimension appears to the left of the semicircle.
Reading order on the drawing is not reading order in the shop. The semicircle above the line does not mean the far side is welded second just because it is drawn above; on a backing weld it is welded first. This is the single most common misreading of the symbol, and it is why experienced detailers add a short note beside it rather than relying on the glyph alone.
Where each dimension goes
- Semicircle position
- Opposite the groove symbol, marking the face of the joint where the backing deposit is laid.
- Size (left of the semicircle)
- Optional dimension controlling how much weld metal the backing deposit contains.
- Contour and finish
- Calls for the backing weld's exposed face to be left flat, convex, or concave, with a method letter where it is dressed.
- Note or tail
- Where the before-the-groove sequence is stated, since the glyph is the same as a back weld's.
Worked examples
Backing weld under a V-groove
“Lay a backing weld in the root first, then fill the 60 degree vee from the front.”
The semicircle above the line is deposited before the groove below it, so the front-side passes fuse into solid metal rather than bridging an open root gap.
Backing weld with its face dressed flat
“Deposit the backing weld, dress it flat, then fill the bevel groove above it.”
The bar beyond the semicircle asks for a flush face on the backing deposit, which matters when the back of the joint has to sit against a mating surface after welding.
Common mistakes
- Treating the semicircle as always meaning back weld and welding the groove first.
- Confusing a backing weld with a backing bar. One is deposited metal, the other is a separate piece.
- Skipping inspection of the backing deposit because it will be covered by the groove passes.
- Assuming the drawing order top to bottom implies the welding order. It does not.
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.