Weld symbol
Melt-through symbol
A filled semicircle on the far side of the reference line calls for visible root reinforcement pushed completely through a joint welded from one side only.
Reads as: Other side: back weld, 1/16 size.
How to read it
Melt-through is the requirement that a weld made from one side visibly comes through to the other. The symbol is a semicircle on the opposite side of the reference line from the weld, drawn solid rather than as an outline. That filled shape is what separates it from the back and backing weld semicircles, which are drawn as open outlines and represent metal deposited from the far side.
The distinction is worth holding onto because the consequences differ completely. A back weld means someone works on the far face after the groove is filled. A backing weld means someone works there before. Melt-through means nobody works on the far face at all: the requirement is achieved entirely by controlling the weld from the near side so that the root pushes through and forms a visible bead behind the joint.
A number to the left of the melt-through symbol gives the height of the root reinforcement above the far surface. Where the drawing needs the far side smooth — inside a pipe carrying product, for instance — that dimension is written specifically and the reinforcement is checked, since too much protrusion restricts flow and too little means the root never fully fused.
Achieving it is a matter of heat and travel control on the root pass. Too little and the root stays unfused; too much and the pool drops through into a burn-through, which is a defect rather than reinforcement. It is the notation most closely tied to welder skill, since no fixture or backing arrangement can produce it for you.
Where each dimension goes
- Filled semicircle on the far side
- Calls for visible root reinforcement pushed through from the near side, with no work done on the far face.
- Height (left of the symbol)
- How far the root reinforcement is allowed or required to protrude beyond the far surface.
- Filled versus outlined
- The filled shape is melt-through; the same semicircle drawn as an open outline is a back or backing weld.
- Contour and finish
- A bar beyond the symbol calls for the root to be dressed flat, which is common on internal pipe surfaces.
Worked examples
Controlled root reinforcement on a pipe butt
“Weld this joint from outside so the root shows through 1/16 inside the pipe.”
The 1/16 to the left of the far-side symbol limits how far the root reinforcement protrudes, which matters where excess metal inside the bore would restrict flow.
Melt-through dressed flush inside
“Push the root through and dress the inside surface flat afterwards.”
The bar beyond the far-side symbol calls for the protruding root to be dressed level, which is common on process piping where the internal surface has to stay smooth.
Common mistakes
- Confusing the filled melt-through semicircle with the open outline of a back or backing weld.
- Reading melt-through as permission to burn through. Excess drop-through is a defect, not reinforcement.
- Ignoring the height dimension and leaving root reinforcement that restricts flow inside a pipe.
- Assuming melt-through can be achieved with a backing bar in place, which by definition prevents it.
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.