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Melt-through symbol: what it means on a drawing

Fillet Gauge editorial teamSources checked 4 min read

The melt-through symbol is a filled half-circle drawn on the opposite side of the reference line from a groove or flange weld symbol. It calls for a weld made from one side that must show visible root reinforcement on the far side. No second weld is made there: the reinforcement comes from the same weld penetrating through.

A V-groove butt joint drawn end on, welded from the arrow side, with the root of the same weld showing as a small bulge on the far side, beside a V below the reference line and a filled half-circle above it with 1/16 on its left
Filled means melt-through: one weld, reinforcement showing through. Numbers are invented.
Use this diagram

How to read it

  1. Find the weld. Melt-through always rides with another weld symbol, most often a groove such as a V, bevel or square groove, or an edge weld on flanged sheet. It is never the weld on its own.
  2. Find the side. It sits opposite that weld symbol. If the groove is below the line, welded from the arrow side, the filled half-circle is above the line, because the reinforcement shows on the other side.
  3. Read the left side. A number to the left of the half-circle is the height of root reinforcement required. 1/16 means the drawing wants the root to stand 1/16 proud on the far side. With no number, the symbol only asks that reinforcement be visible.
  4. Read the contour. A contour mark and a finish letter can go on the half-circle when the root side must be ground or finished, for example before painting or coating.
  5. Read the tail. The process and any notes on the root, such as how it will be inspected, go there.

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Symbols - Welding and NDE

On a welding symbol, what does a circle at the junction of the arrow and the reference line indicate?

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Why a drawing asks for it

Melt-through means the weld penetrated the full joint and you can see it on the far side, without anyone welding that side. That suits joints whose root side can't be reached for a back weld, and sheet-metal flanged joints, where burning through to the back is part of the design.

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Common mix-ups

  • Filled or open. Filled is melt-through: no separate weld. An open half-circle in the same spot is a back or backing weld, a second bead made on the root side.
  • Melt-through or backing. Backing material, such as a steel strip under the root, is a rectangle opposite the groove, with an R inside if it must be removed. It's material, not weld reinforcement.
  • Melt-through or a convex contour. A convex mark is a curved line on the weld symbol's own face. Melt-through is a solid half-circle on the opposite side.
  • Required height or limit. The number says what must show. How much is too much comes from the code or contract, not from the symbol.

Melt-through on the CWI exam

Melt-through, back weld and backing weld marks look alike and are read without a book in CWI Part A, where Symbols, Welding and NDE is at least 10% of the exam. The filled-versus-open detail is small and easy to miss. On Part B, Utilization of Specifications and Drawings is at least 8% of the practical, and root reinforcement is a dimension you may have to check.

Practice it

The welding symbols quiz drills the groove shapes and the back weld mark that melt-through is read against. See how the groove side is dimensioned on the V-groove weld symbol page, and find every supplementary mark on the welding symbols chart.

Filled or open? Check the rest of Part A

Take the free 10-question check. It's weighted to the Part A outline, scored by domain and needs no signup, so you see whether symbols or another domain costs you the most points. The Part A course gives you a lesson on reading welding and NDE symbols and on every other exam subject, two full-length timed practice exams, and a worked explanation with every answer.

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Facts checked 24 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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