AWS A2.4 §7.7 · D1.1:2025

Back Weld & Backing Weld Symbol

How to read the open semicircle symbol — the critical difference between a back weld (after) and a backing weld (before). Placement, sequence, and D1.1 CJP joint applications.

Back / Backing Weld Symbol — Anatomy

V-groove + backing weld

Open vs filled semicircle

Sequence is the difference

Critical distinction: Open semicircle = back or backing weld. Filled (solid) semicircle = melt-through. Both sit opposite the groove weld. The fill tells you which one — look for it.

How to Read

Back vs Backing Weld

The same open semicircle symbol represents two different operations. The sequence determines the name and purpose.

Backing Weld (Before)

A backing weld is deposited BEFORE the main groove weld. It supports the root pass from the reverse side — an alternative to using a steel backing bar. The welder flips the joint, lays down the backing weld, then flips back and completes the groove weld from the primary side.

Back Weld (After)

A back weld is deposited AFTER the main groove weld. It seals or reinforces the root from the back side. The groove weld is completed first, then the joint is flipped and the back weld is applied.

How to Tell Which

The tail of the welding symbol specifies "BACKING WELD" or "BACK WELD". With multiple reference lines (specifying welding sequence), the order of operations determines the meaning without tail notation.

Feature	Backing Weld	Back Weld
Sequence	Before groove weld	After groove weld
Purpose	Support root pass	Seal/reinforce root
Alternative to	Steel backing bar	Backgouge + reweld
Tail notation	"BACKING WELD"	"BACK WELD"

D1.1 context: Most prequalified CJP joints in D1.1 Figure 5.1 specify either steel backing (a bar, not a weld) or backgouge as the method. Backing welds appear as an alternative when physical backing bars are impractical.

Common Questions

Back & Backing Weld Symbol FAQ

What is the difference between a back weld and a backing weld?

Both use the same open semicircle symbol, but the welding sequence is opposite. A backing weld is applied BEFORE the groove weld — it supports the root pass from the reverse side. A back weld is applied AFTER the groove weld — it seals or reinforces the root from the back side. The tail of the welding symbol specifies which: "BACKING WELD" or "BACK WELD". With multiple reference lines, the sequence determines meaning.

Where is the back/backing weld symbol placed on the welding symbol?

The open semicircle is placed on the OPPOSITE side of the reference line from the groove weld symbol. If the groove weld is below the reference line (arrow side), the back/backing symbol goes above (other side), and vice versa. This placement logically represents the weld being on the reverse side of the joint from the primary groove weld.

What is the difference between a backing weld and a backing bar?

A backing weld is a weld pass deposited before the main groove weld to support the root from the reverse side. A backing bar is a physical piece of steel placed against the back of the joint to contain the weld pool. Both achieve the same purpose — supporting the root pass — but one is weld metal and the other is a separate component. D1.1 Figure 5.1 CJP joints specify either backing (bar) or backgouge methods.

How do you tell a backing weld symbol from a melt-through symbol?

The backing/back weld symbol is an OPEN semicircle (unfilled arc). The melt-through symbol is a FILLED (solid black) semicircle. Both sit on the opposite side of the reference line from the groove weld. Open = back/backing weld (a separate weld operation). Filled = melt-through (visible root reinforcement from the primary weld, not a separate pass).

Related Reference

Back Weld & Backing Weld Symbol

Back vs Backing Weld

Backing Weld (Before)

Back Weld (After)

How to Tell Which

Back & Backing Weld Symbol FAQ

More Weld Symbol Pages