How-To Guide

How to Read Welding Symbols

Decode AWS A2.4 welding symbols on structural and miscellaneous metal drawings to verify weld quality and type.

Welding symbols tell fabricators exactly what type of weld is required, where it goes, and how large it should be. These symbols follow AWS (American Welding Society) standard A2.4, and they're essential for structural steel, miscellaneous metal, and pressure vessel work. Understanding the symbol prevents weld defects, rework, and field failures. Every symbol has three main parts: the reference line, the arrow line, and the tail—each carries specific information.

The Three Parts of a Welding Symbol

Every AWS welding symbol has a standard structure:

Arrow Line
Points to the joint being welded. The side of the arrow indicates whether the weld is on the near side or far side of the joint.
Reference Line
Horizontal line with weld type symbols placed above (far side) or below (near side). This is where the weld information lives.
Tail
Contains welding process, specification, or special notes. Omit the tail if no additional information is needed.

Common Weld Types and Symbols

The weld type symbol is placed on the reference line. Here are the most common:

Fillet (∟)
Most common. Joins two surfaces at an angle (typically 90°). Size is shown to the left of symbol.
Groove (—)
Full-penetration weld filling the gap between two pieces. Includes V-groove, U-groove, J-groove variants.
Plug (⊗)
Circular weld through a hole in one plate into another. Used for lap joints.
Slot (▬)
Rectangular weld through a slot. Like a plug but elongated.
Spot (◯)
Resistance weld (electric). Used for sheet metal connections.
Seam (—)
Continuous weld along overlapping edges. Primarily for sheet metal.

Reading Fillet Weld Sizes

Fillet welds are dimensioned by leg size (the two equal sides of the triangle). Size goes to the left of the symbol on the reference line:

Example Fillet Dimensions

1/4" ∟ — 1/4" leg size fillet weld (both sides 1/4")

3/8" ∟ — 3/8" leg size (common for structural connections)

1/2" ∟ — 1/2" leg size (heavier connection, higher cost)

1/4" - 3/8" ∟ — Variable size (increases from 1/4" to 3/8" along the weld)

Larger fillet sizes mean stronger welds but also higher fabrication cost. Designers balance strength with economy.

Groove Weld Symbols and Prep Details

Groove welds require surface preparation (groove angle, depth, root opening). These details are placed around the groove symbol:

V-Groove (V)
45° angled cuts on both sides. Root opening and depth shown.
U-Groove (U)
Rounded groove with parallel sides. Better for thick plates.
J-Groove (J)
One-sided preparation. Often used in T-joints.
Bevel (V flipped)
One-sided V-groove. Commonly used for fillet combinations.

The depth of groove is shown inside or above the groove symbol. Root opening (gap between parts before welding) is shown below the groove symbol with a dimension.

Weld Location: Arrow Side vs. Other Side

The position of the symbol above or below the reference line indicates where the weld goes relative to the arrow:

Below Reference Line
Weld on the ARROW SIDE (near side) of the joint. This is where the arrow points.
Above Reference Line
Weld on the OTHER SIDE (far side) of the joint. The opposite side from the arrow.
Both Sides
Symbols appear both above and below. The joint is welded on both sides.

Why This Matters

Fabricators can only see one side of a joint clearly when setting up the weld. Misreading arrow side vs. other side leads to welds placed on the wrong surface, creating structural weaknesses that may require complete rework.

Supplementary Symbols and Finish

Small symbols added around the weld symbol convey important information:

Back Weld (⏚)
Weld placed on the back side after the main weld (for full penetration).
Finish: C (Chip)
Grinding or chipping required after weld.
Finish: G (Grind)
Weld surface ground flush. Requires full-depth blending.
Finish: M (Machined)
Weld surface machined to dimensions. Only for critical applications.
Contour: Flush (—)
Weld blended to match parent material surface.
Contour: Convex (⌢)
Weld crown slightly raised above parent material.

Reading a Complete Example

Here's how to read a real welding symbol step-by-step:

Symbol: 3/8" ∟ on bottom, 1/4" ∟ on top, with "AWS D1.1" in tail

Weld Type: Fillet welds (both sides)

Arrow Side: 3/8" fillet weld

Other Side: 1/4" fillet weld

Specification: AWS D1.1 (standard for structural steel welds)

What to Tell Fabricators: Weld both sides of this joint—3/8" on the near side, 1/4" on the far side, per structural steel code.

Common Errors in Reading Welding Symbols

Watch for these frequent mistakes on job sites:

Confusing arrow side with other side—resulting in welds placed on the wrong surface.
Misreading fillet size (1/4" vs. 1/2")—cheaper or weaker than designed.
Ignoring back weld symbols—front weld alone may not penetrate fully.
Missing finish or contour requirements—unground welds when grinding is required.
Overlooking the specification in the tail—wrong welding process or rod type used.

Verifying Welds on Site

Before signing off on welded connections, inspect against the symbol:

Check both sides—are welds present where the symbol shows them?
Measure fillet size—does the weld leg size match the symbol dimension?
Verify finish—are welds ground if the symbol specifies finish requirements?
Confirm location—are welds on the correct side of each joint?
Check for defects—spatter, undercut, or incomplete fusion requires repair.

Most structural steel fabrication drawings specify AWS D1.1 inspection requirements. Reference that standard if quality questions arise during installation.

Related Guides

Steel Connection Types
Welded vs. bolted connections and when each is used.
How to Read Structural Drawings
Understand structural plans and elevations.
Structural Steel Shapes and Sizes
Common W-shapes, angles, and plates.
Structural Steel Erection Errors
Common installation mistakes and how to catch them.

Catch Weld Specification Errors Before Fabrication

Welding symbol errors on structural drawings are expensive to fix once fabrication begins. Helonic's AI automatically reviews welding symbols against structural standards and flags symbols that don't match connection design.

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