Weld Symbols & Blueprint Interpretation

A welder who can't read a print is a welder who waits on the foreman to tell him what to do. A welder who can read a print can pick up a drawing, walk to the fit-up, and build the part exactly as the engineer intended - size, type, location, and all. Weld symbols are the shorthand that structural engineers, detailers, and fabricators use to communicate weld requirements on every drawing in the shop. They are defined by AWS A2.4 (Standard Symbols for Welding, Brazing, and Nondestructive Examination), and once you know the grammar, reading them is no harder than reading a map.

This guide walks through the anatomy of a weld symbol, the most common weld types you will see on structural and fabrication prints, the rules for arrow-side and other-side, and the way dimensions, contours, and supplementary notations combine into a complete callout. By the end, you will be able to look at a callout on a print and state the weld type, size, length, location, and any special requirements without guessing.

Anatomy of a Weld Symbol

Every AWS weld symbol is built from the same base skeleton. Knowing the skeleton is the key to reading any variation.

        +----------------+
        | supplementary  |     (optional: contour, finish)
        +----------------+
               |
       weld symbol (above/below line)
               |
tail --- reference line --- arrow ---> to the joint
               |
       weld symbol (above/below line)
        +----------------+
        | supplementary  |
        +----------------+

The key parts:

• Reference line - the horizontal line that carries the weld information. Always drawn horizontal, always read left to right.
• Arrow - points from the reference line to the joint being welded. The arrow side is always the side of the joint the arrow touches.
• Tail - the forked end on the opposite side of the arrow. Contains reference info (spec, process, or "see note 3"). If there is no special reference, the tail is often omitted.
• Weld symbol - the small pictograph (triangle, square, V, U, etc.) that describes the weld type. Its position above or below the reference line carries meaning.
• Supplementary symbols - optional modifiers such as the all-around circle, the field weld flag, the contour symbol, or the finish method.
• Dimensions - numbers placed around the weld symbol that specify size, length, pitch, and depth.

Arrow-Side vs Other-Side vs Both-Sides

This is the rule that trips up every beginner. Memorize it:

• Weld symbol BELOW the reference line = weld on the arrow side of the joint.
• Weld symbol ABOVE the reference line = weld on the OTHER side of the joint (the side the arrow does NOT touch).
• Weld symbol on BOTH above AND below = weld on both sides of the joint.

It feels backwards at first - "below means arrow side?" - but it becomes second nature. Think of the reference line as a mirror of the joint: what you see on your side of the mirror is the arrow side; what you see on the far side is the other side.

Common Weld Type Symbols

You will see these four types on more than 90 percent of structural and general fabrication prints. Learn them cold.

Fillet:     |          (a right triangle - vertical leg on the
            |_          reference line, hypotenuse pointing away)

Square      ||          (two short vertical lines - square-groove
groove:     ||           between two butt-joined plates)

V-groove:   /          (a V - bevels on both plates forming a V)

Bevel        /|         (a V with only one bevel, plate on
groove:     / |          the non-beveled side is vertical)

U-groove:   _/         (a U - single-plate U prep or two J's)

Plug/slot:  [ ]         (a small rectangle - plug weld through
                         a hole, or a slot weld through a slot)

The Fillet Weld

Fillet welds are the workhorses of structural steel - corner joints, T-joints, lap joints. The fillet symbol is a right triangle. The vertical leg sits on the reference line, the hypotenuse points away.

The number to the left of the symbol is the leg length (in inches or millimeters, depending on the drawing's unit block). A 1/4 to the left of a fillet symbol means a 1/4-inch leg fillet. Both legs are assumed equal unless two numbers are given separated by an "x" (e.g., 1/4 x 3/8).

The number to the right of the symbol is the weld length. If no number appears, the weld runs the full length of the joint. A 1/4 to the left and 6 to the right means a 1/4-inch fillet, 6 inches long.

Intermittent fillet welds use a length-pitch notation: 1/4 3-6 means 1/4 fillet, 3-inch weld length, 6-inch pitch (center-to-center). So you weld 3 inches, skip to 6 inches from the start, weld another 3 inches, and repeat.

Groove Welds

Groove welds are used on butt joints where full or partial penetration is required. The symbol tells you the prep type (square, V, bevel, U, J, flare-V, flare-bevel) and the size tells you how deep to weld.

The size number to the left of a groove symbol is the depth of groove (how deep the prep is, or for square-groove, how thick the material is). A number in parentheses immediately after represents the effective throat or the depth of the weld itself if it differs from the prep depth.

Example: 3/8 (1/2) on a V-groove means 3/8 depth of prep, 1/2 effective throat. Most full-penetration welds are called out as CJP (complete joint penetration) - often indicated by leaving the size blank and noting CJP in the tail, or by placing "CJP" in a box in the tail.

Plug and Slot Welds

Plug welds fill a pre-drilled hole to lock two plates together. The symbol is a small rectangle placed above/below the reference line. The diameter of the hole appears to the left. If the holes do not fully fill (a depth less than the plate thickness), that partial depth appears in parentheses.

Slot welds use elongated holes instead of round holes; the pitch (center-to-center of slots) appears to the right of the symbol.

Supplementary Symbols

All-Around Circle

A small circle at the junction of the arrow and the reference line means the weld goes completely around the joint - the arrow no longer needs to point at any one segment because the weld wraps the entire perimeter. This is common on pipe-to-plate connections, HSS-to-base-plate welds, and tube-in-tube penetrations.

Field Weld Flag

A solid black flag at the junction of the arrow and the reference line means the weld is to be made in the field (on site), not in the shop. Field welds get called out specifically because they have different inspection requirements and often different electrodes or procedures than shop welds.

Contour and Finish

The contour symbol (a small arc or flat bar) sits directly above or below the weld symbol and specifies the final surface profile:

• Flush (a horizontal bar) - ground flat to match the parent metal surface
• Convex (an upward arc) - normal convex bead, no grinding
• Concave (a downward arc) - ground concave, often for fatigue-critical joints

A letter above the contour symbol indicates the finishing method:

• C - chipping
• G - grinding
• M - machining
• R - rolling
• H - hammering

So a fillet with a flush contour and G above means "weld the fillet and grind it flush."

Tail References

The tail holds the references that do not fit anywhere else:

• Process - e.g., GMAW, SMAW, FCAW, GTAW
• WPS (Welding Procedure Specification) number - e.g., WPS-103
• Note references - e.g., See Note 4 which would direct you to a note in the title block
• Specification - e.g., AWS D1.1

A typical tail reads: GMAW / WPS-14 or See Note 3.

Walking Through a Complex Callout

Here is a full real-world callout and how to read it:

                 ___
                /       1/4 ______/__________    tail: GMAW/WPS-22
            |
            |  (field weld flag)
            |
           |
                       (triangle)
                                       1/4  6-12

Read it step by step:

1. Fillet symbol BELOW the line - weld on the ARROW SIDE of the joint.
2. Size 1/4 to the left of the symbol - 1/4-inch fillet leg.
3. 6-12 to the right of the symbol - intermittent weld, 6 inches long, 12-inch pitch (center to center).
4. Field weld flag present - this is a field weld, not shop.
5. Tail GMAW/WPS-22 - use GMAW (MIG) per Welding Procedure Specification 22.

Translated to a walk-up-and-weld instruction: "In the field, using the GMAW process per WPS-22, lay a 1/4-inch fillet weld on the arrow side of the joint, 6 inches long, repeating every 12 inches center-to-center."

Common New-Reader Mistakes

• Welding the wrong side. If the symbol is below the line, the weld is on the arrow side. Do not put it on the opposite side just because that is where your gun fits better - confirm with the foreman before depositing.
• Ignoring the field weld flag. Shop-welding a part that was specified as a field weld can mess up camber, fit-up, or shipping assembly - the engineer wanted that joint made after erection for a reason.
• Missing the all-around circle. Partial-perimeter welds on a member specified as all-around are a rejection-ready defect.
• Mixing up length and pitch. 3-6 means 3 weld length, 6 pitch (3 on, 3 off). It does not mean 3 to 6 inch range. Pitch is always center-to-center.
• Guessing at tail references. If the tail says "See Note 4," go read Note 4. Do not assume it is a generic reference.

Practice Drills

Reading symbols is a memory and reps skill. To get fluent:

1. Get a copy of AWS A2.4 (the standard) or a laminated weld symbol chart - keep one in your tool bag.
2. Grab any set of structural steel drawings and walk through every weld callout. For each, write out: type, size, arrow-side/other-side/both, length, pitch, field or shop, any supplementary symbols.
3. Ask the detailer or lead welder to check your reads. Mistakes now are cheap; mistakes on the iron are expensive.

Day 1 Checklist

• Weld symbol chart or AWS A2.4 reference on hand
• Ability to identify: fillet, square-groove, V-groove, bevel-groove, U-groove, plug, slot
• Arrow-side vs other-side rule memorized (below line = arrow side)
• Can read a fillet size and length (e.g., 1/4 6)
• Can read an intermittent fillet (e.g., 1/4 3-6)
• Recognizes field weld flag and all-around circle
• Knows where to find the WPS referenced in the tail

Reading prints separates the laborer from the tradesman. Invest the hours now and you will never be the welder standing at the joint asking someone else to explain it.