Reading Welding Symbols, Joints & Positions
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Here's one that matters more than its name lets on — Reading Welding Symbols, Joints & Positions. If you remember one thing, make it this: The welding symbol on the drawing is the instruction — joint type, weld type, size, length, and which side — and a welder who can't read it can't build what the engineer designed. Do this right and the inspector and the welder are reading the same page.
The five basic joints
Butt (two pieces edge to edge in the same plane), T (one piece perpendicular to another), lap (overlapping plates), corner (two pieces meeting at an edge to form a corner), and edge (two parallel pieces welded along an edge). The joint is the geometry; the weld is what you put in it.
The common welds
- Fillet weld — a triangular weld in the corner of a T, lap, or corner joint. The most common weld in construction; sized by its leg length.
- Groove weld — fills a groove between the pieces: square, V, bevel, U, or J. Can be partial-penetration or complete-joint-penetration (CJP), which must fuse through the full thickness and is what a moment connection or a pipe butt weld needs.
- Plug and slot welds, spot and seam welds, and surfacing for special cases.
The welding symbol (AWS A2.4)
The symbol has a reference line with an arrow pointing to the joint. Information below the line applies to the arrow side of the joint; above the line applies to the other side. On the line: the weld symbol (a triangle for a fillet, a V for a V-groove), the size to its left, the length and pitch to its right, a contour symbol (flat, convex, concave) and finish method, and a tail for notes like the WPS number or "typ." A flag at the bend means field weld; a circle at the bend means weld all around.
Example: a fillet symbol below the line with 1/4 to its left and 3-12 to its right means a 1/4-inch fillet on the arrow side, 3 inches long on 12-inch centers.
Positions
Plate: 1G/1F flat, 2G/2F horizontal, 3G/3F vertical, 4G/4F overhead (G = groove, F = fillet). Pipe: 1G rotated, 2G vertical fixed, 5G horizontal fixed, 6G fixed at 45 degrees. The position is part of the welder's qualification; a 3G qualification covers flat, horizontal, and vertical plate but not overhead.
Going Deeper (Intermediate)
The structural drawings show what the engineer requires; the fabricator's shop drawings translate it into every weld on every piece, and the erection drawings show field welds. The WPS (welding procedure specification) ties the symbol to the process: joint details, process, filler, position, preheat, and parameters. A PQR (procedure qualification record) is the test that proved the WPS works, and a WPQ (welder performance qualification) proves the individual welder can run it.
Weld size is what the engineer designed for, and undersized fillets are a common inspection failure. Weld gauges measure leg size and throat; a fillet one size too small on a long run is a structural deficiency, not a cosmetic one.
Advanced / Pro-Level
Pros read the symbol and the code together. AWS D1.1 sets prequalified joint details for common groove welds that can be used without a separate PQR, with specific root openings, bevels, and backing requirements. They know when backing bars must stay or be removed, when back-gouging is required for CJP welds without backing, and what the effective throat of a partial-penetration weld actually is. On pipe, they know the difference a root pass and a hot pass make and why the root is often TIG or 6010 before a 7018 fill.
They also understand inspection symbols and the testing plan: which welds get visual only, which get magnetic-particle or dye-penetrant for surface defects, and which CJP welds get ultrasonic or radiographic testing.
Practice Challenge
A drawing shows a fillet symbol above the reference line with 5/16 to the left, a flag at the bend, and "typ." in the tail. Describe the weld. (Answer: a 5/16-inch fillet weld on the other side of the joint from the arrow, continuous since no length is given, made in the field, and typical for all similar joints on the drawing.)
In Practice
A welder who reads the symbol lays down the 5/16 fillet the engineer called for. A welder who "welds it good" lays down a 1/4 and the inspector writes it up on 40 connections.
Common Mistakes to Avoid
- Confusing arrow side and other side
- Undersized fillets because nobody checked with a gauge
- Treating partial-penetration and CJP groove welds as the same thing
- Welding in a position the welder isn't qualified for
Takeaway: The welding symbol on the drawing is the instruction — joint type, weld type, size, length, and which side — and a welder who can't read it can't build what the engineer designed.
Educational overview only — not hands-on welding instruction, a safety program, or a substitute for training under a qualified welder. Follow OSHA, ANSI Z49.1, your employer's procedures, and the applicable welding code for your work.