A welding symbol on a drawing is an arrow and a horizontal reference line carrying a code for the weld: which joint (the arrow), which side of it (the symbol below the line means the arrow side, above means the other side), what kind of weld (fillet, groove and so on), how big, how long, and anything special (field weld, weld all around, finish). In the US the rules are set by AWS A2.4 (current edition 2020); internationally by ISO 2553 (2019), which has a notable difference: its "System A" adds a dashed line for the other side. This guide explains how to read both, with the misreadings that matter most on structural steel drawings.
The parts of a welding symbol
| Part | What it tells you |
|---|---|
| Arrow | The joint. It touches the joint line; several arrows can share one symbol when each joint gets the same weld. |
| Reference line | Carries everything else. Below it = arrow side; above it = other side; both = both sides. |
| Weld symbol | The weld type: fillet (triangle), groove welds (square, V, bevel, U, J, flare), plug or slot, spot, seam, stud and others. |
| Dimensions | Size to the left of the weld symbol; length and pitch to the right; groove angle and root opening at the symbol. |
| Supplementary symbols | Field weld flag, weld-all-around circle, backing, spacer, melt-through, contour and finish. |
| Tail | Information with no fixed place in the symbol: process, a welding procedure reference, or "CJP". Omitted when not needed. |
Arrow side and other side
The rule is fixed, whichever end of the reference line the arrow is on: a weld symbol below the line is on the arrow side of the joint, above the line on the other side. The "arrow side" is defined by the joint, not by where you stand: it is the side of the joint the arrow touches. Jason McCool, P.E., writing in STRUCTURE magazine (February 2023), points out a common error here: a weld on a hidden member on the far side of a plate is not shown by drawing the symbol above the line as an other-side weld; it should be referenced in the tail.
Fillet welds
- Size (the leg) is written to the left of the triangle; unequal legs are written as, for example, 1/4 x 3/8, and the drawing must show which leg goes on which member, because the order alone means nothing.
- Length is written to the right. No length means the weld runs the full length of the joint.
- Intermittent welds are written length-pitch, for example 2-6: 2-inch segments at 6-inch centres, the pitch measured centre to centre.
- Chain intermittent welds on both sides have segments opposite each other; staggered welds have them offset, shown by offsetting the symbols above and below the line.
- A fillet weld has no default size. If no size is shown, look for a "typical" size in the general notes, and ask if there is none.
Groove welds: CJP and PJP
Groove welds join members edge to edge or in T and corner joints, with the edges prepared (square, V, bevel, U, J, flare). Two cases matter most:
- Complete joint penetration (CJP) fills the full thickness of the joint. It can be specified by writing "CJP" in the tail, by a groove symbol with no depth or size dimensions, or by a groove symbol with a backing symbol opposite and no dimensions.
- Partial joint penetration (PJP) does not. Its symbol shows the depth of preparation and, in parentheses, the effective throat, for example 1/2 (3/8): the edge is prepared 1/2 inch deep and the weld must achieve a 3/8-inch effective throat (the "E" dimension). A depth without parentheses is only the groove preparation, not the weld size. A groove symbol with no dimensions means CJP for single grooves and symmetrical double grooves; for other joints, look for "CJP" in the tail or ask.
- Flare welds (flare-bevel, flare-V, where a round or bent edge meets another member, as with hollow sections) are always partial penetration, so the effective throat in parentheses is needed.
The root opening is written inside the groove symbol and the groove angle outside it.
Supplementary symbols
- Field weld: a small flag at the junction of arrow and reference line means the weld is made on site, not in the shop. Its direction has no meaning; its position does.
- Weld all around: a circle at the junction means the weld continues all around the joint.
- Backing: a rectangle opposite the groove symbol; an R inside it means the backing is removed after welding.
- Spacer: a rectangle centred on the reference line.
- Melt-through: a filled semicircle opposite the groove symbol, meaning complete penetration with visible reinforcement on the back.
- Contour: flush, flat, convex or concave. Older drawings may also show a letter for the finishing method (such as G for grinding); under the 2020 edition the contour may be achieved by any method unless the drawing says otherwise.
- Broken arrow: on bevel and J grooves, the arrow bends towards the member whose edge is prepared.
Weld all around: where it causes trouble
"All around" is easy to draw and sometimes wrong. McCool's STRUCTURE article notes that AWS limits welding around corners of three-dimensional joints, where the weld throat can be short at transitions and poor fit-up is hidden; continuous seal welds are allowed for reasons such as air- or water-tightness or galvanizing, and then the engineer should specify inspection. On hollow section joints where members are flush, the "fillet" all around actually becomes a flare-bevel weld on the flush sides. When you see the circle on a complex joint, check that it is intended.
Multiple reference lines
Several reference lines on one arrow show a sequence: the line nearest the arrow is the first operation, then read outward. They can include steps such as back-gouging or testing.
AWS and ISO side by side
| Item | AWS A2.4 (US) | ISO 2553, System A |
|---|---|---|
| Sides | Below the line = arrow side, above = other side | Solid line = arrow side; a parallel dashed line = other side |
| Fillet size | Leg size | "a" = throat thickness, "z" = leg length |
| Intermittent | Length-pitch, pitch centre to centre | Number x length (gap), gap = clear distance between segments |
| Process | Letters, e.g. SMAW, GMAW | ISO 4063 numbers, e.g. 111, 135 |
ISO 2553 also has a "System B" with a single reference line, close to the US convention. The biggest trap between the two systems is fillet size: an ISO "a5" weld has a 5 mm throat, which is a leg of roughly 7 mm, so reading "a5" as a 5 mm leg gives a weld about 30% too small. A dashed line on the symbol tells you the drawing uses ISO System A; the drawing's notes should say which system applies.
Design drawings and shop drawings
In US steel practice the structural engineer usually shows the weld requirements on the design drawings (for example "CJP" or a PJP effective throat), and the fabricator's detailer turns them into full joint preparation, using AWS D1.1's prequalified joint details, on the shop drawings. So the symbols on design drawings are often simpler than those on shop drawings. When you review shop drawings, check that each weld meets what the design drawings require.
Common misreadings
- Reading above and below the line by your own position instead of the joint.
- Assuming a default fillet size when none is shown.
- Reading a PJP effective throat as the preparation depth, or the other way round.
- Missing the field weld flag, which changes who makes the weld and how it is inspected.
- Treating ISO "a" (throat) as the leg size.
- Applying "all around" to joints where it cannot be welded properly.
Frequently asked questions
What do the numbers on a welding symbol mean?
The number left of the weld symbol is the size (fillet leg, or groove depth and effective throat); the numbers right of it are the length and, for intermittent welds, the pitch.
What does the flag on a welding symbol mean?
A field weld: the weld is made on site rather than in the fabrication shop.
What does the circle on a welding symbol mean?
Weld all around: the weld continues around the whole joint.
Which side is the arrow side?
The side of the joint the arrow touches. A symbol below the reference line applies to that side.
What does a number in parentheses mean on a weld symbol?
For groove welds, the effective throat the weld must achieve, typically on partial joint penetration welds.
What does a dashed line on a welding symbol mean?
The drawing uses ISO 2553 System A, where the dashed line carries the other-side weld.
Ncored is the desktop PDF editor we built in our architecture studio for drawing sets from AutoCAD, Revit, and ArchiCAD, on Windows and Mac (Apple Silicon). Large steel and structural sets stay smooth to zoom deep into a connection and read a small welding symbol. Cloud a symbol you need to question and leave a comment, and compare two issues of a sheet as a colour overlay to see which welds changed. Markups are saved as standard PDF annotations that open in Acrobat, Bluebeam, Preview and other standard PDF viewers. 14 days free. No signup, no email, nothing to enter.
Related: how to read structural drawings, shop drawings vs submittals, construction details.
All product names, standards and organisations mentioned on this page are trademarks or registered names of their respective owners and are used for identification only. Ncored is an independent product by Noir architects and is not affiliated with, sponsored by, or endorsed by any of them. This page summarises common conventions from AWS A2.4 and ISO 2553 as described in published articles; the standards themselves, the project's drawings and the engineer of record govern. Individual experiences may vary.