10 common welding symbol
mistakes
These are notation and interpretation errors — not welding technique mistakes. Each one trips up engineers, inspectors, and fabricators who read or write welding symbols on technical drawings, especially when switching between ISO 2553 and AWS A2.4.
Every mistake below is grounded in ISO 2553:2019. No guesswork.
Confusing arrow side between System A and System B
ISO 2553 §4.7.2.1, Table 5The mistake
Assuming “below the reference line = arrow side” in all cases. This is only true for System B. In System A, the solid line = arrow side and the dashed line = other side.
Why it happens
AWS uses the same convention as ISO System B, so people trained on AWS assume it is universal.
The rule
In System A, the component of the reference line (solid or dashed) on which the symbol is placed determines the side. Solid line = arrow side, dashed line = other side. In System B, position above or below the single reference line determines the side.
Fillet weld sizing — forgetting the a/z prefix in ISO
ISO 2553 §5.5.1, Table 6 No. 2.1The mistake
Writing just “5” for a fillet weld on an ISO drawing. Without the prefix, it is ambiguous — is it 5 mm throat or 5 mm leg?
Why it happens
AWS uses bare numbers (always means leg size), so people carry that habit into ISO work.
The rule
In ISO 2553, fillet weld size must always be preceded by ‘a’ (nominal throat thickness) or ‘z’ (leg length). The symbol ‘a’ shall not be used for unequal leg fillets.
Misreading the broken arrow line on bevel and J welds
ISO 2553 §4.6.3, Table A.1The mistake
Ignoring the break in the arrow line, so the wrong member gets the bevel preparation.
Why it happens
The break is subtle and easy to miss, especially on busy drawings or small-scale prints.
The rule
For single-bevel and single-J butt welds, the arrow line shall have a break and point toward the member that is to be prepared. The arrow need not be broken if there is no preference for which member is bevelled.
Mixing System A and System B on the same drawing
ISO 2553 §4.3The mistake
Using a dual reference line (System A) for some welds and a single reference line (System B) for others on the same drawing.
Why it happens
Fabricators working across UK/EU (System A) and US/Pacific Rim (System B) projects may default to whichever they learned first.
The rule
System A and System B shall not be mixed on the same drawing. The system used shall be clearly indicated, including units of measurement per ISO 129-1.
Intermittent weld spacing — confusing n, l, and (e)
ISO 2553 §5.3.2, Table 6 No. 1.3The mistake
Confusing the number of elements (n) with the spacing (e), or placing dimensions on the wrong side of the symbol. People forget that (e) is centre-to-centre spacing in parentheses, not edge-to-edge gap.
Why it happens
The notation n x l (e) packs three values into a tight space. The parentheses around (e) are easily overlooked or dropped.
The rule
Dimensions of intermittent welds go to the right of the symbol: number of elements (n), multiplication sign, length of each element (l), spacing between element centres (e) in parentheses. If n is omitted, the weld runs the full joint length.
Staggered vs chain intermittent — misreading the Z symbol
ISO 2553 §5.3.2.2–5.3.2.3, Table 3 No. 13The mistake
Treating staggered and chain intermittent welds as interchangeable, or not recognising the Z symbol that distinguishes staggered from chain.
Why it happens
Both are intermittent welds on both sides. The visual difference on the drawing is just the Z offset symbol between the two rows of notation.
The rule
Chain intermittent: weld elements directly opposite each other. Staggered intermittent: weld elements offset so centres of gaps on one side align with centres of welds on the other. The Z symbol across the reference line designates staggered.
Joint preparation depth (h) vs penetration depth (s)
ISO 2553 §6.5, Table 9The mistake
Assuming the depth of joint preparation (h) equals the required penetration depth (s). They are independent dimensions.
Why it happens
On a V-butt, the groove is machined to depth h, but the weld may need to penetrate further (s) or not as far. People assume one number covers both.
The rule
The depth of preparation (h) shall be followed by the penetration depth (s). Both letters must be included with their values. The depth of preparation can be greater than, equal to, or smaller than the penetration.
Root gap — specifying it on both sides of the reference line
ISO 2553 §6.2, Table 7The mistake
Placing the root gap dimension (b) on both sides of a double-sided butt weld symbol.
Why it happens
It seems logical — if the weld is on both sides, surely the gap applies to both. But there is physically only one root gap per joint.
The rule
The root gap (b) shall only be shown on one side of the reference line, as there is physically only one root gap per joint.
Multiple reference lines — reading the wrong one first
ISO 2553 §4.7.3, Figure 5The mistake
Reading the operations in the wrong order when there are multiple reference lines. The convention is arrow-to-tail, not top-to-bottom.
Why it happens
People naturally read top-to-bottom, but the line closest to the arrow is the first operation, regardless of vertical position.
The rule
The first operation shall be specified on the reference line closest to the arrowhead. Subsequent operations go on lines progressively further from the arrow.
Weld-all-round vs weld-between-points — using the wrong symbol
ISO 2553 §4.5.2–4.5.3The mistake
Using the weld-all-round circle when the weld type or size changes around the joint, or when the weld does not return to its starting point.
Why it happens
People think “weld-all-round” means “weld everywhere” rather than its precise meaning: continuous, same type, same size, full perimeter.
The rule
Weld-all-round requires the weld to be continuous, same type and same dimensions throughout. If type or size changes, use the weld-between-points symbol and separate welding symbols for each section.
Want the full picture?
These 10 mistakes are easier to spot once you understand the full symbol structure. Start with the step-by-step reading guide, or compare the two main standards side by side.
Need to verify dimensions on site? A good set of welding gauges makes quick work of throat, leg, and gap measurements.
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