ISOMETRICResources / Isometric symbolsUpdated 29 Oct 2026

Weld and joint symbols on piping isometrics

Every joint on a piping isometric is marked: a dot for a shop butt weld, a cross or a flag-style mark for a field weld, two short lines for a flanged joint, one short line for a threaded joint.

SymbolIsometricWhat it is

Butt weldA dot on the line marks a butt weld.

Field weldA field weld is made on site, at installation, to join two spools or a spool to equipment.

Flanged jointTwo short lines across the pipe mark a flanged joint: two flanges bolted together with a gasket between them.

Threaded jointA single short line across the pipe marks a threaded (screwed) joint.

Joints and welds

Joints matter more on an isometric than anywhere else, because each one is a piece of work: a weld to make, number, inspect and test, or a flange to bolt up and torque. The shop welds decide where the spools break; the field welds are what is left for site.

Field weld marks vary most between projects. A cross, an open circle or a dot labelled FW are all common, so check the legend sheet.

Frequently asked questions

Which joints and welds appear on a piping isometric?

This legend covers butt weld, field weld, flanged joint, threaded joint. Project legends may add more.

Can I download these symbols?

Yes. Each symbol page has free SVG files, and the hub has printable PDF charts.

From the isometric to the signed test pack.Deskely Testpack builds test packs from the line list and the drawings, records the test in the field and tracks reinstatement to the last spade.

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Joints and welds

Frequently asked questions

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