GI Pipe — Classes, Threading and Where It Still Belongs

What it is

Mild steel pipe with a zinc coating, threaded and coupled. Once the default for all building plumbing in India, now largely displaced internally by CPVC but still the correct specification in several places.

Specification at a glance

Property	Typical specification
Standard	IS 1239 (Part 1) for tubes and fittings
Classes	A (light), B (medium), C (heavy)
Large diameter	IS 3589 for water and sewage mains
Jointing	Threaded, or flanged / welded on larger sizes
Coating	Hot dip galvanised, mass stated
Fittings	Galvanised malleable iron to matching class

Where GI still belongs

Firefighting mains and hydrant systems, where the pipe must survive heat and mechanical damage and where NBC 2016 Part 9 and fire authority requirements govern. Plastic is not acceptable here.

Exposed risers and plant rooms, where impact resistance matters.

External and buried runs where mechanical strength is needed.

Terrace and rooftop lines exposed to UV, which degrades most plastics.

Where projects go wrong

• Class unspecified, so light-class pipe arrives for a medium or heavy duty.
• Threads left uncoated after cutting, corroding from the joint outward.
• GI used for hot water internally where scaling will progressively choke the line.
• Dissimilar metal joints — GI directly coupled to copper sets up galvanic corrosion; a dielectric union is required.
• Fittings of a lower class than the pipe, making the fitting the weak point.

Standards referenced

Steel tubes and fittings under IS 1239 (Part 1). Larger water and sewage pipe under IS 3589. Plumbing services provisions under NBC 2016, Part 9.