GI Pipe — Classes, Threading and Where It Still Belongs

GI pipe is mild steel pipe with a zinc coating, threaded and coupled, made in light, medium and heavy wall classes (A, B and C) under IS 1239. Largely displaced by CPVC for internal plumbing because of scaling, it remains the specification for firefighting mains, exposed risers, buried runs and rooftop lines.

By Dhruv Agarwal · · 1 min read

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

PropertyTypical specification
StandardIS 1239 (Part 1) for tubes and fittings
ClassesA (light), B (medium), C (heavy)
Large diameterIS 3589 for water and sewage mains
JointingThreaded, or flanged / welded on larger sizes
CoatingHot dip galvanised, mass stated
FittingsGalvanised 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.

Standards referenced

  • IS 1239 (Part 1) — Steel tubes, tubulars and other wrought steel fittings (Bureau of Indian Standards)
  • IS 3589 — Steel pipes for water and sewage (Bureau of Indian Standards)
  • NBC 2016, Part 9 — Plumbing services (Bureau of Indian Standards)

Frequently asked

They are wall thickness classes under IS 1239. Class A is light, Class B medium and Class C heavy, and pressure capability rises accordingly. Most building plumbing and firefighting work specifies Class B or C, with Class C used for firefighting mains and buried runs. A specification saying only GI pipe leaves the class to the supplier, and Class A will be the cheapest thing that technically matches the description.

Scaling and corrosion. In hard Indian water a GI hot water line narrows progressively with internal scale until flow at the far fixtures becomes unusable, and rust staining appears at fittings. CPVC does not scale the same way and does not corrode. GI remains appropriate where mechanical strength, temperature or fire exposure rule out plastic.

Yes, and this is where GI systems usually start corroding. Cutting a thread removes the zinc coating at exactly the joint, exposing bare steel. Threads should be coated after cutting, and the specification should say so. Neglecting it means the pipe body outlasts the joints, and the joints are what leak.

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