# 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.

## 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**.

## Frequently asked questions

### What do Class A, B and C mean?

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.

### Why has CPVC replaced GI for internal plumbing?

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.

### Does threading damage the galvanising?

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.

## Sources

- [IS 1239 (Part 1)](https://standards.bis.gov.in/website/published-standards/department-wise) — Steel tubes, tubulars and other wrought steel fittings
- [IS 3589](https://standards.bis.gov.in/website/published-standards/department-wise) — Steel pipes for water and sewage
- [NBC 2016, Part 9](https://www.bis.gov.in/standards/national-building-code/?lang=en) — Plumbing services

---

Source: https://hagerstone.com/materials/gi-pipe