# Galvalume Sheet — Coating, Grades and Service Life

> Galvalume is cold-rolled steel sheet coated with an aluminium-zinc alloy of nominally 55% aluminium, 43.5% zinc and 1.5% silicon, supplied bare or pre-painted as PPGL for roofing and wall cladding. Its service life is decided mainly by coating mass, stated as an AZ class such as AZ150.

## What it is

**Cold-rolled steel sheet coated with an aluminium-zinc alloy**, nominally 55%
aluminium, 43.5% zinc and 1.5% silicon. Supplied bare, or pre-painted as PPGL
for a coloured finish.

It is the default substrate for industrial roofing and wall cladding across
Indian PEB construction.

## Specification at a glance

| Property | Typical specification |
| --- | --- |
| Standard | IS 15961 or ASTM A792 |
| Coating mass | AZ70 / AZ100 / **AZ150** and above |
| Base steel yield | 345 MPa and 550 MPa grades common |
| Thickness | 0.45mm — 0.80mm TCT for roofing |
| Finish | Bare Galvalume, or pre-painted PPGL |
| Paint system | Polyester, SMP or PVDF by exposure |

**TCT** means total coated thickness — always confirm whether a quoted
thickness is TCT or base metal thickness, because the difference is real money.

## Why coating mass is the specification that matters

Two sheets can both be "Galvalume" and differ in service life by many years.
The variable is coating mass:

| Coating | Indicative use |
| --- | --- |
| AZ70 | Sheltered, low-exposure, short-life structures |
| AZ100 | General inland industrial |
| AZ150 | Standard for durable industrial roofing |
| AZ200 and above | Coastal, chemically aggressive, long-design-life |

## What drives the cost

Base metal thickness · coating mass · steel grade · bare versus pre-painted ·
paint resin class, with PVDF above SMP above polyester · profile and rib depth ·
sheet length and transport.

## Where projects go wrong

- **Coating mass unspecified.** The most common and most expensive omission.
- **Thickness quoted as TCT and read as base metal.** A 0.50mm TCT sheet is
  thinner in steel than a 0.50mm base metal sheet.
- **Cut edges and swarf left on the roof.** Drilling swarf rusts in place and
  stains and pits the sheet. It must be swept at the end of every shift.
- **Contact with wet concrete, copper or lead.** Each drives accelerated
  corrosion; isolate the interfaces.
- **Fastener spacing uniform across the roof.** Edges, corners and ridges see
  higher suction under IS 875 Part 3 and need closer spacing.

## Standards referenced

Aluminium-zinc coated sheet under **IS 15961** and **ASTM A792**. Wind loading
on roof and wall cladding to **IS 875 (Part 3)**.

## Frequently asked questions

### What is the difference between Galvalume and galvanised sheet?

Galvanised sheet is coated with zinc alone. Galvalume is coated with an alloy of roughly 55% aluminium, 43.5% zinc and 1.5% silicon. The aluminium gives far better atmospheric corrosion resistance and heat reflectivity, while the zinc retains sacrificial protection at cut edges. For industrial roofing in India, Galvalume typically outlasts equivalent galvanised sheet by a wide margin.

### What coating mass should be specified?

Name it explicitly, for example AZ150, which means 150 grams per square metre of alloy coating counted over both surfaces. Lighter coatings such as AZ70 cost less and corrode sooner. Coastal, industrial and high-humidity sites should be at AZ150 or above. A specification that says only Galvalume without a coating mass is unenforceable.

### Is Galvalume suitable near the coast?

With a higher coating mass and a pre-painted finish, yes, but it is not immune. Within a few kilometres of the sea, chloride loading is severe and a bare Galvalume roof will have a shorter life than inland. Pre-painted grades with a suitable paint system, generous coating mass and correct detailing at cut edges are the practical answer.

## Sources

- [IS 15961](https://standards.bis.gov.in/website/published-standards/department-wise) — Steel sheet, aluminium-zinc alloy metallic coated
- [ASTM A792](https://www.astm.org/products-services/standards-and-publications.html) — Steel sheet, 55% aluminium-zinc alloy-coated
- [IS 875 (Part 3)](https://standards.bis.gov.in/website/published-standards/department-wise) — Wind loads — roof and wall cladding

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Source: https://hagerstone.com/materials/galvalume-sheet