Steel roofing sheets are protected by a metallic coating. The two most common are GI (zinc) and galvalume (aluminium-zinc alloy).
GI (galvanised iron)
A pure zinc coating protects the steel by corroding first. GI is economical and widely available, suiting sheds, temporary structures and budget roofs.
Galvalume
Aluminium-zinc coating combines aluminium's barrier protection with zinc's sacrificial protection. Generally it lasts longer than GI at a similar coating mass and reflects more heat, keeping buildings cooler.
| GI | Galvalume | |
|---|---|---|
| Coating | Zinc | Aluminium-zinc |
| Corrosion resistance | Good | Better |
| Heat reflection | Moderate | Higher |
| Price | Lower | Slightly higher |
Colour-coated sheets
Colour-coated sheets add paint over a GI or galvalume base for a finished look and extra protection. Light colours reflect more heat.
Our advice
For factories, warehouses and homes, galvalume or colour-coated galvalume is usually the best value over the roof's life. For short-term sheds, GI keeps cost down.