Both sheets are steel underneath, and both are sold as rust-resistant. The difference is in the thin metallic coating on the surface, and that coating decides how long the roof lasts, especially in humid, coastal or industrial air.
Standard galvanized: a zinc coating
Galvanized sheet is steel coated in zinc. Zinc protects in two ways: it acts as a barrier between the steel and the air, and it corrodes in the steel's place if the coating is scratched. That works well for years in ordinary conditions. But the zinc is slowly used up as it protects, and once it is gone the steel underneath begins to rust.
Alu-Zinc: an aluminium and zinc alloy
Alu-Zinc replaces the pure zinc with an alloy of aluminium and zinc. As a rough guide, the coating is mostly aluminium, and that changes how it ages. Aluminium forms a thin, transparent oxide layer on the surface which shields the metal beneath, and this layer renews itself if it is scratched. That is why an Alu-Zinc sheet keeps its shine and resists corrosion for so long, even in highly corrosive environments. The result is up to five times the product life of standard galvanized sheet.
The zinc in the alloy is still doing a job: at cut edges and scratches it provides sacrificial protection, the same way it does on galvanized sheet.
Side by side
| Standard galvanized | Alu-Zinc | |
|---|---|---|
| Coating | Zinc | Aluminium-zinc alloy |
| Corrosion resistance | Good in mild conditions | Outstanding, even in harsh environments |
| Expected life | Baseline | Up to 5x longer than galvanized |
| Appearance | Dull grey over time | Spangled silver that holds its shine |
| Can be painted later | Yes, with the right primer | Yes, over-painted on site if colour is wanted |
"Up to" matters: the real figure depends on where the roof is. A roof in a dry, sheltered spot will get a long life from either sheet; a roof near the coast or beside a factory will show the gap much sooner.
Where Alu-Zinc is worth the extra cost
- Coastal roofs in Mombasa, Kilifi and along the coast, where salt in the air attacks zinc quickly.
- Industrial buildings with fumes, humidity or heavy condensation.
- Any roof you don't want to redo, where a longer life saves more than the price difference costs.
Where galvanized is still a sensible choice
For a small structure, a temporary building or a dry inland site, standard galvanized does the job at a lower price. Just be honest about how long the building has to last before you decide.
Our Alu-Zinc range
Our Alu-Zinc Mabati 11/3 is made at our Ruiru plant as an uncoloured, spangled-silver sheet, and our colour-coated profiles use an alu-zinc base too. If you're comparing options for a specific roof, tell us where it is and we'll say which one we'd put on it.