Why coastal buildings need a roof that cannot rust
Salt air does not wait for the building to settle. From the week a galvanised roof goes on near the coast, its zinc layer is being consumed, and the failure it leads to is perforation, not staining.
The zinc layer is a countdown
Airborne salt keeps roof surfaces damp for longer and makes that moisture far more aggressive. The zinc coating on a GI sheet is consumed steadily rather than gradually degraded, and the closer to the shoreline, the faster it goes.
What happens next
Once the coating is gone the exposed steel rusts, and rust on a roof does not stay cosmetic. It works through the sheet, and the first sign is usually water inside the building rather than anything visible from outside.
Why aluminium sidesteps it
Aluminium forms a thin, stable oxide layer that protects the metal beneath instead of consuming it. There is no sacrificial coating to run out. On the Konkan, Malabar and Coromandel coasts that difference decides whether a roof lasts five years or the life of the building.
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