What a GI roof really costs over 25 years
Galvanised iron wins the quotation and loses the ledger. Once the repainting cycles, the patch repairs and the eventual re-sheet are priced in, the cheaper roof stops being cheaper somewhere around year eight.
The zinc on a GI sheet is measured in microns. Everything about the roof’s working life follows from how long that layer survives.
Where the two curves cross
A galvanised roof starts cheap and accumulates cost: repainting every few years, fasteners and flashings replaced as they corrode, patch repairs after each monsoon, and eventually a full re-sheet with the labour and downtime that involves. An aluminium roof front-loads its cost and then largely stops asking for money.
Why the metals behave differently
Galvanised steel is iron protected by a sacrificial zinc coating. The coating is doing a job, and jobs finish. Once it wears through, faster in humidity, faster still near salt, the iron underneath begins to rust and the roof starts failing from the inside. Aluminium has no iron in it. There is no equivalent countdown running.
What this means when you specify
If the building is temporary, GI is defensible. If it is meant to stand for decades, the arithmetic goes the other way, and it goes there sooner on the coast than inland.
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