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Coastal exposure

Coastal exposure is the severity of a salt-laden marine environment, which sets steel coating grade, fixings and warranties (such as Colorbond standard versus Ultra).

By Dan Waring. Last updated 3 June 2026. Verified 3 June 2026.

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Coastal exposure is the severity of a salt-laden marine environment at a site. It grades from severe (within roughly 1 km of breaking surf, and worse right on the beachfront) through moderate to mild and inland. The category determines the steel coating grade, the fixings, and the product warranties you can rely on, such as Colorbond standard versus Colorbond Ultra.

Airborne salt drives corrosion, and it falls off sharply with distance from breaking surf, so two houses a few hundred metres apart can sit in different corrosivity categories. AS 4312 maps these as atmospheric corrosivity zones (broadly C1 mild up to C5 very severe, with the worst marine being “severe marine”). The closer to surf, the higher the category and the more protection everything metal needs.

What it drives on a build:

  • Roof and wall steel: standard Colorbond is fine inland but not within the severe marine zone, where Colorbond Ultra (or stainless, or other upgrades) is needed, and the manufacturer’s warranty is void if the wrong grade is used for the zone.
  • Fixings and flashings: screws, brackets and flashings must match (stainless or heavily galvanised near surf); mixing a lesser fastener with a good sheet gives early rust streaks.
  • Other metals: balustrades, garage doors, fixings, lintels all need their corrosion protection set to the zone.

For a builder the practical points are to establish the coastal-exposure category at specification (distance to breaking surf is the first question), to spec the matching steel grade and matching fixings (the fixing is the usual weak link), and to keep the documentation, because a warranty claim on premature corrosion turns on whether the right grade was installed for the zone. Underspecifying near the coast is a guaranteed callback.

Also known as: Marine exposure, corrosivity category, coastal corrosion zone.

Category: Materials / Durability.

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References

Last updated: 2026-06-03. Verified: 2026-06-03. Quarterly review for currency.

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