The short answer: Aluminum cannot rust, because rust is iron oxidizing and aluminum contains no iron. Steel roofing can only resist salt air as long as its coating stays intact, and coatings are interrupted at every cut edge, trim, scratch and fastener hole. On the Salish Sea that difference decides how the roof ages.
Where this actually matters in Washington
Salt air is not a coastal abstraction here — it reaches deep inland on the marine air that runs the whole inland sea. Bellingham and Anacortes at the San Juan gateway. Whidbey and Camano. Sequim and the shoreline along the Strait of Juan de Fuca. The Kitsap Peninsula and the waterfront communities that face it. Homes in these places live in chloride-laden air year-round, and their roofs are wet a great deal of the time, which is precisely the condition corrosion needs.
The failure mode in coated steel
A steel roof is not corrosion-resistant on its own. It is steel protected by a metallic layer and a paint system, and the protection is only as continuous as the film.
Every roof has interruptions. Panels are cut on site, so the raw steel core is exposed at each cut edge. Trims, hips, valleys and eave details multiply those edges. Fastener holes puncture the coating. Foot traffic and wind-carried debris scratch it. In a dry inland climate those interruptions can sit for years without consequence. In marine air with salt in it, they are the starting point: corrosion begins at the edge and creeps back under the coating, which is why edge rust and lifting paint show up on steel roofs near salt water long before the panel field looks tired.
Why aluminum is a different problem entirely
Interlock panels are 3105-H24 / 3003-H24 aluminum to ASTM B209, at roughly 95% recycled content. There is no iron to oxidize, so a cut edge on an aluminum panel is still aluminum — not an exposed steel core waiting for chloride. The AAMA 2605 70% PVDF Alunar® coating, warranted 30 years, is there for color and finish life, not to hold back rust that the base metal is incapable of.
The rest of the system matters for the same reason: every panel locks to its neighbors on all four sides and every fastener is concealed, so there are no exposed fastener heads on the surface for salt-laden rain to work at over decades. The system is evaluated under ICC-ES ESR-1790 for wind uplift resistance to 177.5 psf — relevant on any exposed shoreline — and carries a UL 2218 Class 4 impact rating. A Class A fire rating is available as an assembly under ASTM E108 with GAF VersaShield® underlayment per ICC-ES ESR-2053; it depends on the full assembly, never the panel alone.
The warranty point most homeowners miss
Read the fine print on roofing warranties near salt water and you will find the pattern: coastal or salt-air exclusions, distance-from-shore limits, reduced terms within so many miles of the water.
The Guardian Lifetime Limited Material Warranty carries no salt-air exclusion. The coverage on a home in Anacortes is the coverage on a home in Redmond: lifetime to the original owner, transferable once, with the second owner covered to 50 years from substantial completion. For a homeowner comparing an aluminum system against coated steel on the water, that is a genuine, checkable difference — and it is worth asking any competing bid to show you their salt-air language in writing.
Who serves the north Sound
Whatcom and Skagit counties are covered from our Bellingham office at 2219 Rimland Dr, Suite 301, Bellingham, WA 98226-8660, (360) 502-4018 — close enough that the estimate visit and any later service call are a short drive rather than a scheduling problem.
Next steps
The salt-air metal roofing page goes deeper on marine exposure, the aluminum roofing page covers the material, and the FAQ handles the questions that follow. Then look at the record: the gallery, reviews, proof, and who we are. When you want a number for your own roof, request a written estimate.
