Coastal material guide
Galvalume vs aluminum on the Maine coast
The most useful documents in this decision were written by the steel manufacturers themselves. Their Galvalume warranties exclude coverage near saltwater, at distances from 1,500 feet (Metal Sales) to two linear miles (Union Corrugating). When the people selling the steel will not stand behind it on the shore, that answers the coastal question better than any sales pitch. Here is the full picture.
What is Galvalume, exactly?
Galvalume is steel sheet coated with an aluminum-zinc alloy. The coating does two jobs: the aluminum content provides barrier protection over the surface, and the zinc content provides sacrificial protection, corroding first so small breaches in the coating heal over instead of rusting. Inland, that combination has made Galvalume the default substrate for residential metal roofing, and industry research on unpainted Galvalume standing seam found service life in excess of 60 years (Metal Construction Association study).
Salt changes the chemistry. In a marine atmosphere, chloride attacks the coating faster than it can protect the steel, and the weak points are exactly where a real roof cannot avoid them: cut panel edges, field scratches, drilled penetrations, anywhere raw steel meets coastal air. The sacrificial system that shrugs off a scratch in Lewiston gets steadily consumed on an island in Casco Bay. That is a qualitative description, and it is the reason the warranty documents below read the way they do.
Where the Galvalume warranties stop
| Manufacturer | Saltwater exclusion | Source |
|---|---|---|
| Metal Sales (AZ55 Galvalume, 45-year warranty) | Does not apply within 1,500 feet of saltwater; marine atmospheres and salt spray excluded | Warranty PDF |
| McElroy Metal (standard Galvalume warranty) | Same 1,500-foot saltwater exclusion, per McElroy’s own coastal guidance | McElroy coastal article |
| Union Corrugating (Galvalume warranty) | Excludes sheets within two linear miles of saltwater marine atmospheres | Warranty PDF |
Exclusion language verified against the linked documents, July 2026. Warranties get revised; the copy attached to your quote is the one that governs your roof.
One nuance keeps this from being a simple anti-steel rule. One manufacturer sells a coastal-rated Galvalume variant warranted far closer to the water than its standard product, which proves the point: the document, not the metal's name, is what covers you. Two panels can both be called Galvalume and carry entirely different coastal coverage, so read the specific warranty rather than assuming (McElroy's coastal guidance).
Why aluminum owns the shoreline
Aluminum does not rust. There is no steel core waiting behind the finish, so a cut edge or a scratch on an aluminum panel exposes more aluminum, not raw steel. That is why coastal aluminum systems are sold with saltwater warranties that steel systems cannot match (industry comparison of coastal paint warranties). The warranty gap is the industry pricing its own corrosion risk, in writing, and it points the same direction everywhere salt air reaches.
For Maine that means the material conversation should come before the panel and price conversation on any house near the water: the working harbors of the Midcoast, the Downeast shore, and the beach towns and islands of the Portland metro. The coastal aluminum roofing page covers the systems themselves.
Expect the two materials to price differently, and treat that as information rather than an obstacle. An installer quoting aluminum on a shorefront house is matching the metal to the exposure; an installer quoting the cheaper standard steel there is quoting a roof whose own manufacturer has disclaimed the site. The cheaper number is not the better deal when the coverage behind it is void on arrival.
How to apply this to your house
Which metal should your roof be? Measure your distance to saltwater, then let the warranty documents decide. Outside every exclusion distance in the table, Galvalume is the proven, economical default. Inside any of them, specify aluminum or a coastal-rated system whose warranty explicitly covers your distance, and keep that document with the contract.
- Measure straight-line distance from the house to saltwater, not driving distance. Tidal rivers and bays count as marine exposure in most warranty language, so read how the document defines it.
- Make every quote name the substrate: aluminum, standard Galvalume, or a coastal-rated product, with the warranty attached.
- If a quote puts standard Galvalume on a shorefront house, ask the installer to show you, in the warranty, why it is covered. The answer decides whether to keep talking.
- If the roof is being replaced anyway, fold the material decision into the replacement scope rather than treating it as an upgrade line.
Whatever the material, file the warranty documents with the signed contract and keep both. A coastal roof warranty is a transferable asset on a house that will likely change hands before the roof does, and the buyer's inspector who finds the right metal with the right paper behind it has nothing to argue about. The Maine Metal Roofing Guide puts this decision alongside snow loads, costs, and contractor verification, in the order a Maine owner actually meets them.
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