Why steel needs a coating at all

Steel is the strongest, cheapest roofing metal, and it rusts, so every steel roof panel is really a sandwich: a steel core wrapped top and bottom in a metallic coating that keeps oxygen and water from reaching it, usually finished with paint on top. The paint is cosmetic and sacrificial sunscreen; the metallic coating is the actual life-support system, and its type and thickness, stamped on every legitimate spec sheet, predict the roof’s lifespan more reliably than any brand name. The two coatings that matter are zinc (galvanized) and 55% aluminum-zinc alloy (Galvalume), and the industry migrated decisively toward the second for reasons this page unpacks.

This is the layer most quotes never mention and most buyers never ask about, which is exactly why our steel roofing guide calls the coating question the smartest one a metal-roof shopper can ask. Consider this the deep answer.

Galvanized: zinc, the self-healing bodyguard

Galvanizing dips steel in molten zinc, bonding a zinc jacket to both faces. Zinc protects two ways: as a barrier, simply covering the steel, and sacrificially, zinc is more chemically active than iron, so at any breach (a cut edge, a scratch, a fastener hole) the surrounding zinc corrodes preferentially, throwing itself on the grenade while the exposed steel stays protected. That self-healing at breaks is galvanizing’s superpower, and why cut edges on galvanized panels resist rust surprisingly well.

The limit is that zinc spends itself: it weathers away at a steady rate, faster in humid, coastal and industrial air, and when the jacket is gone the steel beneath begins to rust. Thickness therefore is lifespan: G-90 (0.90 oz of zinc per square foot, total both sides) is the roofing-grade standard worth insisting on; G-60 and lighter belong on sheds and interior work. Quality G-90 galvanized under paint serves 25 to 40 years in ordinary climates, less near salt.

Galvalume: the aluminum-zinc alloy that changed the industry

Galvalume, developed by Bethlehem Steel in the 1970s, coats steel in an alloy of 55% aluminum, 43.4% zinc and a little silicon. The aluminum contributes what aluminum does best, an inert, durable oxide barrier that weathers far more slowly than zinc, while the zinc in the alloy still provides a measure of sacrificial protection at cut edges. The blend outlasts G-90 galvanized by roughly two to four times in most exposures, which is why virtually all premium steel roofing, and nearly every long panel warranty, is Galvalume-based today. The roofing spec is AZ-50 to AZ-55 under paint.

Galvalume’s trade-offs are few and specific: its sacrificial protection at scratches is weaker than pure zinc’s (the barrier is the star), it must not be used in direct contact with concrete, mortar or animal-confinement chemistry (alkaline attack, where galvanized actually holds up better), and like all coated steel it still concedes the salt-spray coast to aluminum. Everywhere else, it is simply the better jacket.

Reading the specs like a pro

Three lines specify any steel panel completely, and a quote unwilling to state them is telling you something. Coating: “AZ-50/AZ-55 Galvalume” or “G-90 galvanized”, the number is ounces (zinc) or the alloy weight class; lighter numbers mean lighter jackets and shorter lives. Gauge: 26 for homes, 24 for premium, 29 for outbuildings, per the steel guide. Paint: PVDF (Kynar) for decades of color, polyester/SMP for economy, the finish protects the coating that protects the steel, so all three layers matter in order. Bare (unpainted) Galvalume is itself a legitimate, spangly-silver roof with a long record on barns and modern builds.

One more spec habit: warranties on steel roofing are really coating warranties, read whether the years quoted cover perforation (the coating) or just paint fade, and note the coastal-distance exclusions, which are the manufacturer telling you honestly where the jacket loses to salt.

Which should you choose?

Choose Galvalume for essentially every house, and any panel you want to see out its 40-to-70-year potential: it is the default behind quality standing seam, ribbed panels and metal shingles. Choose galvanized (G-90) where panels meet mortar or concrete, over animal confinement, or where budget rules and the building forgives, and step up the zinc weight rather than down. Choose neither, go aluminum, within a couple of miles of breaking salt water. Then let installation finish the job the coating starts: shears not abrasive saws, sealed cut details, no dissimilar-metal contact, per our installation guide.

It is a rare corner of roofing where one spec-sheet word buys decades. Ask it, and the rest of the metal decision falls into place. We quote coatings explicitly on every metal job across our service areas.

Galvanized vs Galvalume FAQs