If you've spent any time comparing metal roofing quotes, you've likely seen three coating names used almost interchangeably: GI, Galvalume, and ZAM. They are not the same coating, they do not perform the same way over 20 years, and the difference is bigger in a coastal climate than almost anywhere else. Here's what each one actually is.
GI — Galvanized Steel
GI is steel coated with two layers of pure zinc (about 99%) through a continuous hot-dip process. Zinc protects steel the way it always has: it corrodes preferentially, sacrificing itself so the steel underneath doesn't rust. It's the most common coating on roofs today because it's cost-effective and well understood — but zinc alone has a finite protective life, and once it's consumed at an edge or scratch, the steel beneath is exposed.
Galvalume — 55% Aluminum-Zinc Alloy
Galvalume swaps pure zinc for an alloy: roughly 55% aluminum, 43.5% zinc, and a trace of silicon for adhesion. The aluminum forms a passive barrier that resists oxidation and handles heat better than zinc alone, while the zinc content still provides some sacrificial protection at cut edges. The result is generally longer service life than GI in most climates, though the aluminum-rich coating protects cut edges less aggressively than a zinc-dominant one.
ZAM — Zinc-Aluminum-Magnesium
ZAM is the newest of the three: a ternary alloy of zinc, aluminum (roughly 6–11%), and magnesium (roughly 1–3%). The addition of magnesium is what sets it apart — it forms a dense, stable corrosion-product layer that actively "self-heals" at cut edges and scratches, rather than simply resisting corrosion passively. In independent testing and in the field, ZAM consistently outperforms both GI and Galvalume in coastal, high-humidity, and industrial environments, which is why it's increasingly specified for silo storage, cable trays, and coastal architectural panels where edge corrosion is the real long-term failure point.
Which one should you choose?
| Grade | Corrosion Resistance | Best For |
|---|---|---|
| GI | Good | Standard roofing, most budgets |
| GL | Very Good | Coastal air, longer service life |
| ZAM | Excellent | Harsh/coastal, self-heals at cut edges |
For a standard inland residential re-roof, GI remains a perfectly reasonable, cost-effective choice. For coastal properties, high-humidity regions, or anywhere cut edges will be exposed to weather over decades, Galvalume is a meaningful step up, and ZAM is the strongest option available today. The right answer depends on your specific site conditions and how long you need the roof to perform without touch-up — not on which coating is newest.
