If you're comparing metal roofing quotes in Miami-Dade or Broward, you'll see two coating names over and over: galvanized (GI) and Galvalume (GL). Inland, the difference is a footnote. Within a few miles of salt water, it's the whole story.
What each coating actually is
Galvanized steel is coated in nearly pure zinc. Zinc protects steel sacrificially — it corrodes first, so the steel underneath doesn't. Galvalume replaces most of that zinc with aluminum: roughly 55% aluminum, 43.5% zinc, and a trace of silicon. The aluminum forms a passive barrier that resists oxidation far longer in salt-laden air.
Why coastal air changes the math
Salt accelerates zinc consumption. A G90 galvanized panel that would shrug off decades inland can show edge corrosion dramatically sooner on a barrier island. Galvalume's aluminum-rich coating holds up materially better in the same exposure — which is why most coastal specifications in South Florida call for AZ50/AZ55 Galvalume or better.
Where galvanized still makes sense
Galvanized remains a reasonable choice for inland projects, budget-driven jobs away from the coast, and applications where panels are fully painted and maintained. The point is not that GI is bad — it's that the right answer depends on distance to salt water, and a quote should say which coating it prices.
How to read it off your quote
Look for the coating designation, not the marketing name: G90 means galvanized at 0.90 oz/ft² of zinc; AZ50 and AZ55 mean Galvalume at those aluminum-zinc coating weights. If the quote doesn't state a designation, ask. The mill certificate that ships with the order should match it.
