Compare Galvalume (AZ) and galvanized (GI) steel coil to choose the right coated coil for roofing and outdoor service, and learn what coating weight to specify.
Galvalume and galvanized coils both protect steel with a metallic coating, and on a drawing they can look like the same product. They are not. Galvalume coats the steel with an aluminum-zinc alloy that resists heat and weather differently from the pure zinc of galvanized coil, and the choice changes both service life and price. Selecting the wrong one can mean a roof that rusts early or a project that overpays for a coating it does not need. This guide explains how to choose between them for exposed service.
What the Two Coatings Are
Galvanized coil, often marked GI, coats the steel with essentially pure zinc. Galvalume, often marked AZ or GL, coats it with an alloy of roughly 55 percent aluminum, about 43 percent zinc, and a small amount of silicon. The aluminum in Galvalume forms a stable oxide that resists oxidation at higher temperature, while the zinc provides sacrificial protection at cut edges. The result is that Galvalume generally gives two to several times the corrosion life of an equal-weight galvanized coating in atmospheric exposure, along with better heat reflectivity.
When Galvalume Is the Better Choice
For roofing and wall cladding, especially in warm, humid, or moderately corrosive climates, Galvalume is usually the better long-term choice. Its longer service life and better heat reflection lower building operating costs, and it accepts color coating well for pre-painted roofing. Galvalume is also preferred where the surface will see elevated temperature, such as near heat sources, because the aluminum oxide withstands heat that would attack a pure zinc coat. For exposed building envelopes designed to last decades, the higher upfront cost is normally recovered through longer life.
When Galvanized Coil Is Sufficient
For general fabrication, ductwork, internal framing, and parts where the coating will be painted or where service is mild and protected, galvanized coil remains a sound and economical choice. Where cut edges are protected or the part is sheltered, the sacrificial behavior of pure zinc is adequate, and the lower cost and wider availability of galvanized coil make it the practical specification. It is also preferred where the application needs maximum sacrificial protection at damaged edges in buried or soil contact, where Galvalume's aluminum-rich coat is less active.
What to Specify on the RFQ
Distinguish the coating clearly, marking GI for galvanized and AZ or GL for Galvalume, and never accept a quote that names only galvanized when Galvalume is required. Specify the coating weight, such as AZ150 for Galvalume or Z275 for galvanized, and the relevant standard. For prepainted coil, also state the paint system and color, since the coating and paint are ordered together. Because the two coatings can look similar in a photo, confirming the coating type and weight on the mill test certificate before shipment is the simplest way to prevent a costly substitution.