Galvalume

Galvalume is a metallic coating applied to steel sheet or coil, typically 55% aluminum, 43.5% zinc, and 1.5% silicon by weight. The silicon acts as a metallurgical bonding agent, preventing the aluminum and zinc from separating during the coating process. This composite coating is applied by continuous hot-dip coating, where steel passes through a molten metal bath at around 840 degrees Fahrenheit. The result is a steel substrate protected by a layered metallic system that combines the corrosion resistance of both aluminum and zinc.

The coating typically ranges from 0.6 to 1.5 mils per side on construction-grade material, measured in ounces per square foot (common specifications are 90, 120, or 150 ounces per square foot). Galvalume coatings are softer than traditional galvanized zinc coatings, which means they dent and scratch more easily during handling and installation. This property makes it essential to protect cut edges and damaged areas during construction, as exposed steel will rust unless sealed with touch-up paint or sealant.

Performance and Application

Galvalume steel performs well in most climates, with superior corrosion resistance compared to bare steel or older hot-dipped galvanized coatings. However, it performs poorly in harsh coastal environments with salt spray or in chemically aggressive settings like feedlots or paper mills. The aluminum component provides good aesthetic qualities, resisting red rust staining better than traditional zinc coatings. Because of the softer surface, fingerprints and light scuffing are more visible, making cosmetic control important for visible architectural applications.

Common uses include metal roofing, siding, gutters, downspouts, flashing, and structural framing in agricultural buildings. The material gained wide adoption in the 1970s as an alternative to traditional galvanizing, particularly for roofing applications where its lighter weight and workability offered advantages. Modern Galvalume is sometimes called AZ series, referring to the Aluminum-Zinc composition, while older terminology sometimes conflates it with similar coatings like Galvanneal.

The coating can be painted or left bare, though bare Galvalume will gradually develop a weathered gray patina over 10 to 20 years depending on climate. Fasteners used with Galvalume should be stainless steel, hot-dipped galvanized, or Galvalume-coated to avoid galvanic corrosion at dissimilar metal interfaces. Touch-up after construction damage is critical; many job-site failures stem from bare steel edges left unprotected at cuts, holes, or mechanical fastener penetrations.