When Galvalume and galvanized steel are bent or folded, microscopic cracks form in their thin coatings. When water enters these cracks, it forms streaks of rust. The aluminum in Galvalume lowers the protective qualities of zinc so galvanized steel is better able to withstand scratches and cuts. For this reason, galvanized steel is more often used for metal shingles, shakes and tiles that have a lot of bends, folds and corners, and Galvalume is typically used for roofing panels joined by seams, commonly called “standing seam” panels. Galvalume panels are most often found on low-slope roofs of industrial buildings. Residential users are sometimes attracted to the bright, shiny quality of its unpainted form.
Size and Cost
Standing-seam Galvalume roofs are usually 24-gauge steel, .024 inch thick. Galvanized metal shingles, shakes and tiles are typically made of 26-gauge steel, .018 inch thick, or 28-gauge steel, .014 inch thick. Standing-seam galvanized roofs are usually 24-gauge steel, .024 inches thick, or 26 gauge. The strength of galvanized steel makes it good for areas prone to hail. Galvanized steel is less expensive than Galvalume.
Galvalume is coated with 55 percent aluminum, 43.5 percent zinc and 1.5 percent silicon, necessary to prevent the coating from becoming brittle, according to the website Steel Mills of the World. Galvalume is better than galvanized steel for roofs in coastal areas subject to salt spray. Galvanized steel designated G-90 means that it contains .90 oz. of zinc coating per square foot. G-30 and G-60 provide less protection and you should avoid them for your roof, according to the ClassicMetalRoofs.com. The G rating does not measure thickness of the metal.
Unpainted Galvalume keeps its shiny metallic finish longer than galvanized steel. After about five years, it darkens and takes on a mottled appearance. Most galvanized shingles, shakes and roofing panels are covered with a base paint, usually PVDF (polyvinylidene fluoride), a high-end paint most often used on metals. PVDF paints, marketed in the U.S. as Kynar or Hylar, adhere better to galvanized steel sheets than to Galvalume. In 20 years, PVDF paint usually wears down to 30 percent of its original thickness and exposes bare metal in 40 to 50 years. After that, you should spray metal surfaces with acrylic paint every three to five years.