GALVALUME: Ideal For Metal Roofing
By Michael R Slattery
Galvalume sheet is an ideal material for metal roofing in Miami because of its extraordinary outdoor corrosion resistance and resulting long life. It can be readily rollformed into a variety of panels, as well as formed and stamped into tile and shingle facsimiles. It can also be factory painted to impart color and extend durability. Most important, it can be used in modern day metal roof designs which are strong, yet light weight. And unlike conventional nonmetallic roof systems, Galvalume® sheet roofs won't crack and peel when subjected to the elements and are 100% recylable.
The product is a 55% aluminum-45% zinc alloy coated sheet steel developed by Bethlehem Steel and sold commercially under the trademark Galvalume®, starting in June 1972. Bethlehem Steel later licensed other major steel companies to produce and sell the product using its patents and technology. In North America, the Galvalume® trademark is used by Bethlehem Steel, Dofasco Inc. (Canada), National Steel Corp., U.S. Steel and Wheeling-Nisshin, Inc. In Central and South America, Industrias Monterrey (Mexico) markets the product under the trademark Zintro-Alum, while Galvak S.A. de C.V. uses the trademark Galval. Steelscape, Inc. uses the trademark ZINCALUME®. All of these products are the same generic 55% Al-Zn coated sheet steel made by the same process using the same technology.
Both Galvalume® and galvanized sheet steels are made by a "continuous hot dip" process. Coils of cold rolled steel are welded end-to-end and processed continuously on the coating line at speeds up to 600 feet per minute. These continuous sheets are first fed into a molten coating bath contained in an open top, brick-lined heated pot. The sheet then passes around a roller submerged in the coating bath and exits the bath vertically, pulling out with it an envelope of the coating material. Most modern production lines have a coating thickness gauge feed-back control, which automatically adjusts air pressure and position to ensure that a uniform coating is applied. Various finishing steps are carried out to complete the process, after which the sheet is wrapped around a reel into a coated steel coil.
Galvalume® sheet is used for two broad categories of metal roofing: nonresidential and residential. For nonresidential roofing, bare or unpainted Galvalume® sheet is used for low slope, concealed fastener standing seam roofs and for steeper slope screw-down roofs with exposed fasteners. Prepainted Galvalume® sheet is mostly used for steep slope architectural roofing and mansards to enhance the appearance of the building using color, form and texture. For residential roofs, pre-painted Galvalume® sheet is generally used, although some architects and home owners use the less expensive bare Galvalume® sheet with its bright metallic surface. Galvalume® residential roofs can be new or retrofitted over old roofs.
Galvalume® sheet has good heat and sunlight reflectivity. Its reflective surface makes it excellent for roofing in hot areas like Miami. During the summer it makes buildings cooler by efficiently reflecting away sunlight and reducing the amount of heat transmitted into the building. Likewise, it makes buildings warmer in the winter by reflecting inside heat from the underside of the roof back into the building. This good thermal reflectivity translates into energy savings by reducing cooling costs in the summer and heating costs in the winter. Tests have shown that only half as much heat from solar radiation is transmitted into a building fitted with a Galvalume® roof compared to a galvanized roof, and almost one-third less heat compared to a clay tile roof.
Galvalume® vs. Galvanized
Galvalume® sheet has been successfully used in metal roofing applications for more than 25 years; it has been evaluated in outdoor R&D tests for well over 30 years. Based on these tests, in which corrosion weight losses were measured and compared with galvanized, Galvalume® sheet is projected to outlast galvanized sheet (with an equivalent coating thickness) in various atmospheres by up to nine times. Likewise, recent inspections of 82 low slope Galvalume® roofs up to 22 years old in the eastern U.S. confirms the R&D results. These roofs are in excellent condition and are projected to last 30 to 40 years before requiring major maintenance.
Popularity Still Growing
More than 30 years after its development, Galvalume® roofing panels are used everywhere. An estimated 40 billion square feet of Galvalume® sheet steel now covers buildings in all kinds of climates and environments throughout the world. The popularity of Galvalume® metal roofing continues to grow among architects, roofing contractors, businesses and homeowners.
This RMI Article includes exerpts from an article at steelroofing.com