All Steel Roofing has a protective barrier on both sides of the sheet called a metallic coating which protects against rusting. This is underneath and separate from any paint which imparts color to the product. There are two types of metallic coatings used:
Galvanized: This is 100% Zinc in various thicknesses depending upon the product usage.
Galvalume or Zincalume: A mixture of aluminum and zinc. (55 percent by volume aluminum)
Metallic coatings “sacrifice” themselves to protect the iron (Fe) in steel from oxidation when exposed to the air and moisture is present. Zinc is a more “active” metal than iron so it oxidizes first and forms a protective barrier — zinc oxide, before the iron (Fe) in the steel can become Ferric Oxide (rust). When zinc is combined with aluminum to form Galvalume there is even more protection in most circumstances.
Different Levels of Protection
There are differing amounts of metallic coating used on sheet steel depending upon its final product application. For agricultural applications, galvanizing levels called G-40 or G-60 may be used quite successfully. For houses, the MRA recommends a minimum of G-90 be used. This is in accordance with the Guidelines for Residential Metal Roofing published by the Metal Construction Association. The greater the number, the longer the protection against rust will last.
The numbers G-40; G-60; and G-90 refer to the ounces of zinc per 100 square feet of sheet steel coated (top and bottom). G-90 will have 90 ounces of Zinc- 45 ounces on each side per 100 square feet of roofing sheet steel.
Galvalume has a designation AZ-50 or AZ-55. These are equivalent levels to G-90 galvanized product. However, in many years of exposure testing Galvalume has proven to be up to three times more effective in preventing rust from appearing on the sheet steel.
Aluminum sheet does not require a separate metallic coating barrier since aluminum oxide, when it forms is not noticeable in most cases.