Steel is an excellent material choice for metal roofing and siding. It offers an outstanding blend of performance, affordability and end-use flexibility. While corrosion can be a performance concern with raw steel, roofing and siding products are coated with a high-performance metallic coating to provide long-lasting corrosion protection. This coating is applied in a controlled, rapid process, which creates a tightly bonded and consistent finish. Metallic-coated material can be used in building design bare or painted. Paint is applied following the metallic coating process.
(Figure 1 – The application of the metallic coating)
How to effectively manage corrosion performance on steel roof and siding products
Common Metallic Coating Variants
The two common coating types are galvanized and aluminum-zinc, also known by the trade name Galvalume or ZINCALUME. Galvanized coatings consist entirely of zinc, whereas aluminum-zinc coatings consist of a specialty mix of 55% aluminum.
As a protective element, zinc provides a sacrificial barrier for the steel underneath. This ensures the coating will corrode preferentially to the base steel. However, as it is sacrificial in nature, it is consumed gradually over time. Aluminum by comparison, provides a protective barrier which prevents the spread of corrosion. Aluminum-zinc coatings combine the benefit of preferential corrosion, while slowing its spread, to provide longer-lasting corrosion protection and superior cut-edge and edge-creep corrosion resistance.
Metallic Coating and Specification
Metallic coating specifications will vary based on the type of coating and the amount of coating applied, called coating weight. The thickness of the metallic coating will directly impact the longevity of the corrosion resistance. For galvanized material in metal roofing and siding applications, optimal specification thicknesses are G60 and G90. The number denotes the coating of zinc per square foot. For example, G90 reflects 0.9 ounces per square foot. For galvanized material, corrosion resistance is typically directly proportional to the amount of coating, in that a G90 product will be 1.5 times more resistant compared to a G60 product.
The appropriate aluminum-zinc (Galvalume) coating weight for metal roofing and siding applications is AZ50. AZ50 refers to 0.5 ounces of the coating per square foot. This offers the optimum mix of performance, warrantability, and formability (ability to be formed into a panel). Due to the differences in performance and density between the two coating types, the numerical terminology does not represent an equivalency, i.e., AZ50 is not the same as G50.
Warranty Considerations
Galvanized products typically do not carry a corrosion warranty. Licensed aluminum-zinc products (Galvalume or ZINCALUME) will carry a corrosion warranty of 20 to 25 years, often referred to as a substrate warranty. it is important to note that the substrate warranty is independent of any additional finish warranty provided by the paint system.
Common Coating Weight Variance
Some specifiers may consider lowering the metallic coating thickness to save on product cost. This is not recommended. All licensed aluminum-zinc products must be produced at certain coating weights to carry a product warranty. The aluminum-zinc coating in thinner variants (such as AZ35) can provide pathways for corrosion to spread to the base steel.
Some specialty applications, such as those within proximity of airborne chemicals or salt spray, may require alternative coating thicknesses. Thicker coating weights can impact other factors such as formability and cost. In these situations, specifiers should consult their preferred product manufacturer before selecting a coating. For these environments, a high-build paint system with a thicker primer may be a more suitable alternative as metallic coating production lines offer limited flexibility to produce output on a smaller, project-specific scale.
Other Design Considerations
Aluminum-zinc coatings offer superior corrosion resistance in most applications. However, when used in conjunction with a premium paint system (such as a PVDF system), both coatings will provide long-lasting corrosion protection in most environments. There are some environments in which aluminum-zinc is not a suitable coating, namely those that are highly alkaline such as animal confinement settings.
Other important tips to avoid premature corrosion with metallic coated products is to ensure the surface does not trap standing water, such as roof slopes below 1/4:12. Additionally, designers should ensure these products are not installed in direct contact with dissimilar metals in corrosive environments, or galvanic corrosion can be induced. The most common example of dissimilar-metal induced corrosion is the installation of noble metals, such as copper or stainless steel fasteners and accessories, in direct metal to metal contact with a metallic-coated steel in a severe marine environment.
Richard King is the product manager at Steelscape, Kalama, Wash. To learn more, visit www.steelscape.com.