Metal Roofs – Aluminum vs Galvanized/Galvalume
January 26, 2017 | By Bryan Rusch | Filed under: Articles, Education, Metal Roofing, Technical
All Metal Roofs Are Not Created Equal
The benefits of metal roofing in New England cannot be disputed. Although not all metal roofs are created equal, nor are they installed in the same manner. We are going to cover a vital item that is highly significant when it comes to residential applications. That is the base metal used for the roof.
Base metal, when used in the context of construction, is the primary component used in construction material, from the sheets of roofing exposed to the elements, right down to the fasteners contained beneath the roof. When used as part of a roof assembly, the base metals can be steel, aluminum, copper, zinc,or a combination of base metals. All base metals have distinct properties which contribute to advantages and disadvantages in longevity and appearance when applied in various climate zones.
We will focus on two widely used base metals used in roofing: steel and aluminum. Steel is a hard, strong, gray or bluish-gray alloy of iron with carbon and usually other elements, used extensively as a structural and fabricating material. Aluminum is a silvery-white, soft, nonmagnetic, ductile metal. Both metals can be used for roofing. Steel can be used for structural roofing and architectural roofing. Aluminum is primarily used for architectural roofing. Architectural roofing does not require structural integrity in the construction assembly.
Galvanized and Galvalume Steel Roofing
It is important to note that base metals can be combined, coated, or layered with additional metals in a variety of bonding processes. Two popular roofing materials that use this process of binding two or more dissimilar metals are referred to as galvanized and galvalume, each of which use steel as the base metal. Iron or steel can be coated with zinc; this is known as a galvanized metal. The zinc acts as a protective layer to iron or steel to prevent rust. Galvalume is aluminum and zinc coated steel. The combination of aluminum and zinc is used once again as a protective layer to the steel. Steel exposed by itself without other protective metal coatings such as aluminum or zinc turns a reddish- or yellowish-brown flaky coating of iron oxide that is formed on iron or steel by oxidation, especially in the presence of moisture. It is what we refer to as “rust.”
Rust has been called “the great destroyer” and “the evil.” The Pentagon refers to it as “the pervasive menace.” It destroys cars, fells bridges, sinks ships, sparks house fires, and nearly brought down the Statue of Liberty. Rust costs America more than $400 billion per year—more than all other natural disasters combined. (reference: RUST: the longest war)
What Happens When the Protective Coating Wears Off?
Everyone has come across rust at some time in their life. We have all seen it before. Rust is unsightly. Shiny spokes on a bicycle wheel, or a dented corner panel on a car, all show the common enemy of rust becoming evident eventually. The question is would you want to see rust on your home’s roof in the future? Which base metal assures no rust whatsoever over the course of the roofs life? The only one of the three base metals we have looked at that is rust-proof is aluminum.
Metal working requires tools which bend, cut, and shape metal roofing components. Roof valleys and ridge caps are bent, roof edges are cut, and panels are formed and shaped depending on the particular style desired. All of this metal work by mechanics contributes to stresses on the metals and exposure of edges where a base metal may lose the protective coating, be it another metal or paint finish which is meant to keep it from rusting. This is particularly true for galvanized and galvalume roofs where the base metal steel is now exposed to the elements. Warranties for galvalume and galvanized roofing will call out a certain acceptable amount of rust from any edge that is cut within a specified period of time. Do you want to look up at a newly installed metal roof and wonder when the rust will show up, the paint finish will start peeling, and rust will begin running down your siding? I suspect not. So if that is true, you will want to choose to have an architectural aluminum roof installed, as aluminum does not rust. Aluminum, as you remember, is used to coat steel in the galvanized and galvalume products. You also do not want the fasteners holding your roof on, which may also be galvanized undergoing deterioration.
The photo above shows galvanic reaction occurring, through the paint finish no less, of a steel based roof underneath the flashing of a drip edge above. This would not happen in a Kynar resin paint finished aluminum roof.
The photo above shows the drip edge and rib bend in the galvalume standing seam roof of the same home. If you examine the photograph closely, you will see the paint finish has chipped away underneath the drip edge where the steel base metal has begun to deteriorate (rust).
Galvalume metal roof failure on commercial building
It is common for many buildings to use galvalume roofing as an initial cost saving measure. In the photo above we are beginning to see the failure of the paint finish along the seam of the bent metal. The failure is either in the paint resin, or the manufacturing process when the finish was adhered to the metal coil. Galvalume has a coating of aluminum over the steel and will likely take more time for rust to occur at the exposed locations of the panel. However, wear occurring along the roof’s drip edge where all the water and snow run off will eventually wear away the aluminum coating once the paint finish fails, and rust will ensue.
Only Aluminum Offers Long-Lasting Protection While Remaining Aesthetic
Paint finish warranties are key to understanding the expected longevity of a galvanized or galvalume metal roof. Aluminum roofing comes with paint finishes, as well, and have their own warranties. The big difference is that aluminum will oxidize/chalk if exposed to the elements, but the aluminum will not look unsightly and continue on with a rapid oxidation process (deterioration/rust), as steel will.
Be sure to visit Classic Metal Roofs website to see non-rusting roofs made with aluminum for your next architectural “metal” roof.