Rust: Steel Roofing Can Be Protected
Steel roofing can offer certain desirable attributes for some homes, and often at a lower price point. But how do you keep a ferrous metal like steel from rusting?
1. Protect cut edges
2. Buy the right quality of steel
3. Ensure that a steel roof is right for your environment
Cut Edge Protection for Steel Roofing
Over the years, we have heard all kinds of explanations for how cut edges are protected on steel roofing. Most of them have been misleading at best. First of all, it’s important to realize that there really is no difference between factory edges and edges made from cuts on the job site. Both are ultimately at risk of rust. Field-cut edges, due to the lack of precision that often exists on job sites, are at considerable risk of rust. The key to a rust-free edge is a sharp and crisp cut that exposes as little bare carbon steel as possible. Read on to find out how that steel is protected from rust.
Steel Roofing Quality Matters
The steels used in metal roofing have a corrosion-resistant metallic coating on both sides of the core carbon steel. This coating is usually an alloy of zinc and/or aluminum. Steels are categorized or graded based upon the amount of this metallic coating that is on the steel. The more of this coating that is on the steel, the better cut edge protection will be. I am a fan of G90 galvanized steel for corrosion resistance. While painted AZ50 Galvalume steel also meets industry standards, the coating on this metal still has less protective zinc in it than galvanized steel has.
When a cut is made in the steel, bare carbon is exposed but so is the corrosion-resistant metallic coating. When rainwater washes down the cut edge, zinc ions from the exposed metallic coating go into solution with hydrogen from the water and are deposited on the bare carbon steel edge. Over time, this builds up and protects that edge from rust.
Steels that have more zinc in the metallic coating, therefore, will get this protection faster and to a larger degree. Cut back on the zinc such as with alloys that are heavier on aluminum than zinc, and you’ve reduced the potential for cut edge protection.
This same protection needs to take place on scratches made to the paint or metallic coatings on the metal, around the holes created by any fasteners through the steel, and also to areas where the metal is folded too tightly, and small cracks occur in the coating – something that leads to what is called Tension Bend Staining. It is this protective nature of the zinc that prevents rust on steel roofs.
My company recently analyzed an offshore-produced steel roofing product that, according to best practices here in the USA, did not even have enough zinc for cut edge protection on an interior product, let alone an exterior product!
Steel Roofing and Corrosive Environments
In corrosive environments such as salt coasts, an additional problem occurs in that the salt acts as a catalyst that can set off iron oxide (rust) on the cut edge very quickly – before the zinc has had a chance to build up sufficiently to protect the edge. This is why I do not recommend steel roofing in the proximity of salt or brackish water coastlines. For ultimate safety, I suggest a two-mile limitation.
For situations where cut edges are going to be very vulnerable, aluminum or copper can make far better substrates than steel.
Todd Miller has spent his entire career in the metal building products manufacturing industry. He is president of Isaiah Industries, an organization recognized as one of the world’s leading metal roofing manufacturers. Todd is currently Vice President of the MRA (Metal Roofing Association) and a Past Chair of MCA (Metal Construction Association). Through his website, he strives to raise the bar on standards and practices to provide property owners with the best possible products for successful roofing projects.
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