Resisting Rust: Metal Roofing Offers Optimum Corrosion Resistance

By Edward Thomas, Vice President and General Manager, Follansbee

Today’s modern metal roofing materials – with their innate ability to lend sleek, classic visual appeal to most any building envelope – are ideal for providing unique aesthetic appeal to most any structure. But it is their variety of inherent functional benefits – like corrosion resistance – that make metal roofing an equally practical option for structures in coastal regions.

Because structures along the shore are continuously subjected to the salty blasts and high winds of the marine environments they’re located in, specifying highly corrosion resistant exterior building products is vital to the performance of the building envelope. For these areas, metal roofing is a sensible option because of the unsurpassed corrosion and moisture resistance that it can provide, weathering the extreme rain, snow and salty blasts of any marine environment.

Metal roofing has a long history of performing well in coastal applications, including these four examples of Follansbee roofing. Above is an historic mixed-used development restoration project in Stonington, CT. (click images to enlarge)
A beach house on Chesapeake Bay.
A home in Boca Grande, FL.
A beach home on Figure Eight Island, NC.

Depending on the manufacturer, most metal roofing products normally undergo a series of tests to prove their ability to withstand extreme weather conditions. The American Society of Testing and Materials (ASTM) is one commonly recognized standard for measuring the effectiveness of building products in these circumstances.

For example, ASTM B117 Standard Practice for Operating Salt Spray (Fog) Apparatus measures the performance of metal roofing in continuous salt spray conditions. This involves testing in a controlled corrosive environment that produces relative corrosion resistance information for specimens of metals and coated metals exposed in a test chamber. ASTM B117 testing covers the apparatus, procedure and conditions required to create and maintain a salt spray (fog) test environment, to ensure accurate test results. The test is effective in that it samples to conditions that are actually more corrosive than usual “real world” exposure. This is because the test uses sodium chloride in de-ionized water, which usually lacks the moderating effects of other dissolved salts like those containing calcium and magnesium, which tend to be somewhat protective.

ASTM D4585 Standard Practice for Testing Water Resistance of Coatings Using Controlled Condensation gauges the effectiveness of metal roofing when exposed to continuous condensation. Because water can cause degradation of coatings, knowledge of how a coating resists water is helpful in predicting its service life. Failure in a condensation test may be caused by a number of factors including a deficiency in the coating itself, contamination of the substrate or inadequate surface preparation. The test is therefore useful for evaluating coatings alone or complete coating systems. Condensation tests are used for specification acceptance, quality control and research and development of coatings and substrate treatments. A coating system is considered to pass if there is no evidence of water-related failure after a specified period of time.

When properly installed, stainless steel metal roofing products coated with alloys comprised of a mixture of zinc and tin will provide the highest level of corrosion and condensation resistance available. To date, these types of materials have surpassed more than 26,000 hours of ASTM B117 and D4585 testing with no signs of red rust.

Other alloys, coated over metals like zinc, aluminum and copper, can also provide enhanced corrosion resistance, and will normally pass between 2,000 to 3,000 hours of salt spray testing.

In addition to providing optimum corrosion resistance, metal roofing is also resistant to high winds and carries a Class “A” fire rating, functioning as an all-purpose, high-performance addition to the exterior building envelope. Its variety of added sustainable features – like energy saving benefits and the ability to be fully recycled at the end of its long lifecycle – additionally make it an environmentally friendly option that is perfect for either coast.

Edward Thomas is vice president and general manager for Follansbee, the only manufacturer of pre-painted or natural terne roofing and a leading supplier of metal roofs for new and retrofit commercial, residential and historic projects. Contact Ed at (304) 527-1260 or ethomas@follansbeeroofing.com. For more information on Follasbee roofing, go to www.follansbeeroofing.com.

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