“Metal Roof and Wall Panel Components in Contact with Preservative-Treated Lumber,” a position paper approved by the Glenview, Ill.-based Metal Construction Association, addresses frequently asked questions regarding metal roofing fasteners and alkaline-copper-quaternary- (ACQ-) treated lumber. ACQ is a water-based wood preservative that protects against decay and insects. The copper in it, however, can accelerate the corrosion of zinc-plated fasteners. One of the most common causes of rusting steel fasteners is galvanic corrosion, which occurs when dissimilar metals are present, an electrical contact path between the metals exists and an electrolyte is present. All three are the case when metal roofing fasteners are installed into ACQ lumber. To minimize corrosion, use similar metals. Doing so will reduce the amount of corrosion that occurs to the least noble material. Fasteners have various coatings on them, which fall into two categories: barrier, such as Kynar paint, or sacrificial, such as zinc. Many barrier coatings will provide six times more corrosion resistance than standard zinc-plated fasteners. They protect the base metal from corrosion as long as the coating is not chipped, peeled or cracked. A sacrificial base coat often is used under the barrier topcoat. The Southern Pine Council, Kenner, La., has stated carbon- and stainless-steel fasteners are acceptable for use in ACQ-treated lumber. These fasteners also are recognized by the MCA and the Rosemont, Ill.-based National Roofing Contractors Association. For more information about metal roof and wall panel components in contact with pressure-treated lumber, visit www.metalconstruction.org/pubs/pdf/PreservativeTreatedLumber_072808.pdf.
