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Q.I am considering installing steel roofing over 2x4 pressure-treated purlins. Can the chemicals used to treat lumber attack steel roofing?

A.Paul Fisette responds: I would not recommend using pressure-treated purlins under steel roofing without taking special precautions. A metal can corrode when minuscule amounts of electricity travel through an electrolyte connecting dissimilar metals. Water is an electrolyte. Since condensation is very likely to collect on the underside of metal roofing, you have an electrical conduit waiting to be plugged in.

Most pressure-treated wood is treated with chromated copper arsenate. The metal present in the treatment chemical (copper) is dissimilar to the metal roofing (steel). I would be concerned that galvanic corrosion could degrade the metal roofing or fasteners.

Galvanic corrosion occurs when electrons move away from an anode toward a cathode. This electron transfer causes the anode to degrade, while the cathode stays intact. If you remove one of the three components (the anode, cathode or electrolyte), you can prevent corrosion.

The easiest way to avoid this potential problem is to use untreated purlins. If you insist on using pressure-treated purlins — for example, for termite protection — you could paint the purlins or the back of the roofing to break the electrical connection. Perhaps installing a plastic spacer or a strip of self-adhering roofing membrane between the metal and pressure-treated purlins would be a more practical way to separate the dissimilar metals.

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