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.
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