A. David Keener responds: To answer your first question: Purlins are always a good idea over asphalt shingles, for a couple of reasons. They allow you to create a flat roof plane, and ensure that you have solid wood to screw into where you need it. Also, asphalt shingles can be corrosive to metal roofs, so it’s a good idea to separate the two materials.
Make sure you find a roof panel that is designed for use over purlins; some panels have to be installed directly over a plywood deck. Space the purlins according to the manufacturer’s recommendations.
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