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Q.I am bidding a job where the owner wants to install a new metal roof over a double layer of asphalt shingles. The roof has solid lumber rafters and plywood sheathing. Should I install 1x4 horizontal purlins over the asphalt and attach the metal roof to those? Should the purlins be pressure-treated?

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.

As for the size of the purlins, 2x4s are the best choice. Their 1 1/2-inch thickness gives the screws a greater pullout resistance, making the roof much stronger against high winds. If you use 1x4s, you’re getting only half as much screw penetration. Whatever purlin you use, it must be adequately attached to the roof deck and rafters.

It’s probably not necessary to use pressure-treated purlins except in very hot, moist environments. But because most preservatives are corrosive to metal, if you do use pressure-treated wood, you must install a layer of roofing felt between the wood and the metal.

David Keener is an engineer with Fabral, a manufacturer of metal roofing and siding based in Lancaster, Pa.