A.In my view, purlins are the best choice for a shake roof. Shakes last much longer when installed on purlins or skip sheathing, because they can dry from the back as well as from the top. Plywood is used more often these days because it is faster to install.
Plywood roof sheathing on a sloped roof (particularly on low-slope roofs) can provide some diaphragm action if the roof framing is rigidly attached to the wall plates. However, since this is seldom the case, most light-frame structures are designed to be strong enough without relying on a roof diaphragm.
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