- Q.For shake roofs, can I
substitute purlins for plywood roof sheathing? Does the
plywood serve as a diaphragm on a sloped
roof?
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.