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.