On Tue, 30 Oct 2007 15:25:27 GMT, Anthony
wrote:

I would highly recommend lathing the roof. With no air moving between the
tin and shingles, moisture will build up and creep behind the shingles. 1"
x lathing should work.

How do you (and the poster from Oregon) propose to have air movement
under the metal roof and above the asphalt shingles without creating
bat heaven there? The guys who installed my (similar to R-Panel) roof
left out the sealers on the part under the solar collectors, and the
bats moved in and multiplied. I like having bats around, but after a
few years the smell was unbearable.

I ended up having to disconnect and remove the solar collectors,
remove a large area of roofing, clean up the mess, and re-assemble
everything. You can believe I sealed and caulked every possible
opening, no matter how tiny. It was amazing to see how small a space a
bat can squeeze through.

Granted, my metal is not installed over a layer of asphalt, or any
other moisture barrier. I wouldn't expect the tab shingles to be a
problem, but there is probably a relatively continuous layer of
roofing paper under them that might be enough of a barrier to cause
condensation. If it was my roof, I'd strip off the asphalt before
applying metal. Solves the moisture issue, lets you seal the metal
tightly against living things, and avoids the smell of roasting
asphalt oozing into your house when the sun shines on the metal.

Loren