wrote:

Here in the southern US, not leaving a place for ventilation will cost
you a roof job about 10 years before it should.

Why would you say that? Seems unlikely to me, if there's no chance of
condensation from moist house air leaking up under the roof.

Because that's how cathedral ceilings are built base on years of
construction experience...

You might say the same about shark repellent in Kansas :-)

Nick