I just replaced to house roofs. Put steel on both. Put the steel right on the asphalt shingels. No worry of condensation cause there is no air space.
You will never catch me putting asphalt on a building again. One roof was 16 years old and the other 11 and the shingles were junk and flying off with the wind.
Our barn was tinned with nails in 1954 and still looks great. Screws will hold tighter.