This post finally prompted me to get registered. We are renovating a 1930's home with the original metal shingle roof over firing strips. If we had used asphalt shingles, OSB or plywood decking would have been installed over the strips. But, we chose a metal roof for is long life and high solar reflectivity. It was installed over the original firing strips and satisfied the manufactures requirements. By the way this house had zero insulation and this is the main subject of my reply. I try to keep abreast of the latest techology on home constuction and energy efficiency and am applying it as part of the renovation. Section R806.4 of the 2006 IRC allows for unvented conditioned attic assemblies. In upstate SC and western NC (Zone 3), air-impermeable (closed cell) insulation can be applied directly to the underside of the roof deck. With our rough sawn 2x6 rafters, we sprayed 3" of foam at R-7/per inch plus 3" fiberglass at R-13, for a total of R-34. Also, we sprayed the dormer side walls and gable ends with 1" foam plus fiberglass. This area will eventually be finished for living space. We have not used the HVAC zone in this area and have found that it stays within 4 degrees F of the first floor. So even if it was just an attic, there would be major energy savings and the life of an asphalt roof would be longer due to a lower deck temperature. I could go on and on about all the other things, but I will spare you at this time.