Underlayment and also Steel Roof-- Steel Roof Covering 101 with Todd Miller
http://dominickqgwl954.bravesites.com/entries/general/roofing-replacement-vs-repair-work--7-elements-to-take-into-consideration
Still will certainly require a remedy for condensation, but those options are well documented and your builder will have choices for you. Metal roofings last much longer and are much less susceptible to leaks.