Underlayment and also Steel Roofing-- Steel Roofing 101 with Todd Miller
http://cruzovee627.jigsy.com/entries/general/owens-corning-tiles-duration-how-much-time-do-they-last-
Overlap the tiles the same amount you would if you were nailing them to the roof without the air vent, that is, just sufficient to cover the nails. Get rid of all the roof shingles caps from the roofing ridge,