Download some installation guides from various manufacturers. The ones I read all directly prohibited putting it down on shingles. The thermal expansion scratches the metal underneath causing corrosion. The guides I read seemed to prescribe exactly what marshmud described. That, and placing insulation board in between furring strips (2x4’s screwed to rafters) to displace volume of air that will contain water vapor and condense underneath. The guides also go to a great deal of detail on the screwing patterns around the perimeter (key to preventing uplift and peeling off from wind), and a great deal of detail on the various flashing and sealing. I think even if you are hiring it out it’s a good idea to locate and download 2 or 3 of those product brochures/installation manuals. After educating yourself you can better decide who is a quality installer.