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.

EDIT: BTW depending on how your house is built, finding the rafter centerline at the ridge and eave as you progress across the roof length may not be trivial. The one roof I did was very time consuming because it was not laid out on 16” centers and varied from ridge to eave. I had to probe with screws to establish each rafter and snap a chalk line for each rafter. A professional no doubt knows more tricks than me, but I could see how someone competing on jobs would avoid doing that if possible and the customer didn’t make them.