TT roof questiion, any caprenters here?

In my rig the roof sheet was slightly above the rafters in certain locations. Fiberglass batt had been inserted in those areas, between the rafter top surface and the roof sheet. I can only think this was done to maybe eliminate squeaks or to afford a thermal break. I wouldn't reef down on the screws more than necessary. If you have a 1/4" metal piece, a 3/8" wood sheet, and a 2 1/2" screw, you are getting nearly 13/4" of thread into the rafter, considering the gap. That should be plenty if you are using enough screws and are also using an adhesive sealant underneath the metal.