insulation

I recently purchased a tiny house (30x12). The walls are currently insulated and electric has been run, otherwise empty shell. It currently has a shingle roof which I plan to switch to metal within the next year or so. The roof pitch is kind of barn style where it has an initially steep pitch and then a really low pitch to the center of the roof.

Questions: I am trying to decide if I can use rigid foam directly under the roof decking, 4 inches thick? Can I put the metal roofing directly over the shingles?

Thanks.

Yes, you can use foam. It will require code covering for flame resistance, but it can be used.

You can put the metal right over the shingles with an appropriate slip layer to protect the backside of the metal.