Get those shingles off, get a good view of your decking, make sure nothing is rotten.
If all good, cover with #30 with rag felt, mark the rafters and commence to nailing on 1x4 or 2x4 lattice.
Allows for a solid piece of meat to fasten your new metal roofing to.
I've done a couple dozen like this^^
That screwing metal to shingles is a half ass deal. Only if youre lucky will any screws hit a random rafter. Otherwise your punching into brittle 3/8" to 1/2" plywood.
About like zipping screws into a pumpkin rind.
