Underlayment when replacing metal roof

My father's house was built in 1911 and has a standing-seam metal roof. Due to neglect, sections of the roof began to leak. I have band-aided the roof with Henry 209 mastic and Henry 555 silver seal paint. I am now researching replacement of that roof, possibly doing it myself. The current roof does not have a typical deck of plywood under it. There are just boards going horizontally across the trusses. There are gaps between those boards where you can see the metal roof from the attic. The gaps are typically 1/2" to 3/4" wide. I am thinking that I should put down plywood then tar paper, or something else, over the plywood, before installing the new metal roof. My questions are: 1. Should I pull the metal off and put the plywood over those boards going across the trusses, or should I remove the boards and put the plywood directly on the trusses? I am thinking I can go with 1/2" plywood if I can install over the existing boards, but maybe 3/4" if I have to rip up the boards. 2. What should I put between the plywood and the metal roof? Is tar paper OK, of should I use something else? Thanks, Chris

1/2 CDX should be fine with or without the boards depending on the rafter spacing. I prefer a synthetic underlayment underneath metal roofs as compared to felt but both will work just fine. If the skip sheathing is okay, I would not remove it and it is entirely possible (depending on the spacing) to go direct overtop of it. You don't want to skimp and especially if you are going to go back with metal but pictures are helpful in this case.