Underlayment and fasteners for tongue and groove roof.

Hello everyone, I am making roof repairs on my lakehome (new to me 3 years). It has tng 1x6 (high quality wood, no knots, etc. I have to replace most of the eaves. Keep in mind, I have spoken to a local builder/GC and he said to only replace the boards that are bad with 3/4 ply. I plan on putting metal and he advised also placing horizontal purlins. Now....my confusion is with the underlayment and type of nails. I have read so much to the point where I am about to just go old school with 30 lb felt and be done with it (with cap nails).

I have read to use permeable, not to use permeable, only use nonpermeable if you don't have moisture, etc. Also read don't use synthetic but do all peel and stick.....with all this it's very confusing. I did have whirlybirds with small vents around the eave but plan on converting it to a soffit to ridge vent system. My current plan is to install non permeable synthetic with cap nails but have read it will wick water through, especially with the boards.

Also, how deep ring shank should I use for the ply?

Opinions, questions, comments?

No need to use a permeable underlayment. The roof isn't designed to "breathe" at the roof layer anyway.

I would use one of the synthetic underlayments indicated for high temperature metal applications if it were my home.

You just need to use an underlayment that is specific for metal is all.

Nails should be as deep as the plywood or decking, plus at least an inch for penetration. Deeper is finer as well. You just don't want to over penetrate if you have an finish ceiling but longer is better than shorter here.

That's a relief Eric, I did already buy some synthetic underlayment that is approved for metal (was hopine i didn't have to return it). I also went ahead and got 16d ring shank nails. Though I'm confused because the shorter ones say framing and mine say decking. Is this because most people don't use 3 1/2 inch nails?

Ring shanks are great here. The decking is already attached via code at the time. These additional fasteners will just beef up the connection.