I'm rennovating an attic space and hope to top it off with some nice wood. But right now, I have planking as a subfloor, then another layer of planking on top of that. It's old wood, so these are just about a full inch thick each.

The question is how thick should the next layer be? I want to smooth things out because the boards are pretty rough, but I don't want to add a ton of weight, either. Should I go thinner than 1/2"? I was going to try Advantech, but I can't find it under 5/8.

This is a strange, 11'x35' room, by the way.

If you rough sand the rough boards, you wouldn't need anything. If they are too rough,you might want to sand before putting anything over them, including ply. Can't tell from here. This is where rental sanders come in very handy.