I own a 120 yr old row house. The bedroom floors are the original 3/4" x 2 1/2" pine directly attached to joists. The floor has been sanded and refinished twice. They are in bad shape (big gaps, splintering) and cannot be reasonably refinished again and the new flooring is already purchased.
I plan to lay new 3/4" oak hardwood flooring. What do I need for underlayment? Can I just lay the new floor perpendicular to the existing floor and parallel to the joists? Or should I support it with 3.8" plywood or OSB? Or should I just lay it diagonally?