Hi, I have a 1925 bungalow, and I gutted my bathroom, and pulled out some decimated tongue and groove flooring which was under tar paper which was under 3 layers of linoleum. I'm left with some 8" wide boards (5/8" thick) that are on a 45 degree angle across the joists. (The floor joists are 16" on center). These boards are under the flooring and walls of the entire first floor. These 8" boards are 3/4" below the fir flooring of the hallway.

I want to install tile, so my question is about underlayment. Do I need plywood or can these boards serve that purpose? If I need to put plywood on top, how thick? Cement board--how thick?

Also, I am putting in a clawfoot tub. Should I put the underlayment down first, drill holes for plumbing later, or make holes before putting down the underlayment?