I am putting 3/4 BC over my 5/8 CDX Subfloor. I am installing 3/4 Solid Oak Bruce T&G. Do I nail/screw the 3/4 Underlayment to ONLY the subfloor or to the joists. I have done many ceramic tile floors but this is my first Hardwood Floor.

I gotta ask WHY first ? FYI, 5/8" plywood subfloor is adequate to install a 3/4" solid floor if you lay it perpendicular to the direction of the floor joists. So, are you intentionally raising up the height of the flooring? If so, then attach the 3/4" plywood with glue and either screws or ring shanked nails installed in a grid 12" OC.

I dont want to just go with the bare minimum. I also dont want to have a step down when I put my ceramic in the other rooms. Screw it every 12"... Thanks. I will get on it!