I will be installing 3/4 prefinished oak naildown flooring in our bedrooms. I have removed the carpet and 5/8 particle board. I am going to add another layer of 1/2 plywood over the existing 1/2 subfloor. I have been told to lay the new plywood in the oposite direction to the bottom layer and put it down with screws. What should be my spacing on the screws? I was also told that it wasnt necessary to screw the second layer to the joist but to just screw it to the bottom layer. Is that correct? Thanks

I'm not sure what the NWFA says about this but I installed this floor a couple years ago and structurely, it's stable. The only call back I had was they wanted a maintenance coat one year later.

Install your new ply-wood in the adjacent direction of the existing sub-strate.

Use 1 1/2" screws in a 2X2 square pattern..

And using a premium sub-floor glue such as OSI ( found at HD or Lowes ) between the two sub-floors in a herringbone pattern would be a good idea