"Top layer" plywood subfloor fastened to decking

Question

I just tore out a ton of particle board subfloor that was previously used for carpeting. The entire house has decking (2x6 planks) running on a 45 degree angle over joists with 3/8" plywood over the decking, with the exception of a few rooms where I tore out the particle board so just decking there. I'm trying to figure out how to attach more 3/8" plywood to the decking where I tore out the particle board. We are installing t n g hardwood eventually.

1. Should I try to drive decking screws through plywood into the joists, or can I just glue and screw into the decking?

2. Does the plywood need to be oriented perpendicular to the joists, even with the decking under already? Or should I orient the plywood so that it will run perpendicular to the hardwood we will install (so, parallel to the joists..)

3. Should I take the time to level the plywood subfloor over the decking, or should I just wait until installing the actual hardwood tongue and groove to worry about leveling (or "flattening, as I suppose it will be)?

Answer 1

The 45 degree decking is the primary sub-floor. It's a technique that was widely used in the past, but has subsequently been replaced with simply dropping sheets of plywood down.

In answer to your questions:

1) You can glue and screw to the sub-floor.

2) In this case, it doesn't matter. The sub-floor is carrying the load diagonally. However, if there were no sub-floor, then you would want to go perpendicular. See this discussion on the subject.

3) Always - always - always level as you go. Fix the level at the soonest possible opportunity or it can be amplified by an order of magnitude.