I am planning to install a cork floor (glue down tiles) and have been working at preparing the floor. The subfloor is plywood over constructed joints that are approximately 24 inches on center (spacing is actually slightly erratic), but at least due to the type of joist, I have the 3-1/2 width of a 2x4 rather than just the narrow edge to screw into. I had to level the subfloor using Henry universal leveling compound (about 1/2 inch thick in the worse areas). I am now planning to screw down 1/2 inch thick plywood (fully sanded birch faced on both sides) using deck screws.

My questions are these:

How much space should I leave between sheets of plywood. Currently, the house has very low humidity due to the forced air heating system. According to the cord floor manufacturer, the space between the sheets need to be filled with putty to prevent damage to the cork tiles. How worried should I be about possible buckling of the underlayment?

2) I have laid out the sheets of underlayment so that major seams are on top of joists. The underlayment runs parallel to the joists and perpendicular to the existing subfloor). Besides screwing the underlayment into the joists (which are 24 inches on center). how many rows of screws should I have in the areas where the underlayment is being screwed only into the subfloor?

you have done a great job.make sure the screws are galvinized (coated so they wont rust) use a penny as your spacer for the 4' x 8' sheets of plywood,use floor patch(cement based) to fill the gaps and cover the screws.one screw every 6" in every dirction is about right.and then every 3" around the edge. plywood won't expand and contract. good luck