I'm remodelling our upstairs bathroom - this includes installing bamboo flooring. The floor joists are spaced at 19" - 20" intervals. Over these are nailed 3/4" planks that were a floor for carpeting. Some nails have popped up but can be put back into place by hammering them. I do plan to add 2 1/2" self-tapping flat-head wood screws that counter-sink themselves to the planks such that I have a relatively flat surface on which to overlay with a layer of plywood. The current planked subfloor appears to be in relatively good shape.
My Question: What is the minimum plywood thickness I should be using? Also, please specify the type of plywood I should be using.