Sub-flooring for oak

Question from Dan: I want to replace carpeting with prefinished 3/4" x 3 1/4" solid oak flooring in my late sixties home that has 2x6 tongue and groove sub floor. The sub floor planks run the same direction as the intended direction of the prefinished boards and are separated by as much as 1/4" in some boards as well as differing heights of 1/8" in many places. What would you recommend to flatten this type of sub floor? Adding a sheet wood product is a problem, because adjacent tiled floors are only 7/8" above the sub floor. Thanks for your help. 

Jake's Answer: Dan, I have never seen a sub-floor of the type you describe. I assume this sub-floor runs perpendicular to the floor joists and that is the same direction that you should run your new hardwood floor. The direction and the separation of the boards do not concern me. The difference in heights does concern me. You have to figure out a way to get the boards re-nail or screwed so they are all even or you will have to install an underlayment, preferably ½” plywood. Make sure to gap the plywood at all edges to allow for some expansion and contraction. Where the new hardwood meets up with the tile floor, you will need to install a transition strip.