I have an old house and when I look up between the joists in the basement, I see the underside of the tongue-and-groove wood plank floor. In most of the main floor, the floor is just that – planks, still strong but aesthetically challenged on top. In the kitchen and basement, it’s been layered over with light plywood, then layers of tile, sheet lino, what-have-you. Upstairs, they layered over the entire upstairs. I believe that all areas augmented with plywood were first water-damaged. They all feel a bit spongy in places (lots of places). First question: for the areas where I have only the wood plank floor – can I lay new hardwood floor over it (the thin stuff that they say you have to put over plywood), or do I really have to put plywood down first? Second question: In the areas where there are multiple layers, I intend to pull everything off (I need to run new wiring in all those areas anyway). Can I replace all flooring materials with 1″ boards crosswise to the joists?

I am not grasping the entire question so I will respond with what I think you are asking. For the pulling up of the upper floor and wanting to use 1 inch boards. You would be better to lay down 4×8 3/4 inch osb tounge and groove sub floor, then a flooring of your choice. For the covering over the “planks” if you are referring to laminate flooring (glueless/floating floor). If the flooring boards are in good condition and you have a minimum of 3/4 inch you could go over them with the laminate. When in this situation I generally opt to lay down at least another 1/4 inch of 4×8 luan prior to installing the laminate floor. I cannot remember what the code is, but I generally try for 1 inch minimum of total flooring material. The floor will only be as good as what is under it. I hope this helps.