My home is approx 26 years old (I have only owned it 2 years) and the sub-floor used was kind of subpar in my opinion. I believe it is a 1/2" OSB. It is very creaky and has developed some waviness. Most of the first floor was tiled and the builder applied the tile mortar right to the thin OSB. As you can imagine, 26 years later, we have a lot of cracked, broken and loose tiles. Anyway, we will soon be tearing out all the tile, to replace it with a bamboo product. In addition to improving the look, I want to make sure we have a floor that feels and sounds solid. My question for the board members is, would I be better off tearing up the subfloor and laying down a fresh new layer of 3/4" ply, or just adding a thinner layer of ply (1/2") on top of the existing subfloor? Doubling up on top of the existing subfloor would certainly be a lot easier than trying to cut the old plywood out (around walls, doorways, etc), but I am concerned that it would flex and conform to the imperfections of the original floor, and make our finished bamboo floor wavy again. And for clarification, I am not talking about a significant wave. Where the floor was tiled, the subfloor seems to have a little bit of sag between the floor joists. The tile has not yet been removed, so I cannot measure the sag but it would be micros of an inch. Just enough to see that it is not perfectly flat when the light hits it the right way.