I live in a small condo (built in 1979) and recently removed the old carpet in preparation for installing "wood" laminate (1/4", + pad). The sub floor is 3/4" plywood, non-T&G, and many of the joints are un-level (even on some of the ends where sheets meet on a joist!) up to 1/4". The surface is wavy in some areas, arching up (!) between joists. Some areas are also "hollow"-feeling, as if the sheet has a large void, or has de-laminated and the top layer has bubbled upward. I assume that these symptoms are due to some combination of the following:
1) my unit is on the 1st-floor of a four-story building and was exposed longest to the elements (during its original construction)
2) it is immediately above a moist crawlspace
3) it is part of a price-controlled, town-subsidized housing program that was cheaply and shoddily constructed
So, what to do? Cut out and replace all the sub floor? Replace it piecemeal? Or, leave it and add a new layer over the subloor (either as add'l subloor OR underlayment)?
Adding to the dilemma:
1) I was considering either ceramic tile or poured-concrete veneer for part of the unit and, while I haven't fully researched this, I assume I have to add to the sub floor, or add underlayment, to these areas and thus might want to increase thickness under the laminate flooring as well, in order to equalize net height between the areas
2) because of the housing category (price-controlled), I get no return on my investment. I want to do this job correctly, but also cheaply!