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Wavy subfloor

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!

Re: Wavy subfloor

Hard to believe that you'd be 1/4" off on some joints. I think you must be correct in that there was a ton of water absorbed by some of the plywood and not others. If possible, you may want to get under the floor to see if there was any waterproof membrane used. If you don't stop the moisture from coming in on that side, you could have the same problem later on, but with your finished flooring at risk.

You could rent a floor sander and try to grind down some of the high spots and try to level them off. For the low spots, you can get a floor filler and level them off that way too.

Sounds like you have a real job there.

Good Luck.

Blue RidgeParkway
Re: Wavy subfloor

perhaps your subfloor wasn't nailed down but stapled, either way moisture, carpet cleaning with water not extracted, or whatever it seems wavy. the hollow sounds could be that it has lifted or knots inside in the layers now broken up.

You might try finding and marking out the joists with a string line and making a small pilot hole with a drill bit and then screwing down some deck screws to reattach the floor to the joists. You may have two layers of plywood screwing them without pre-drilling a narrow pilot hole may not draw them together and to the joist and just tear up the top layer so be sure to drill the right size pilot hole for the screws.

afterwards you can use a floor patch leveling compound. you may need to "paint" the areas with a binder first.

Re: Wavy subfloor

I didn't get the definitive answer I hoped for , so I'll try again w/ more detail.

My floor is 3/4" plywood, not T&G, and there is no blocking. (Oddly, my upstairs neighbor HAS blocking). It is over a moist crawlspace that I have access to. The joints btwn sheets, on joists (2X10), are uneven up to 1/8". The height of the joints btwn sheets on the long side are off up to 1/4" because the plywood is alternately warped both up and down. The plywood is nailed down to the joists - it seems that the problem is not (much) due to lifting, but rather to swelling/separating/collapsed voids in the plywood itself. There is one area - a few square feet - where it seems that the top ply of a sheet is separated and bubbled up; I can feel a definite "spring" when I step on it. I can see that the primer on the plywood is covering the lifted edges, suggesting they were (mostly) present during construction, so I don't think the warping is due to the moisture in the crawlspace, but rather to original construction conditions and/or lousy plywood. Putting a level perpendicular to the joists a few feet away from a beam shows quite a difference in height (as if the joints were not crowned properly or consistently). Not all of the sub floor is bad.

My current thought: pour a self-leveling compound on top of the current plywood (after screwing down the bubbled-up area). The self-level compound would correct for both plywood waviness and mis-matched joist heights. Then glue and screw down to the joists a layer of 3/8" or 1/2" OSB on top of this (staggering joints in both directions relative to the original ply), to give me a total sub floor thickness of at least 1 1/8", which should be adequate for a floor of tile or concrete veneer, although the room I'm currently working on will only carry wood laminate.

One problem that I've already run up against is I can't find a self-leveling compound that is non-cementitious, that I can screw down thru. Otherwise, is there any problem with this method? Should I add blocking before I start this? If so, what height is necessary - is 2X4 or 2X6 sufficient? Why does my upstairs neighbor have blocking yet I don't? Do I need to worry about the moisture in the crawlspace? If so, what to do? Staple vapor barrier to the bottom of the joists?

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