Self leveling and plywood


I've researched this extensively without a clear conclusion, so time to reach out to the people who have probably experienced it.
I have a plywood subfloor that has several uneven parts. My plan is to (1) level the worst spots with self-leveling cement; (2) add another half inch layer of flooring plywood glued and nailed to the joists. Then I will felt the remaining uneven spots as necessary, and staple hardwood flooring.
The self-leveling products do not lend themselves well to plywood I know, and it certainly doesn't seem recommended to add hardwood directly over SLC. But I'm hoping that my "sandwich" approach will work. I have previously dealt with such problems by shimming and using varying layers of felt. But this hasn't always worked very well - it's very difficult to deal with differences of more than a 16th or so. In this instance I am looking at some differences as great as 1/2". I should add that the uneveness was caused by a previous structural issue that has been fixed but that left a residual gap or two.
Any thoughts gratefully accepted.
Cub.
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cubby wrote:

Are the underlying floor joists that far out? How large are these places that are so bad and how large is the entire floor area. What is joist spacing?
If there are really 1/2" low spots you'd be advised imo to use bulk material to fill in the worst of the valleys if it's a sizable area.
There's no reason it wouldn't work although I'm not sure I wouldn't be more inclined to fix/level the joists and lay whole new 1" T&G subflooring instead; that being more so the larger the total area.
Think this is one would have to see/study to decide on actual plan of action...
--
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Here is a better idea...
Why not actually fix the structural issues by adjusting the settled floor joists... Adding more crap on the top of an unevenly settled sub-floor is only asking for more settling after you go through all the trouble of making the floor temporarily level...
Finding out why your floor has settled unevenly and solving that problem is the actual solution to your issue...
~~ Evan
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As per original post, structural issues have already been fixed. This was where an addition joined the main structure, and at the intersection (with a dividing wall between) the floor either side was probably never level. There was also some sag on one side, which like I say has been addressed. Now the dividing wall has gone, support underneath replaced and there are no residual structural issues, but there is an unevenness that I need to fix. It's about 20 feet long, varying in uneveness from nothing to 1/2" in one small patch but mostly within 1/.4"
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cubby wrote: ...

But how wide???? IOW, what's the total area w/ the problem and why after the structural repair is it still uneven and what, precisely is it that is uneven?
Too little information for real analysis...
As noted in other response and hinted at here, I'd still be thinking "repair the joists and then the subfloor will have a level starting point (at least within what normal leveling can handle). The SLC trick is primarily for poured slabs...not that it can't be used elsewhere, but I'd still want to see the situation before committing.
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Umm, perhaps the structural issues are partially repaired but not totally fixed, if they had been totally fixed the floor would have been returned to an almost level condition...
Complete the repair...
~~ Evan
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On Sat, 12 Jun 2010 20:01:48 -0700 (PDT), Evan

From what I understand, this is a transition between the original house and an addition, and there USED to be a wall there, which is now gone - so there is every chance the two floors NEVER matched up. But shimming the subfloor level WOULD be the best way to fix it. Put a 2X4 beside the floor joist and jack it up until the floor is level, then screw the 2X4 to the loist. Repeat wherever the floor is low. Differences of 1/8 inch or so rerspond very well to a floor sander.
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