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
Any thoughts gratefully accepted.
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
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
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...
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"
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.
Umm, perhaps the structural issues are partially repaired but not
totally fixed, if they had been totally fixed the floor would have
returned to an almost level condition...
Complete the repair...
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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