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Old 03-06-2018, 05:42 PM   #1
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Really hate to ask - how much leveling can we do with thinset / concrete patch

I have done a number of projects around my houses over the years and always try to do things properly. If a floor is not level I will typically rectify it by using a self-leveling underlayment, sistering joists, etc.; however, I was out of town last week and my well intending father-in-law tried to help with a toilet that had started to run constantly. Apparently he correctly diagnosed the issue and installed a new fill valve. Unclear what happened next and my wife seems to have amnesia, but long story short the bathroom flooded and smells terrible.

The bathroom has been on the remodel to do list since we moved in as it has carpet and is dated. The carpet has a pad underneath over the concrete slab. The padding is completely soaked through so I have already torn it out and the smell is gradually abating. I intend on installing tile on the floor for now and putting off the rest of the remodel until other projects currently in process can be completed. This is just a 1/2 bath so the floor is only 10' x 4'; however, the concrete slab is not flat (or level) with various high / low spots of 1/4" - 3/8" and the low spots are relatively small (max is 8" x 3"). I am planning on using relatively large tiles (12 x 24), but open to suggestions.

Because I typically try to remediate the root cause, I have not had experience trying to use thinset to get the substrate suitable to install tile. My current plan is for unmodified thinset over the concrete slab, a layer of ditra, versabond (or do I need to use a thinset that is unmodified vs. lightly modified), and tile. Is leveling 1/4" - 3/8" using thinset even feasible? If not, could I use a concrete patch with some sort of adhesive? I would prefer not to use a self-leveling concrete on such a small area, but could if necessary.

Any thoughts are greatly appreciated.

Last edited by tomormatt; 03-06-2018 at 06:11 PM.
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Old 03-06-2018, 06:22 PM   #2
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Depends on how bad it really is. A small crown that's 3/8" high can be ground down easy enough. A 3/8" dip is really more than what thinset mortar is made to fill.

If it's just bad all over, I might try a self-leveling compound over the whole floor. You'll really want the floor as flat as possible when installing tile that large.

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Old 03-06-2018, 06:33 PM   #3
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Believe it or not SLC is perfect for what you need to accomplish. With a small area, that floor will be glass flat with probably one pour.

Why the Ditra?
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Old 03-07-2018, 12:12 AM   #4
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SLC it is. I am not opposed to it, but my parents are coming in a couple days and I was trying to accelerate the project so hoping to save time (i.e. curing) by doing it one step.

I may be off base on this and maybe it is unnecessary, but I have always used Ditra between concrete and tile primarily as a decoupling membrane. Our house is hopefully settled (30+ years old), but we do get a few shakes from time to time.
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Old 03-08-2018, 08:33 PM   #5
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Sounds like you've used slc before.

Ditra can't hurt, but may not help.

Thinset isn't designed for floor leveling and when installed thicker than design, it tends to shrink and crack creating other problems.

Once you've got the floor uncovered, it may need to be cleaned up. Water must be able to be absorbed. Sometimes, it will have some crud on it (sealers, adhesives from the carpeting, etc.) that can affect absorbtion, and thus become a bond breaker.

If the peaks aren't too bad, as said, you can grind them down. SLC over a slab can be applied to a feather edge, but it won't do that on its own...it needs help. Think pancake batter...it doesn't continue to flow and fill the pan, it has a meniscus or edge.

You need to asses the slab for cracks...it they're wider than about 1/8" or are at different levels, tile may not be in your future.
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Old 03-09-2018, 06:23 AM   #6
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There are levelers out there that will allow you to set your tile in as little as an hour after you pour them. Ours is Ultraplan Quicktraffic. Remember that you'll need to prime the concrete first. And of course we'd recommend a mortar designed for large and heavy tiles.

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Old 03-09-2018, 08:32 PM   #7
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Thanks for the input. Have used SLC before and am going to use it now. The floor needed some clean up for sure. Not as bad as the last room I did on this level that had vinyl adhesive that took lots of work with a steamer to get off, but the carpet had glue under the nail strips.

Got wiped out by a flu bag and my parents are not coming next week now (combination of not wanting to get my flu and it being crappy weather in Southern California all week) so I have some time. I had thought about trying to do thinset in thin layers to avoid the shrinking / cracking issue, but no reason not to do it properly now that I have some time.
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