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Unread 08-29-2022, 06:23 PM   #1
Aaron McN
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Trying to even out subfloor low spots with SLC, question about sloped floor

Hi all,

I've lurked the forum for a month or so and finally decided to ask a question now that I'm ready to go ahead with tiling our foyer.

My local tile place only gave me one option, which was self-leveling compound. My foyer is roughly 8' wide by 12' long.

Over the 12', to the best of my calculations, the floor drops 1 1/4" from the back wall to the door.

I used a level and a laser to find the spots that are not as flat as they probably need to be and the dips (green section in my photo) get to about 1/8" lower than the higher spots where I measured.

I'm wondering if self-leveling compound would be too runny and run all the way down to the door and not help me with the low spots on either side of the higher joist in the middle or would it be workable within the areas and just need some coaxing out to the lines I drew out?

Other thoughts
•It's an hold school house and we had to have somebody sister some of the joists in the foyer due to rot. It looked great to me but it's not as flat as it looks to the eye, that is for sure.
•The work done is extremely solid. Even lightly jumping up and down did not get the laser beam from the level to even jiggle slightly. I believe it's two layers of plywood. 3/4" with 1/4" on top.

Thank you so much for even just reading my post.

Aaron
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Unread 08-29-2022, 09:38 PM   #2
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Welcome, Aaron.

Let's see if I've got a correct grasp of the situation: Your floor is flat within 1/8th" over the 12-foot span, but it slopes 1 1/4" over that same span. Is that correct?

You do understand that your tiles care not a whit about the floor not being level, they care only about it being flat? The larger the tiles, the more they care.

If I'm correct in my understanding of your situation, a SLC would not be a good choice. But first, let's determine if I'm understanding what you've got.
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Unread 08-29-2022, 09:40 PM   #3
jadnashua
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Tile doesn't care about slopes, it cares about flat. You might want things level, but on an old building, that can present its own problems.

There are essentially two classes of SLC. The more common one would literally, mostly flow to the low point. A misconception about SLC...it often doesn't literally flow and level itself...it takes some assistance. A second thing, most (not all) SLC require a minimum depth over a wooden subfloor (i.e., cannot go to a feather edge).

There is a form of SLC that might work, and as a class of things is referred to as thixotropic. That means that it flows when you agitate it, but stays when you stop...IOW, it could actually be made to form a slope. Again, most of those still require some minimum depth, though, and may not work to go to a feather edge.

I guess it sort of depends on what is happening to the rooms adjacent to the foyer. If you were to level it, you'd have a potentially very significant step.

Hindsight is tough, but if you had installed your sisters such that they were all installed so you had them if one flat (not necessarily level) plane, you could have then installed the subflooring to those new sisters and ended up with a flat floor that would have been easy to tile.

The only other thing I can think of would be to install an unbonded mud bed, but that would make the step out of the foyer even worse as it needs to be a minimum of 1-1/4" thick, then the thickness of the tile.

See what others may come up with...
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Unread 08-30-2022, 05:33 AM   #4
Kman
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I'd start by removing the 1/4" plywood and see what the 3/4" looks like. You don't want the thinner plywood under a tile floor anyway.

So what kind of tile are you planning to install? And what is the longest unsupported span of the joists in the area to be tiles?
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Unread 08-30-2022, 02:26 PM   #5
Aaron McN
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Thanks everyone! I'll address each response below:

CX -
"Your floor is flat within 1/8th" over the 12-foot span, but it slopes 1 1/4" over that same span. Is that correct?" Yessir. I talked to the place I bought my tiles from and I tried to explain that I thought a cement-type patching compound may be the best for me (I was watching Sal Dibiasi on youtube which gave me this idea). The rep looked at me like I was crazy and said all they have is SLC for that type of thing. I reluctantly brought two bags home and commenced on more research. The type is TEC Level Set 200

jadnashua and Kman-
It was 3 years ago we got the subfloor fixed and I now remember that I told the guy who was helping us with some of the larger projects that we were going to put click vinyl flooring in so he did two layers of 3/4 inch plywood to bring it up close enough to the 3/4 inch hardwood floors that are in the two adjacent rooms.

The tile we got is a 10mm rectified porcelain tile.

I did calculations to the best of what I can figure out and if I were to remove one layer of the 3/4" plywood and fill the whole foyer floor with SLC, the tile would come up to roughly 1/2" under the door and only 1/3" under the weather stripping.

So, I don't know what to do. I thought that screeding some type of patch material to fill the low spots in the plywood would be my best bet, but the local store was of no help. I don't think SLC is my best option at all but I feel like I have no other option. I do know that TEC offers patching compounds though but I didn't know that until I got home and started looking up the company.

If I were billing my thoughts by the hour as to how to get this done, I'd be a very rich man...

PS. photo of my ditra is attached (not to the floor, just to this post lol). I had it all cut and ready to go.
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Unread 08-30-2022, 03:18 PM   #6
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Quote:
Originally Posted by Aaron
I believe it's two layers of plywood. 3/4" with 1/4" on top.
That's what Kevin was concerned about, Aaron. Appears you're now saying you have two layers of nominal 3/4" plywood subfloor. We really need to know which is correct. You do not want any 1/4" plywood of any kind in your subfloor package.
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Unread 08-30-2022, 03:27 PM   #7
Aaron McN
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It is 100% two layers of 3/4" plywood. I know if I kept that second layer we'd have a fairly hefty transition piece going to the hardwood as he did it for us to have next to no transition with the originally planned vinyl plank.
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