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Unread 02-07-2022, 11:34 AM   #1
Tim in Delaware
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LFT Thinset as floor patch?

I have a plywood subfloor that needs major flattening (but not leveling, so I can’t use SLC). The floor is quite solid, installed on I-Joists on 16” centers and has no soft or spongy areas that I can detect. I could go into detail about why if anyone’s interested, but basically I have a question about floor patch. I’ve read several places not to use thinset as a floor leveling compound, but the rationale seems to be because “thinset is meant to go on thin, and floor patch is designed to patch floors.” However, the various large-format tile thinsets say they can go on from 3/32” to ¾”, which is perfect for my needs. The various floor patching cements brag about how quickly they set up – like 10 minutes or less – and I really can’t deal with that fast a working time. I’ve worked a lot with thinset, and it seems very forgiving, easy to trowel, with a nice long working time, and seems to stick tenaciously to plywood.

So, can I use LFT to fill in large depressions in a subfloor? Or is there a floor patch product that would meet my needs?

Full disclosure: while I’ve benefitted greatly from this forum’s advice for many tile projects, I plan to install “luxury” vinyl plank over this floor. FWIW, the Coretec stuff that I want to use is 9/16” thick with a cork base, is 9” wide, and comes in random lengths of about 30, 60, and 90”. So, it needs reasonable flatness for it to float properly.

Thanks for any advice (except “use tile instead of that plastic crap!”)
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Unread 02-07-2022, 12:10 PM   #2
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Tim, you can still use the self leveler on your plywood. The stuff really isn't all that self leveling, it still needs to be coaxed.

The challenges in using a thinset mortar is at least 2 fold; because it is so sticky it is difficult to pull a straight edge over it, and the thicker it is the more it shrinks.
If I recall correctly my memory is excellent, but my ability to access it is intermittent.
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Unread 02-07-2022, 07:12 PM   #3
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Are you just trying fill depressions? Ardex(Henry) feather finish would be my first choice if you didn't have very many, if you have a bunch then I would use planipatch
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Unread 02-08-2022, 09:02 AM   #4
Tim in Delaware
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Thanks for the replies. I have some Feather Finish ($20 for a 7 lb. bag!), but yes, I have a lot to fill. Ideally I'd like to screed it to make a gradual slope over about a 12 foot distance. That's why all these fast-setting things scare me. Would Planipatch screed any better than LFT (i.e. less sticky as Dan referred to?)

Can Dan or anyone expand on how to "coax" SLC? I've done self-leveling exactly once - in a small bathroom - and it was challenging to get it flat, so I know what you mean when you say it's not really self-leveling. I've also read dozens of horror stories about DIYers and SLC (just Google "how to remove SLC"). If I work quickly with the SLC, can I screed this to achieve a gradual slope? I have a friend who is lending me a couple of leveling rakes for the SLC - is that how I coax it to a gentle slope?

I confess I'd be happy to pay a pro to do this leveling - I'm 65, and 50lb bags are heavier than they once were - but that's not easy to find, unless I want to pay them to install the flooring as well, which adds a lot of cost, and I really want to do the flooring myself. Any tips on how to find a pro to handle this would be great as well.
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Unread 02-08-2022, 03:48 PM   #5
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Tim, I've used thinset and SLC to fill low areas and I always use a straight edge to pull over and cut the mud. Mark the floor so you know where it needs to be filled. As you spread the mud, force it into the wood, then build it up a little extra so you'll have mud to screed off.

The RS - SLC will set faster and I never use thinset over 1/4 inch thick.

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