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Unread 01-12-2021, 04:56 PM   #2
jadnashua
Veteran DIYer- Schluterville Graduate

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Join Date: Jul 2004
Location: Nashua, NH
Posts: 14,285
As I understand it, none of those options would meet industry standards.

With the planks, if you want them to remain, you need at least 1/2" plywood installed on top of it.

None of the manufacturers suggest using thinset to level or flatten a floor...they all want the floor flat first. A LFT thinset is called for when the tiles themselves are not flat or are irregularly shaped (like maybe some slate), otherwise, if the tile back is flat and the floor is, too, even on a large tile, you don't need all that much thinset. While all cbu manufacturers call for thinset underneath their boards, they all also call for screwing them down. That's not an option if you have slc underneath. So, if you want cbu, that goes down first (it will bend to follow the subfloor), then you flatten it, then tile.

SLC is sort of a misnomer...it often needs some help to actually self-level. And, there are some versions that are called thixotropic (fluid when moving into position, static when you stop; i.e., it stays in place where you put it), that actually can be used to build a sloped surface.

Should you decide to cut out the planks, you could add some cleats to the sides of the joists to make everything all line up nice and flat. Then, you could install say 3/4" ply on top of that. You could then use 1/4" cbu, or a membrane like Ditra (which is thinner), and tile. That would give you the thinnest floor. Don't know what the outside area is, but it may allow less of a transition, which may be worth it to you. If you cut out the planks, you'll probably need to install some blocking for the part remaining underneath the walls and to support the edges of any new subflooring you might install.
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Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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