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kschueler 01-12-2021 01:53 PM

Bathroom remodel - leveling floor
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I am redoing a bathroom in a house that was built in 1942. There was an existing tile floor that I removed. It looks like the previous installation was done with SLC. I found a cracked Joist in the basement and have repaired it by sistering an new board. Unfortunately, at the time I did know the floor above was not flat. The floor is low in the middle by ~3/4" tapering out to 1/4 over a 4' span.

I checked the Deflecto calculator and am good for a new tile installation. I sketched up a couple different options (pdf attached).

I have had two different contractors come out to get quotes for leveling the floor (Need level for shower pan, everything else could be flat). One contractor said he would self level and then install backer board over top (Option #4). The other said he would put in wood strips, use medium set to level out then put backer board on top (Option #3). I would prefer not to use SLC so I am leaning towards Option #1, but want to make sure this is not a bad idea. Looking at doing the work myself, but want to make sure I am not doing something stupid :bonk: that will cause a problem in 10 yrs.

Thank you for any advice you have.

jadnashua 01-12-2021 04:56 PM

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.

cx 01-12-2021 05:25 PM

Welcome, Kevin. :)

What Jim said.

I would eliminate both those contractors from contention on the job. Neither appears to be at all knowledgeable of the requirements of either the ceramic tile industry or the product manufacturer's installation instructions. Not a good sign.

I'm a bit confused about what you intend to install. In one of your drawings you indicate "old tile floor" and in another you indicate "current floor slate." Both those floors now exist, or is the "current" floor the one you intend to create?

I can't find anywhere you indicate the spacing of your floor joists. If they are on 16" centers you barely meet the requirements for a ceramic tile installation, but if the slate is your intended tile you've got a good bit of work to do.

My opinion; worth price charged.

kschueler 01-12-2021 08:58 PM

Thank you for the assistance.

The current floor is the state the floor is in now. The joists are 16" on center. Just measured the span and it is actually 8'6", not 10'

So if I want the thinnest floor the best option is to strip off the 3/8" plywood and the 1" oak boards, level the oak joists and then install 3/4" plywood, Ditra, then tile.

cx 01-12-2021 09:09 PM

You understand that the joist span we're looking for is the unsupported span? Has nothing to do with the size of the room where the tile installation will take place.

For a ceramic tile installation, that would be one way.

My opinion; worth price charged.

kschueler 01-12-2021 09:16 PM

Yes, I understand. The span is 8'6" (length of joists). There are 8 of them all parallel to each other for a distance of 10'.

cx 01-12-2021 09:57 PM

OK. And I had misread state in your drawing to be slate. I see you're planning only ceramic tile.

Bottom line: You gotta remove everything down at least to the board subfloor.

If you can stand to remove the board subfloor, that would give you the opportunity to flatten or/and level (only the flattening is required) the floor at the joist tops, the very best place to do that work. If not, you might consider a reinforced mortar bed of a minimum of 1 1/4" thickness with welded wire mesh in the vertical center. That would give you the very best substrate for your tile installation and allow you to make the surface near perfectly flat and level if you want it level.

If you don't want to do the mortar bed, you must, as Jim pointed out above, install a minimum of nominal 1/2" plywood over your board subfloor before you can do anything else in regard to flattening or leveling. It would be possible to use the SLC you mentioned. That material is not really self-leveling, but it can be caused to be level and flat if you work it correctly. And you could then use that as your tile substrate or you could add a tiling membrane of some sort if you prefer. Different manufacturers have different requirements for minimum thickness and reinforcement when installing the product. And each of them is exceeding proud of his product as reflected in the pricing.

My opinion; worth price charged.

kschueler 01-15-2021 11:13 AM

Thank you!

Just wanted to thank everyone for the advice. After talking it over with my wife (the laundry room in the basement is below the bathroom). We are going to brace up the floor to cut the span in half (trying to level it a little at the same time) and then pour self leveler over the backer board. I'll let you know if I run into any issues.

jadnashua 01-15-2021 07:42 PM

If you decide to use something like screw jacks on poles, you might try doing this over several days, raising it a little bit more each day to get it where you want. Wood lignin acts sort of like a slow flowing solid...under stress, if you try to move it slowly, it flows...go too fast, and it can break.

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