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Old 01-10-2018, 05:04 PM   #1
ONC_ben
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Planning 100sf kitchen floor - Underlayment Prep

Hello all, have enjoyed some great tips from this forum before my first floor tile installation in a tiny bathroom, but now I am facing a bigger floor in a more visible room, the Kitchen.

I would like to have option of using large format tiles if i can get my underlayment up to spec for them. If I cannot do that, I will go with 12 inch tiles.

Kitchen is 10' x 10'

Floor joists are 2' x 10' (nominal), 16' on center, with unsupported span of 11' 1". Joists are Southern Yellow Pine No. 2.

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Deflecto tells me = L/644 , good for ceramic tile, insufficient for natural stone.

Southern Forest Products Association span tables tell me max unsupported span meeting L/360, 30 psf live load, 10 psf dead load is 15' 8"

House has been raised after hurricane flooding, and joists are easily accessible.

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Subflooring is tongue and groove 3/4" ply.

Floor is not level, and I won't really be able to check for flatness until I empty the room

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House is in eastern North Carolina.

I *THINK* that about covers the info you guys always ask new posters, so now here is my first question as I plan the install: (I bet it won't be the last)

I will sand the subflooring to clean up all the crp that has gotten on it from kitchen use, and sand down any extreme ridges at seams, but I have learned from this forum that the issue of "flatness" must be determined after I have installed the 1/4" Cement Backer Units (Durock).

Based on my experience tiling the little bathroom, I am almost certain the surface of the durock underlayment will NOT be as flat as necessary for 16"+ tile formats.

I assume the only way to get sufficient flatness will be to use either self leveling compound or "feathering" compound on top of CBU. I have not looked into the feathering compounds, because at first glance it requires skill and experience I don't have.

I have seen a lot of YouTube DIY and pros showing how they apply SLC.

Since many of these, including "pro" contractors, show how to pour SLC on plywood with no lathe installed, which I know is wrong, I am hesitant to follow their instructions on how to spread the SLC, which they apply only to low spots, spreading out to zero thickness at edge of low spots.

The packaging of the TEC Skill Set SLC they sell at the Borg says that it "can be applied from a 1/16" to 1" inch depth in a single application."

I read this to mean that it must have a minimum depth of 1/16" inch... which I suppose to mean it cannot be poured into only the low areas and "feathered" at the border of the low spot, as I see many pros doing on YouTube, since this would result in a skim-coat area that is not 1/16" deep.

Does this mean that I must set up a form around the entire 100 sf floor, and pour in SLC until there is at least 1/16 inch depth over the *highest* place on the floor?

Am I reading too much into this? Is it acceptable to feather SLC from low spots into the flat areas of the CBU? If not, then I'm gonna need a lot more $LC, exceeding current budget for the installation.

Please, please please tell me it is OK to fill spots and feather it out.. or some other solution where I don't need 100 sf coverage with SLC at minimum 1/16" depth.. but only if that is true.

Thanks in advance.

-Ben
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Old 01-10-2018, 08:58 PM   #2
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Ben,

Look into Ardex Liquid Backer Board. No metal lathe needed.
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Old 01-10-2018, 09:24 PM   #3
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Changing my mind about SLC ..

Thanks Paul.. that stuff looks great, might have been an option before I purchased 8 sheets of durock, all the screws, tape and thinset

As it is I need to work with what I have already got.

I wasn't worried about lathe, because from all I understand it is not a good idea to put SLC under CBUs.

I just need to figure out how to get the surface flat above the CBUs I will install.


The more I look into SLC, the less I believe it is the proper product for this application.

I had been thinking it would be easy way to avoid trowelling & screeding mud. But now I'm not sure it is more work and expense than I need.

I am now willing to do trowelling, screeding, and maybe some grinding down of mud to create flat substrate.

I of course will not know what kind of variations from flat I have to deal with until I install the CBUs and take a straight-edge to them. Assuming I will have gaps between 1/8 inch and 1/4 inch in places, what type of mud should I use to flatten the surface of Durock before moving on to the tiling stage?

I don't entirely understand why it is so important to level the surface before applying large format tile mortar with a 1/2" x 1/2" notched trowel... I would think the troweled mortar would be enough to take care of these differences, but John Bridge says it is necessary and that is enough for me.

So I suppose the mortar I use to level out the surface of the installed CBUs should be something other than thinset mortar?? Durock does not say anything about this.

Time to read up on the "feathering" compounds..
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Old 01-10-2018, 11:14 PM   #4
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answering my own questions :)

I have not found anything on YouTube or my product manufacturer sites about flattening a CBU installation for floor tile...

But I have stumbled across a bunch of stuff on prepping CBU walls for tiling, including flattening over the CBU.

Seems the thing to do is just use a long straight-edge to identify dips, then trowel in an appropriate depth of thinset mortar with appropriate sized notched trowel, screed that off, let it set, then fill in the ridges with a flat trowel, finishing trowel, and possibly a finishing float. Let that set, check again for flatness, repeat until flat.

Sort of like (well, a *lot* like) flattening a drywall installation.

I can do that. This is not the first time I started with the most complex approach to a problem, only to later discover the simplest.

Biggest cost on this will be the time required to let the mortar set between coats. But at my rates, that = $0 :P

If I am wrong about all this, please feel free to tell me how wrong I am
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Old 01-10-2018, 11:43 PM   #5
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The proper material for that method is not thinset, but a flashing compound or SLC
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Old 01-14-2018, 06:45 AM   #6
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clearly SLC would not work for wall prep for tile... so if prepping flat wall for tile, pros use thinset mortar, seems like it would be good for floor.

as long as result = flat, why not use the mortar I have on-hand?

What is benefit of "flashing compound" versus other mortar?
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Old 01-14-2018, 02:46 PM   #7
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All of the "flashing compound" I have looked at emphasize that it is pliable, rubberized, elastomeric, flexible, etc.

That hardly seems the right stuff to provide support to large format ceramic tile flooring. I would think that is exactly the kind of substrate that would lead to cracked and failed tiles.
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Old 01-15-2018, 06:03 PM   #8
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Took me a couple days of scrubbing the subfloor with a TSP/bleach solution once I emptied the kitchen to get rid of all the years of spilled spaghetti sauce, etc ... a bit longer than I planned on (surprise, not)

Tomorrow I will sand out spilled paint & high subfloor ridges, add decking screws where needed, block of and fill moribund pipe and electrical holes, find and pull all the old staples and nails that escaped my tear-out, etc, etc.

Then it will be time to lay down thinset and CBU. Then I can check for flatness, and at that point have to have final plan for flattening.

Have looked and looked at "flashing compounds," which I understand are used to flash roof protrusions and other openings to exterior... They all seem to be rubbery coatings not designed for resisting compression, and I just can't see how that would be advisable in this application... I suspect I am missing something.

As a thought experiment, I am unable to come up with any reason thinset would not work to flatten small low spots... after all Durock specifies to use latex modified thin set to embed tape along the edge seams, and edge seams are built-in low spots.

So if I will definitely be flattening the CBU edge seams with latex modified thinset, and feathering that out to zero thickness, why would it not be appropriate for other low spots?


Again, my problem with the SLCs is most of them require filling floor to minimum depth over the highest spot. That means

a) 100 s.f. coverage, and

b) since floor is not level, would end up using a lot of product and multiple pours to meet the minimum depth at the highest end of the floor.

Ardex liquid backer board looks terrific, but again, I would end up with a thick pour, since floor is out of level, and apparently an expensive one. Also, nearest dealer is 3 hour drive away (and another 3 hours back)

Since all I need is "flat" not "level," the leveling products all seem to conflict with my goal... using them in spots would make little level areas that are not in plane with the rest of the floor, necessarily meaning the floor would be neither level nor flat, unless I fill the entire 100 s.f. with the stuff.

This is small kitchen in a small cottage home. It just feels like installing ardex would be overkill for this project.

I tore out the original tile so long ago, I don't remember what size tiles were installed by builder, but pretty sure they were 15" + ... and I DO remember there were a number of cracked corners.

If I cannot find a reasonably inexpensive, simple and dependable method of flattening floor sufficient for LFT, I will reluctantly go with 12 inch tiles, and deal with hassle of sweeping and mopping all those dang grout lines.
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Old 01-15-2018, 06:12 PM   #9
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Why not check for flatness now, before you put down the backerboard. First, especially since the structure is accessible, you might be able to make any big corrections there, perhaps even to the point of not needing SLC. If the subfloor is flat, adding the backer shouldn't lose flat. Since you recognize that level isn't the goal, you may well achieve flat with structure.
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Old 01-16-2018, 06:19 AM   #10
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Ripping out existing subflooring (which is sound), and messing with planing down joists, re-installing subflooring (which goes under bottom plates of structural and partition walls) is WAY beyond my commitment to large format tile.

12 inch tiles will work.. It is just that I would like to update style to at least the late 1990's with larger tiles.

Demo and remodelling after hurricane has already far exceeded insurance payments, so if I cannot do it for reasonable cost, and not more than two more weeks without kitchen, then it isn't in the cards.

John Bridge says level after CBU, and that makes sense to me. I don't want to rebuild the joists and subfloor for a 10 x 10 kitchen in a cottage.

I just cant help thinking there is a way to do this without further demo and structural engineering.

Won't a little more thin-set and some screeding do the trick? I don't understand why it needs to be so complicated
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Old 01-16-2018, 06:21 AM   #11
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Correction: *John Bridge says flatten after CBU*
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Old 01-16-2018, 05:46 PM   #12
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I don't know for sure that the consensus would be flatten after CBU... you'd like to lay the tile on the CBU, otherwise why did you even use it? IF I am not mistaken, a SLC can go right over the plywood. I am failing to see what the CBU gives you if you put it down and then cover it with more goop. I wait for more expert opinions!

I'd like to see a reference to flatten after CBU, it just doesn't make sense!
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Old 01-17-2018, 03:02 AM   #13
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I did not say that it is consensus to flatten after installing CBU, only that John Bridge says to flatten after installing CBU.

The Ceramic Tile Education Foundation (CTEF) says that flattening should be done before installing CBU
https://www.ceramictilefoundation.or...yment-for-tile

But here on this forum John Bridge and others point out that installing CBU over levelling compound means you will end up driving screws/nails in a grid at least every 8 inches in both x and y directions, and every 6 inches along CBU panel edges *after* the leveling compound has cured.

My gut agrees that doing that is bound to, at the very least, compromise or, and I think this more likely, completely destroy the stability of the leveling layer.

to me, the point of the CBU is that it is quick and easy, and most of the CBU will be flat after installation, but i'm pretty sure I will have to correct some isolated low spots to reach specs for large format tile.

I *could* learn how to lay down a mud substrate over the 100 s.f. area to be tiled.. and adopt a new career. But that is not my goal. I just want some tiles in the kitchen

But because I am almost guaranteed to have *some* low areas, I need to plan on how to deal with these.

I had to take another day off from the project today, so maybe in two days I will know how much variation I have after I lay the CBU.
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Old 01-17-2018, 03:13 AM   #14
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... and I agree that it makes no sense to cover the entire surface of the CBUs with leveling compound.

After much research I think leveling compound is the wrong thing for this problem, since I have no desire to achieve actual level, I just want flat, and in this case they conflict because subflooring is not level.

But if I did think leveling compound was the way to go, you are right, no need for CBU.

Limited skills +limited experience + limited time + limited budget + materials already purchased = CBU that will need "spot" flattening.
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Old 01-18-2018, 04:35 AM   #15
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sanded spilled paint and drywall compound off the subflooring, and sanded down some high ridges at seams today, and spot-checked for variance with short level as I went along... its looking pretty flat so far (fingers crossed).

Got some DAP Bondex floor leveler to fill some "divots" in the subflooring from the demo.

Have some bondo to fill moribund plumbing and electrical holes. Just need to put in some 3/4 inch ply piecies under the holes tomorrow (down in the crawl space) and do the filling from up top.

mighty cold, sleeting and snowing, maybe not tomorrow.
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