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Unread 10-09-2019, 03:32 PM   #1
psharkauburn
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Kitchen Floor issues - flattening for LFT

First post, please bear with me, going to give as much info as I can think of so sorry for the length. Hoping a couple experts can weigh in and help me through some options I'm thinking about.

Project: I have a 400 sqft area of kitchen/living room that I am preparing to put a large format 12" x 24" porcelain tile on that previously was a combination of linoleum and carpet.

Background and work done: Subfloor was approx 1-1/8 inches, floor joists 16" OC, and I planned on installing 1/4" fiber cement tile backer board (eg. hardibacker) per manufacturers instructions (thinset + screws) to the subfloor as my substrate to install tile to. After taking up the carpet / linoleum I discovered a field of particle board (not OSB) which no one condones having under tile. My subfloor was really 1/2" plywood + a 5/8" particle board underlayment on top which I know wasn't going to be strong/stiff enough for a lasting tile job. I removed the 5/8" particle board and replaced it with 5/8" plywood that I laminated via glue + screw to the existing plywood subfloor - making sure to stagger joints vs. the original plywood and not screw thru into joists. After finishing this, the resulting subfloor is noticeably stronger and stiff, and seems within deflection tolerances for the tile - YAY! After getting out the laser level and kind of mapping out the room, I realized what I don't have a is a flat floor. Out of the 400 sqft, there is essentially a birdbath going on with about 160 sqft depression towards the center of the room with a max of about a 3/8" low spot. This low spot seems worst as it meets a big long brick hearth for a fireplace - my gut says the weight of the fireplace + 45 years of house settlement caused the 3/8 sag over time. I am just finishing the brick hearth removal now.

Problem: I'm trying to determine a reasonably cost effective way to flatten (and in this case level) the floor out. What i'm trying to avoid is pouring 400 sqft of self-leveling compound (SLC) to some minimum thickness, the size of the room makes that both expensive and very difficult due to narrow product open time (we'd be talking like 40+ 50lb bags). I've come up with a handful of potential scenarios but can't settle on which seems most appropriate or have gotten conflicting info.

1. Use SLC to fill in just the birdbath portion, let it cure, then install cement backer boards (cbu) on top of now flattened floor. Dealing with 160 sqft of SLC is more reasonable than 400, but most seem to have minimum thicknesses and we'd be talking about 3/8" max tapered to a featheredge. Also CBU requires attachment via thinset + screws and manufacturer doesn't condone screwing thru SLC. I've had this option recommended to me but gut says the order of SLC then CBU is bad.

2. Use SLC to fill in the birdbath portion, try to put the CBU on top while it's still wet. This seems very difficult, as stepping on the CBU + fastening with screws would squish SLC out all the edges. Same as previous, gut says bad installation order.

3. Install CBU to the entire subfloor (easy to follow manufacturer directions on this part), then fill in the birdbath with SLC on top of the cement board. Still worried about minimum thickness aspect or if it even applies as the substrate is now more like cement instead of plywood. Another thought was to install 1/2" CBU in the lowest areas (instead of 1/4") to essentially minimize the volumn of SLC needed to fill the birdbath.

4. Install CBU per manufacturers specs, then screed out a leveling bed of medium bed LFT mortar to fill the birdbath (same LFT tile mortar I would use for tile attachment - specs show 3/4" thickness max and i'd be 1/2 of that). Let that mortar cure and then install tile on top like normal - the field would be mostly CBU substrate along perimeter with leveled cured mortar as substrate in the leveled portion. I like this option best (but realize that really doesn't matter), already have CBU, LFT mortar is cheaper than SLC, and I can screed at a more reasonable pace or in lifts vs the big SLC all at once operation. Still curious if using a mix of 1/4" and 1/2" CBU is ok to minimize volume of birdbath needing to be filled. Is this mortar to mortar attachment ok - the mortar leveling bed (cured) then the same mortar used as thinset for tile bonding?

5. Screed out and level the birdbath first using medium bed mortar. After cure, install CBU on top per manufacturers specs. Ok to screw thru that mortar bed during CBU attachment?

I'm also talking myself into opening the ceiling below to install solid blocking between joists. Probably overkill, but no real downside beyond the decision to tear into the ceiling that I can think of. Cheap way to add some more stiffness to the floor I think. Looking for some good advice / steering toward the best option or ones I haven't thought of. Any help greatly appreciated!
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Unread 10-09-2019, 05:15 PM   #2
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Welcome, Steve.

You don't mention your joist structure and that's an important consideration in determining whether your floor structure is at all suitable for a ceramic tile installation.

Of your plans number 1 through 5, all but number 4 are non-starters and number 4 would only be acceptable if you used an SLC or other suitable patching material to flatten the floor on top of your properly installed CBU.

But first let's talk about your joist structure.

And a little more about just how your installed your second layer of subflooring. Did you use a full spread of wood glue to do the "laminating?" Are the second layer panels oriented with the strength axis perpendicular to the joist structure?

Adding between-joist blocking will do nothing at all to improve your joist design deflection. If you need to stiffen your joists, I would recommend you remove the subfloor to do that, which also gives you the option of bringing the joist tops into plane to help your flatness situation.

This brick hearth was built on top of the wood-framed floor structure?

My opinion; worth price charged.
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Unread 10-09-2019, 07:08 PM   #3
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CX, thank you for the reply!

House was built in the 1970s. Joists are 2x10s with a 14ft span on 16" centers, the room is about 28 feet long. There is not another story above the kitchen.

For the 2nd layer of plywood I used a full spread glue I squeegeed out and used what felt like an obscene amount of screws per sheet. The sheets were oriented perpendicular to the joists (same orientation as original layer) with joints offset/staggered. I had come across the 'Position of underlayment to prevent cracked tile and grout' by Woeste & Nielsen article from a couple reputable sources (including this site) and it made sense so I followed it best I could.

I was definitely under the impression that solid blocking between joists would help stiffen the floor and improve between joist deflection - just made sense helping spread any loads across neighboring joists. Underneath the kitchen is a semi-finished basement with 12"x12" ceiling panels stapled to furring strips on the underside of the joists. Figured I could basically take out an entire row of them down the center of the span and do the blocking, MAYBE even get the ceiling panels back in place without utterly destroying them (those fiber panels just seem to shred so easily).

Yep, the hearth was on top of the particle board/plywood subfloor. It was approx 12 feet long, about 16" deep and 6" high (2 layers of bricks); attaching a pic. I kinda liked it, but we have toddlers and wife wanted it gone so you know... Gives us more open space and allows us to modernize the look, plus I got to get a new demo tool for the job

Name:  IMG_3389_800px_Hearth.jpg
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Unread 10-09-2019, 08:21 PM   #4
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If your joists are of good grade and species, the structure meets code and tile industry deflection requirements for a ceramic tile installation, but you're certainly not overbuilt. That said, you'll be adding a good bit of dead load to the floor with your installation.

Your subfloor, as described, should be adequate as well and your proposed between-joist blocking just isn't gonna buy you enough of anything to be worthwhile in my opinion. Any builder who would start a subfloor with half-inch, square edge plywood is very unlikely to have glued it to the joist tops. With that in mind, it's at least possible that if you elect to remove the ceiling below you could raise the center of those joists a bit and sister them to perhaps alleviate a bit of your center area sag. May be more work than it's worth, but that would be up to you.

I doubt that removing that hearth will make any notable difference, but it's not a bad idea, anyway. Surely the fireplace is not also supported by the floor framing, right?

I would want a look at the joist structure just to see why the sagging took place at all before I committed to adding more dead load to the floor.

As for the flattening above the existing subfloor, if you plan to use a CBU as your tiling substrate, you must do all your flattening or leveling on top of the CBU. If, instead, you elect to use a membrane of some sort as your tiling substrate, you must do all your flattening or leveling on the subfloor before installing the membrane. Either method is perfectly acceptable.

And for tiles the size you're planning to use, the industry standard for flatness is no deviation from intended plane of more than 1/8th" in ten feet nor 1/16th" in two feet. That's a very, very flat floor and you'll be glad to have it when you start setting those big tiles. The only two ways I know of to actually achieve that degree of flatness is with a properly executed SLC installation or a mud bed, but you can try whatever you think will work for you.

My opinion; worth price charged.
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Unread 10-10-2019, 05:57 PM   #5
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@cx - the fireplace is supported by the subfloor; it's a firebox with brick surrounding the face (or brick used to surround it).

Thank you for confirmation on the ordering, CBU then leveling. That's what made the most sense to me, I just had a couple recommendations (one from a manufacturer) going the other direction that didn't sit right with me. Trying to put CBU on top of leveler (cured or wet) seemed suspect.

If you can re-check my option #3 because it sounds like you endorse that one; CBU then SLC to level on top of it. #4 was a similar process just subbing out the SLC for mortar that I could hand trowel/screed. And any input on mixing CBU thicknesses to minimize the volume of leveling fill needed? Seems like it'd reduce dead load a bit as well as save $, but does seem hackish - anyone with good/bad experience with this?

New question thinking about dead load made me think of, do folks endorse NOT tiling under kitchen cabinets? I can definitely just put extra plywood/cbu under the cabinets to bring it up to finished floor height - definitely would reduce dead load around the room a bit. Quick googling looks like folks are split on cabinets then floor and vice-versa, and when its separate trades doing things I get order my come into play just for logistics, but this is all me on this project.
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Unread 10-10-2019, 07:13 PM   #6
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I would absolutely not consider the different thicknesses of CBU, even if installed correctly. You'll end up with distinct linear separations of the thicknesses of the filler material on top of the CBUs, which will very likely result in distinct linear cracks.

Perhaps I misread your #3, thinking you were leveling with thinset mortar again. You can do #3 if you use a SLC that permits feathering to zero thickness and manufacturer recommended for use over CBU. Keep in mind that while SLCs don't really self level, 'specially over a pourous CBU, but will try to some extent, which may not be good in that application. Use of one of the patching compounds and screeding to fill the void without trying to level the floor may be a more practical solution. Your tiles don't give a rat's patooti about level, they just want a floor that's flat. Very, very flat in the case of your 12x24-inch tiles.

My opinion; worth price charged.
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Unread 10-10-2019, 08:49 PM   #7
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Quote:
. if you use a SLC that permits feathering to zero thickness and manufacturer recommended for use over CBU.
Ardex feather finish meets those requirements
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Unread 10-10-2019, 09:09 PM   #8
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@smifwal - thank you, i'm seeing the same on Ardex's site along with GPS and SD-P for trowelable options. Not sure of a supplier for the product though, and the site doesn't seem to point me towards one...

Also seeing TEC 327 - VersaPatch that looks to fit the criteria as well, and I can source at a *fairly* local Menards.
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Unread 10-11-2019, 05:05 AM   #9
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Where part of Missouri are you in?
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Unread 10-11-2019, 08:00 AM   #10
psharkauburn
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In Hannibal - northeast part of MO.
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