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Unread 10-09-2019, 03:32 PM   #1
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Join Date: Oct 2019
Location: Missouri
Posts: 5
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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