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Old 08-06-2018, 11:12 AM   #1
DIY4Fun
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Suggested Approach for Laying Large Tile on Uneven Floor

I've been reading this forum as much as possible recently (both using the search function and Google). I'm in the process of remodeling a relatively small powder room (22 sqft). I pulled up the existing tile which was set on top of a mortar bed (thick set) of approximately 3/4". After removing the tile and mortar, I was initially planning on adding 1/2" to 5/8" AC exterior grade plywood to raise the floor.

I was going to use liquid nails or similar adhesive and deck screws into the subfloor and not the joists (received conflicting info as to whether to drill into the joists as well). I purchased Schluter Ditra and unmodified thinset. I will be using 17.5" x 17.5" ceramic tile for this powder room. The joists are 16 oc and appropriately rated for tile (based on the available online calculators).

However, after removing the mortar bed, I've noticed that a portion of the subfloor (which is made of 2" x 6" I believe) that runs along the wall is not level with the rest of the floor. As highlighted in red in the attached picture.

I know the floor should be level prior to using Ditra and tiling. I also thought of returning the Ditra and using cement board, which would require a level floor as well. My current alternative is the relatively involved process of using SLC (along with the membrane and lathe that are required) or using a mortar bed as the previous installer used.

I would appreciate this board's advice or pointing me in the right direction.

Questions:

1. Should I use a SLC or mortar bed in lieu of using plywood and Ditra or CBU?

2. Because the joists are level, can I simply proceed with laying plywood and adhering it to the subfloor with appropriate adhesive and deck screws?

3. The bathroom walls (I've since removed the drywall on two sides) are structurally resting on the portion that is sloping. Since I cannot remove these floor boards, is SLC or mortar bed the only option?

4. Should I attempt to replace the portion highlighted in yellow? It appears that it is depressed due to the tension of the framing and perhaps rot? It's not entirely clear.

Thanks for your consideration.
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Old 08-06-2018, 12:20 PM   #2
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Welcome, Andres.

1. A reinforced mortar bed may be the easiest method of correcting what you have there. That would require a cleavage membrane and then a minimum of 1 1/4" of deck mud with welded wire reinforcing mesh in the vertical center.

2. It matters not at all whether your joists or subfloor are level, it matters only that the subfloor is flat. You could add plywood over the joist structure you have, but you'd need to find a way to support the edges that I think you are telling us are lower than the joist tops. Not sure how you would do that without removing the existing subflooring. Do you have access to this floor from below?

You do not want to use any kind of adhesive between your layers of subflooring. You might use a constructive adhesive on the joist tops if you elect to install a new subfloor.

My opinion; worth price charged.
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Old 08-06-2018, 01:57 PM   #3
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Thanks, CX. I was thinking of going with a mortar bed, but I don't have enough room for 1 1/4" of mud. The previous installation did not, I believe, use this much mud. As a result, I would like to match the height of the adjacent floor. I have enough room for 1/2" or 5/8" of plywood.

1. I do have access to the floor below as I can remove the drop ceiling tiles in this area. Would you suggest removing the subfloor planks adjacent to the wall?

2. Would 5/8" of mud bed be enough if I used the cleavage membrane and welded wire reinforcing mesh?

3. Alternatively, could I just use self leveling compound because it can handle up to an inch depth?

Thanks again.
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Old 08-06-2018, 02:14 PM   #4
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1. If you can raise the support under the wall that's sitting on those planks to flatten your floor, that would be helpful. Removing them will not otherwise improve your situation.

2. No.

3. Not over what you show in your photo. You would need at least a layer of 1/2" exterior glue plywood over what we see there before you could do anything other than a reinforced mud bed.

My opinion; worth price charged.
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Old 08-11-2018, 07:34 AM   #5
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CX, could 1/2" plywood, then 1/4" hardiebacker, and then self-leveling compound on top of backerboard to level-off the subfloor, be a possibility in this situation?

If applicable, it could potentially take off at least 1" inch of height from the finished floor.
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Old 08-11-2018, 10:30 AM   #6
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If you wanna do something like that I'd recommend you use nominal 3/4" plywood instead. No advantage at all to the CBU in that application and some disadvantages.

You'll then need to follow the SLC manufacturer's instructions for placing over wood framed floors, which usually requires a minimum thickness and a reinforcing mesh.

My opinion; worth price charged.
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Old 08-11-2018, 11:33 AM   #7
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Can OP put slightly more thinset on the red area and put 1/2 cement board over the whole area. You need thinset under cement board anyway so I'm not sure why that wouldn't work. Then you can apply whichever ditra is needed to bring the floor level with the outside floor.

Hopefully a pro will chime in.
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Old 08-11-2018, 12:03 PM   #8
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OP can do anything he wants with the floor, Ali, but the CBU manufacturer does not approve of installing his product over sawn wood subflooring even when it can be done correctly.

My opinion; worth price charged.
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Old 08-11-2018, 12:15 PM   #9
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Ahh. Ok. Didn't recognize the difference between sawn floor vs osb or ply. Good to know, thanks.
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Old 08-12-2018, 05:32 AM   #10
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what is a sawn wood subfloor?
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Old 08-12-2018, 07:07 AM   #11
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Quote:
Originally Posted by Fred
what is a sawn wood subfloor?
1x's or 2x's.
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Old 08-12-2018, 11:16 AM   #12
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From my recent reading, seems the main issue is expansion and contraction. The 1x and 2x wood just is dimensionally stable enough. That's why I love this site. A wealth of info.
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