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leveling plywood floor before hardibacker is installed + confirm

13 years ago

Hi, thanks again to Mongo for great answer to my previous questions!

I have a couple of questions about the floor. The area is 5x8, and I pulled up the old tile which was mortered to a very thin layer of plywood that had been stapled to the subfloor of plywood (looks like chipboard).

I plan to install 1/4 hardibacker to the subfloor before tiling with porcelain 12x12 mixed with 6x6 in a pattern.

The problem is that the subfloor dips about 1/4 inch along the egdes of two opposing walls. Also, in the middle of the area where the vanity cabinet will sit, there is a seam in the plywood that is uneven. The builder had left this portion untiled since it couldn't be seen.

I need advice on how best to flatten out this floor. I understand that is doesn't really have to be "level", just uniformly flat so the tile won't get stressed and crack.(By the way the old tile was 4x4 and had been there 11 years with no cracks).

1. If I leave the under-cabinet area untiled, do I need to fix the floor dip on the other wall?

2. Assuming the dip needs to be fixed, can i glue shims to the sub floor?

3. Can I use self leveling compound? The instructions for LevelQuick say it can used only for exterior grade plywood, but not particle board. What about my "chip-board" plywood? Can it be primed first, then use SLC? Would the SLC run under the wall? Any other issues with using this stuff?

4. Could I use a small amount of the thinset that will be used between the plywood + hardibacker instead of SLC, and use it to level out the subfloor dips, then let it dry/cure, then thinset and install the hardi later?

5. From reading other posts I want to confirm that UNmodified (dry-set)thinset should be used between the plywood + hardibacker. (Both Mongo + Bill recommend, right?)

6. Once hardibacker is in place, do I use modified or unmodified thinset for installing the porcelain tile?

7. On the shower walls, I will also use hardi, and same tile. However, I want to put in a 4 inch stripe of glass tile running through the field. What do you recommend for the mortar/thinset for this tile? Home depot has something labeled for glass I assume would be ok.


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