Trying to tile an unlevel floor


Old 10-20-12, 01:48 AM
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Question Trying to tile an unlevel floor

Hi, I am new to pretty much everything DIY but am trying to learn so that I can fix up/repair/renovate my house myself. Any help is greatly appreciated.

I am attempting to tile my kitchen floor but am running into some problems.
First, the kitchen floor had a hump running down the joist in the middle of the floor. I removed most of the OSB subfloor and found that the joist was causing the hump and was about 3/4"-1" higher than the other joists. So I ground it down and installed 3/4" plywood.

Then I noticed the plywood I just put in was still raised up and sloping from the joist I just ground down. The unevenness didn't seem too bad so I went ahead and put down some thinset mortar and began laying the cement board on top and screwed it down.

After laying my 4th board, I placed a tile on top of one just to make sure it lay flat. When I pushed on the edges of the tile, the other side would raise up slightly (I would guess around 1/16").

Fearing the unevenness of the cement board and raising of the tile would make tiling the floor impossible, or end up cracking the grout and/or tile, I got frustrated and quit for a couple days. When I resumed working, I took up the cement board then spent 2 1/2 frustrating days scraping up the dried thin set mortar.

I also noticed that the original OSB subfloor that I didn't cut out is raised up to 1/8" above the plywood I installed, but then is flush with the plywood in other spots along the same joist. Should I chisel it down?

So...What can I do about evening up the subfloor? I bought some self leveling underlayment (Henry 565 FloorPro) but it was $36/40lb. bag, and the directions say it only covers 43 sq.ft. at 1/8" thickness. My kitchen is about 120 sq.ft. so if I had to level the whole floor with it, it would cost about $120.
Could I just use more of the thinset mortar to even out the low areas, or would the cement board still lay unevenly?

Kitchen-120 sq.ft.
Thinset Mortar-Mapei (it's polymer enriched!)
Subfloor-3/4" plywood and 3/4" OSB
Joists-16" on center
Tile-12"x12" porcelain
Cement Board-1/4" UltraBacker

Any help is greatly appreciated!
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Old 10-20-12, 12:36 PM
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Join Date: Jul 2007
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Allfy16, welcome to the forums, don't you just love it when somebody else's poor craftsmanship rears its ugly head only when you try to renovate. I live with these types of issues on a regular basis. It's important to do what you have done, sit back and evaluate the situation from all angles. I'm also glad that you decided to tear out the backerboard you did install so that you could begin anew with a different gameplan.

Looks like you have one high floor joist. You can remove the subfloor and shave the high joist down. Cheaper than trying to run SLC (self leveling compound). But lets look closer at your install.

Optimally, your want a minimum of 1 1/4" of subfloor (1/2" + 3/4" on top), then your 1/4" backerboard and finally your tile. If the OSB you refer to is actually partical board, you need to remove all of it. Particle board is not stable enough for tile as any moisture will cause it to degrade. It may explain the difference in height of the new vs the old. Its also possible that the house got a lot of rain during its construction. OSB will swell under those circumstances. If that is the case, you need to remove more OSB and replace with new.

As floor joist run the whole length or width of the room, if the math works out, you can start the tile install at the highest point (above the joist). Place a grout line over the joist and work out from there, the grout line will mask the floor hump somewhat so you don't run into a tile that wobbles.

Use your floor patch material to feather out the adjustments from joist to joist and SLC as needed to smooth things out. And last, get a 3/8" x 1/4" thinset mortar trowel. It will leave a deeper bed of mud and allow you a little more play room to "float" the tiles to make up for the height differences of the subfloor.
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