Flooring Tile - installing porcelain tile, question about flatness of subfloor

I'm working on a small bathroom project, with installing porcelain tile. I had more trouble than I expected getting the subfloor leveled out.

the subfloor is all wood 2x6s. there was 1 corner that one of the boards bowed up slightly, so I did a combination of sanding and filler to level it out. I had another section around the toilet that was a little uneven, so did the same thing there.

I then installed concrete backer board over the entire floor, and am now working on a dry run.

but I'm finding that some of the tiles rock slightly. maybe a 1/16 of an inch or so. I think some of the backer board seems aren't perfectly flat, so I'm going to take a belt sander to those. in other cases, it looks like the floor is still just slightly uneven.

I know having a solid subfloor is the most important part, so I want to make sure I do it right. but I'm not clear if these slight wobbles are something I should be concerned about, or whether the thin-set will be enough to even that out.

but I'm finding that some of the tiles rock slightly. maybe a 1/16 of an inch or so. I think some of the backer board seems aren't perfectly flat, so I'm going to take a belt sander to those. in other cases, it looks like the floor is still just slightly uneven.

You don't say what size tile you are using. The floor should be flat to within no more than 1/4" out of plane in 10'. Depending upon the tile size, and the trowel you use, you can probably accomodate the slight rocking of the tile. This sometimes requires a larger notch trowel and some professional setting skills to get the floor flat with little to no lippage.

the subfloor is all wood 2x6s. there was 1 corner that one of the boards bowed up slightly, so I did a combination of sanding and filler to level it out. I had another section around the toilet that was a little uneven, so did the same thing there.

I then installed concrete backer board over the entire floor, and am now working on a dry run.

Your installation is likely to result in failure. Cement board cannot be installed over 2x6 planking. The planking is very unstable as each board moves independently with changing moisture conditions and this movement will likely cause cracked tile and grout. You need to install at least 1/2" of exterior glue plywood over the planking. Then you can install your cement board in a fresh bed of thinset and screw it down according to manufacturers instructions.

You don't say what size tile you are using. The floor should be flat to within no more than 1/4" out of plane in 10'. Depending upon the tile size, and the trowel you use, you can probably accomodate the slight rocking of the tile. This sometimes requires a larger notch trowel and some professional setting skills to get the floor flat with little to no lippage.

got it, the tiles are about 12" square.



Your installation is likely to result in failure. Cement board cannot be installed over 2x6 planking. The planking is very unstable as each board moves independently with changing moisture conditions and this movement will likely cause cracked tile and grout. You need to install at least 1/2" of exterior glue plywood over the planking. Then you can install your cement board in a fresh bed of thinset and screw it down according to manufacturers instructions.

yikes, ok. I think the boards were 2x6, but even if they were 2x8, it sounds like that wouldn't matter, the same problem will apply. I'll pick up the plywood and work on installing that under the backer board.

also looking closer at manufacturers instructions for the backer board.

thanks for the help.