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ol rick 01-04-2012 06:14 PM

Floor backerboard high spot!
I have a problem with newly placed 3x5 cement backerboard. One piece I installed with screws and mortar has a slight ridge or raised seam where it adjoins the next piece lengthwise, so it is 5' long. This ridge must have happened when I did not get screws into the board close enough to its edge. So yesterday I was about to lay more 12 x 12 ceramic tiles (Peru, 1/4" thick) when I saw and I felt each tile rock back and forth as I pre-placed each one. UGH! I tried wet sanding the tape and mortar joint where this ridge is found and it helped lower the ridge some, BUT STILL each of those five tiles will rock 1/8" or so. Oh can I lay these tiles without causing them to break later?

Ol Rick

joecaption 01-04-2012 06:19 PM

Could have just used thin set and a wide sheetrock knife to level it out, or just used a thicker layer of thin set as you were laying it.

Bud Cline 01-04-2012 06:22 PM

That's why you never install the seam tape ahead of time and allow the thinset to dry.

The quickest fix for your problem is to remove the seam tape and scrape the thinset down to the cement board surface.

Then, apply new tape just ahead of installing the tile.

JazMan 01-04-2012 06:26 PM


Could have just used thin set and a wide sheetrock knife to level it out,
So, how is that gonna help? He's installing ceramic not vinyl. Darn ceramic tiles just refuse to bend. :whistling2:

I suggest knocking that area down a bit by sanding or removing the tape, maybe replacing part of the bad area. In the end you may end up with some lippage and a lesson learned for next time.


ol rick 01-04-2012 06:27 PM

OK - let me see here - I did use thinset beneath each sheet of b-board then put the tape along the seams as I went. But should have waited 24 hrs before taping them?

But now to correct what has happened with the seam - I should remove the tape and all thinset down to the b-board, then set the 12 x 12 tile on THAT, but tape just before the tile? I guess I don't see how that will help level the surface so my tile won't rock any.

Ol Rick

JazMan 01-04-2012 06:30 PM

It all depends on what is causing the high spots.


ol rick 01-04-2012 06:32 PM

I sure that the high spot is one continuous ridge along the 5' length. It is where one b-board's edge is very slightly higher than the one next to it.

Ol Rick (distressed)

Bud Cline 01-04-2012 06:41 PM

Installing seam tape and the related thinset ahead of time and allowing it to dry causes high spots all-along the seams. Then when you apply more thinset to install the tile the tile wants to teetor.

Installing cement board and allowing the thinset to purge slightly from under the edges of the boards makes adjacent boards stand tall because the thinset that is purging from under the boards has the moisture wicked-away rapidly from the dry cement board. This creates a firm ridge for the next (adjacent) board to sit on thereby raising subsequent boards as they are installed. The thing to do when installing cement board is to scrape along each edge to remove any purging before placing the next board.

One of these two issues is your problem.

The only way to remedy the problem is to scrape (grind) the top of the board a little.

Adding additional thinset under a teetoring tile is NEVER the way to remedy this problem.

ol rick 01-04-2012 06:52 PM

Bud Cline - thanks
Now I think I got it: Remove the tape and the hardened thinset I used to apply the tape at this 5' seam down to the board surface. Then scrape the cement b-board down a hair to make even with the adjoining board??

Then once the two edges are even, install the thinset I used for b-boards to re-set the tape?

What is best tool to scrape the b-board top surface?

Your earlier help with the b-board phase of my home project was very good. I am glad I asked about this too because I was really thinking of using a thicker layer of latex mortar to set the 12 x 12 tiles to take care of the teetor! Thanks again. Ol Rick

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