# How do I remove thinset from cement board?



## Leoskee (Dec 19, 2007)

The tile job in my bathroom was a complete failure. Three years after the job was done the tiles have started to crack. I pulled up a piece and realized that it was not attached to the thinset. I started to pull up more tiles and realized that they all have the same problem. The thinset was laid down but the porcelin tiles did not stick. 

How do I remove the thinset so that I can redo the job? Is there a way or will I have to remove and reinstall the cement board? Thanks.


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## crankcase (Sep 21, 2010)

I had to redo a floor from the knucklehead that had the house before me. He just set the tiles directly on the OSB subfloor It was cracking the tiles on every seam. I used a 4" angle grinder with the most agressive sanding disks I could find I think it was 36 grit, get a couple disks The grout seemed to be a little tougher than the thinset to grind. Even with a shop vac running with one hand and grinder in the other it made a horrible mess. So contain the area. If you have the white stuff (thinset or mastic I can remember anymore it will be easier to distinguish from the cement board, It works fast, use a sweeping crescent motion and keep the grinder moving or it will dig in to your existing cement board. When you think your done, sweep or vac the area and double check that you didn't miss any spots.


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## Ron6519 (Mar 28, 2007)

You can grind it off or chisel it off. Before you start tiling, I would find out the reason it failed the first time. Are you sure the support framing and subfloor are up to the job?
Ron


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## retired guy 60 (Jun 23, 2009)

There are oscillating tools such as the Multimaster which can use a rasp like tool to remove the thinset but this would be a very slow and arduous task depending upon the size of your bathroom. A grinder might also work but the dust will be miserable. I would probably just remove the cement board. But you have to ask why was the tile job such a disaster? Was the subfloor unsound? Was the thinset not mixed properly or applied according to directions? Even if you could remove the dried thinset without disturbing the cement boards, would you be setting yourself up for another tile reinstall in three years? It is very important that you determine what went wrong so you can do the job properly. If you need follow up info, there are some very knowledgeable pros at DIY Chatroom who can answer any of your questions.


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## Leoskee (Dec 19, 2007)

So if the floor has some give it will cause the tiles to pop off? There is a beam that makes a squeaky sound when the winter comes around. But looking at the way that th tiles just popped off made me think that the thinset/mastic that they used did not hold. The bottom of the tiles look like I just took them out of the box. 

The area near the tub is the part that has cracked tiles. About 3 rows of 12 inch tiles. The rest of the bathroom floor seems fine.


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## Leoskee (Dec 19, 2007)

I want to add that the guys that did the job effed up a lot of things in my house. I have a feeling that the problem is with the way that the tiles were set as opposed to a structural problem. For example, the guy they had install the tile used regular grout where the tub meets the wall. About 6 months after the remodeling job I had water leaking under the tub because all of the grout cracked from settling. I had to redo that and using caulk. He did the same where the floor tiles meet the wall tiles. That grout is cracked up too. 

If it were a structural problem, what would I look for.


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## retired guy 60 (Jun 23, 2009)

Yes, if the floor has some give, the tiles may very well fail. So can the grout. If they used mastic ( I think you originally mentioned thinset) and the floor got wet (think minor flood) the mastic will give way since it is not truly waterproof. There are ways to determine if the floor is solid enough by measuring deflection. I am not high tech. I consider the thickness of the subfloor, the condition of the subfloor, the material it is composed of, the size of the joists, the spacing of the joists and then come up with a conclusion based upon all of the above.


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## Leoskee (Dec 19, 2007)

All I know is that the floor originally was made of cement. During the demo they broke up the cement. The building inspector did not like the way one of the beams looked and had them bolt two new beams to the bent one. On top of this they put down 3/4 plywood. On top of the plywood they put down cement board. The floor looked pretty good and structurally sound. But then again, I am not a contractor. 

The substance that they used to set the tiles was in powder form. They mixed it and used a trowel. Not sure what the difference with mastic is. Im assuming that mastic would be some sort of glue like compond. All I know is that the tiles are clean underneath. Nothing held on. But then again, only the area around the tub is having the problem. 

I will take some pics tomorrow and post them. Hopefully that will help.


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## Ron6519 (Mar 28, 2007)

Leoskee said:


> So if the floor has some give it will cause the tiles to pop off? There is a beam that makes a squeaky sound when the winter comes around. But looking at the way that th tiles just popped off made me think that the thinset/mastic that they used did not hold. The bottom of the tiles look like I just took them out of the box.
> 
> The area near the tub is the part that has cracked tiles. About 3 rows of 12 inch tiles. The rest of the bathroom floor seems fine.


It would help if you could be precise in your description of the methodology of the installation and the floor structure. Terms like, "thinset/mastic" are two different materials. Which one did you use?
You would need 2 layers of plywood, screwed and glued to the joists. The cement board should have been thinsetted to the plywood and screwed to the plywood.
Ron


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## Leoskee (Dec 19, 2007)

I didnt do the job. I hired a contractor to do it. I am now in the process of trying to fix it myself.


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## nakamis (Oct 2, 2010)

thanks for the info man


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## Ron6519 (Mar 28, 2007)

Leoskee said:


> I didnt do the job. I hired a contractor to do it. I am now in the process of trying to fix it myself.


Hard to fix if you don't know what was and wasn't done. It's possible that the structure is okay and the reason these tiles failed was that the thinset had gotten too old or was mixed improperly. But when you tile a room, you tile deep into the room and work your way out. 
Ron


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## retired guy 60 (Jun 23, 2009)

I have to agree with Ron. It is difficult to diagnose a condition like this w/o knowing what was done. From your description, thinset was used and a bad joist was sistered to provide a fix to a faulty joist. Also, it appears that the floor originally had a "mud" job before it was demolished to put down 3/4 inch ply. I also feel that 3/4 inch plywood does not provide the rigidity that tiles require. At the very least another layer of 1/2 inch exterior grade plywood should have been added to the 3/4 inch plywood.
While there is likely a structural problem due to inadequate subfloor and possibly failure to apply thinset under the cement board, it is very likely that the thinset used to apply the tiles was not mixed properly or was very old because the tiles you removed should have a hard coating of thinset on the backside and from what you described there was a total failure to adhere to the tiles.
Just two other things seem relevant. First, you mentioned water under the tub. This could easily weaken the already inadequate subfloor and may have resulted in wood rot. Secondly, tubs filled with water and an average human body are heavy and the front of the tub resting on the plywood with all the extra weight might explain the flex of the wood at that spot. Up and down movement of the subfloor, however slight, does not make for a satisfactory tile job.


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## Leoskee (Dec 19, 2007)

I cant get the pics to load. Can someone help? Thanks. 

So should I rip up the cement board, add another 3/4 plywood layer, reapply cement board and do the tile job over?


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## Ron6519 (Mar 28, 2007)

Leoskee said:


> I cant get the pics to load. Can someone help? Thanks.
> 
> So should I rip up the cement board, add another 3/4 plywood layer, reapply cement board and do the tile job over?


At the top of the,"Building and Construction" forum there are instructions for posting photos.
I wouldn't tear anything up unless you know the reason for the tile failure.
Ron


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