Is It Common for Builders to Skip Thinset Between Plywood and CBU? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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01-10-2017, 09:27 AM
I hired a contractor to remove 900 ft^2 of tile and lay ~1600 ft^2. The majority of the work was on the ground floor slab and done by a skilled group of tilers. But the Contractor planned to have his worker do the upstairs bathroom which I did not like; so I closely watched his work. After just 3 tiles I stopped work and said I would complete the job. Three week have passed and yesterday was my 1st chance to work on the tile. I found one of the tiles was significantly raised from the other two so I popped it up and started to chisel out the thinset. I was surprised that a 8"x32" porcelain tile came up in one piece and had very little thinset adhered to the tile. If one was this way the other 2 are probable also not good, so I decided to remove the sheet of CBU and start over.

As I was preparing for the work I read the CBU instructions (CX would be proud of me) and my old tile books; and found that my contractor and the original house builder had not followed the instructions which required thinset between the plywood subfloor and the CBU. I can tell the original builder didn't do this since the drywall joint compound is covering most of the plywood (it needs to come up). I missed this fault initially since my previous tile work was 12 years ago and was directly to a slab and CBU/Redguard shower walls with a purchased floor tray.

How common is it for builders/contractors to skip using thinset to adhere the CBU to the plywood subfloor? What is the expected result in skipping that step - too much flexing and cracked tile/grout etc? I want to know how hard I should push on my Contractor. This rework is trivial but we are about to start the master bathroom rebuild and I don't want sub-par work on it. In the past, he has done excellent work on the garage and patio addition.

Second question: Why didn't the thinset (Mapei Ultraflex 1) adhere to the tiles? After 3 weeks inside a house, I would think that it should be fully cured. You can see in the photo that the trowel lines are clearly visible; so I know that he did not press enough to really set them. Is this lack of adherence a common problem from not damping the CBU before spreading the thinset or using too dry a thinset mixture? I don't want to make the same mistake since I am using the same material. Please don't get on me for using large tiles in a small bathroom; but my spouse wanted to use the same tile as was installed on the ground floor. So I have to plan my cuts to eliminate "U's".

Thanks for any help you can provide.


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Houston Remodeler
01-10-2017, 09:33 AM

1- Sadly, yes

2- exactly

3- Following the manufacturers instructions is critical, and a good idea no?

4- Thinset mixed to dry, thinset allowed to skin over, CBU pulled too much water out of the thinset, tiles not back burned, poor troweling techniques, dust or water contamination on the back of the tiles, contractor not knowing his trade.

5- See #4 and watch this video (

01-10-2017, 07:31 PM
Applying the thinset in a swirl pattern doesn't give you as much coverage as combing the thinset in straight lines.

01-10-2017, 08:15 PM
Paul and Justin,

Thanks for confirming that I was correct in stopping work. As I removed the CBU, I found it had two extra joints with 3" filler strips to extend the panel and no fiberglass mesh tape over the joints. I changed the orientation of the CBU and eliminated the narrow strips. The only bright spot in his "work" was that it was so poor that I did not have a hard time removing it.

I will follow the approved methods including the straight line troweling for the large tile.


01-10-2017, 09:40 PM
When bedding the tile, if the trowel lines are not straight, it's hard to bed the tile properly, especially if it is larger. You should lay the tile down, then rock it back and forth across those lines to spread them out to complete the proper coverage.

From the back of the tile, it's obvious there was improper coverage. Industry standards call for 100% of the edges and at least 80% (100% should be the goal) of the rest of the tile to be covered when used on a floor.

01-10-2017, 10:51 PM
There isn't 100% coverage but there's plenty of coverage in my opinion. For one or more of the reasons stated above, the thinset didn't stick. I'd say he spread too much at a time and the thirsty Hardiboard caused it to skin over on top.

01-19-2017, 06:33 AM
Jim & Davy,
Thanks for your comments. I definitely will wipe the Hardiboard down with a damp sponge; I saw that it was real thirsty when setting it down to the plywood sub-floor. I will also pull up the first tile after bedding it to assure myself that I am getting full coverage and the thinset is the proper consistency.

Thanks for confirming that his work was not sufficient.