Tiling over cracked concrete

[tileman]

Hi all, I have a kitchen floor to tile, it is a concrete floor of which 95% is already tiled, the room is about 6m long and is a double L, like a Z.
It has to be tiled over the existing tiles, the problem is there are two fine cracks through the tiles that run from one side of the room to the other side.

The first crack is about 2.5m into the room, the second is about 2m further on, I suspect they are caused by joints in the concrete floor.

The customer doesn't want expansion joints or Silicon and I doubt that the tile joints would drop on the cracks anyway.

I think the answer to this problem is backer board, but I have never used it, what is the best method and materials for this job;
1. which primer for the tiles to be tiled over.
2. which backer board and what thickness.
3. Which adhesive to fix the backer board and tiles.

Regards john

 

[Gazzer]

Sounds like a job for Ditra matting or Dural CI to me.

 

[grumpygrouter]

Sounds like a job for Ditra matting or Dural CI to me.

Me too Neale.

 

[bigandy]

Ditra it is for me. Explain your fear and explain what a decoupling membrain is.

 

[pjtiler]

deffo matting:yes:

 

[DHTiling]

I think you need to remove the tiles and see wots wot first.......

 

[White Room]

The screed may have gone hollow in certain areas causing cracking

 

[Perry]

or even better PCI Pecilastic ete
Movement

 

[Alberta Stone]

In situations like that I would try to remove a few tiles here and there and rap test in plenty of spots to be sure they could be tiled over.
If they can be tiled over you should use a diamond grinder to make the surface rough enough for the thinset to key in.
If there is excessive lippage in the existing tiles you can either grind them flush or use a premium thinset with top grade admix to skim coat, but thinset can only be used if the variation is very small, no more than 1/8 of an inch, any thicker and you need a self leveler.
Mapei primer L works real nice.
Use it straight in such an application, roll it on and let it dry before setting tiles, but you need to set them that day, optimally.
Any cbu will do, they guys here can advise better as to which brand.
If you are only using them to bring up the height of the untiled areas then you should measure what the difference is and use a spf to bond them in place but use weights to keep them laying flat as you can't use mechanical fasteners to hold them down while the thinset cures.
The kind of thinset will be determined by the kind of tile you are installing.
Gaz should have a good recommend.

 

[tileman]

Hi Dave, thanks for your reply, I am quoting for this job and want to get it right, it's in a brand new kitchen and they don't want me to lift tiles, the floor is solid, the cracks are very fine, but go from one side of the room to the other, my concern is that if I tile direct onto the existing tiles, the new tiles will crack also, so I think I should use 3mm Detra matting, what do I use to prime the existing tiles, what adhesive do use to stick the Detra down.
regards john