Tiling on top of Tiling

[Keith]

Hi Guys. Just been to look at a floor tiling job. 31m2 Entrance hall through to the kitchen,diner and utility needs tiling. The floor is concrete. On that is 10mm plywood. On that are tiles. these have been down for a few years. What I am trying to understand is these tiles have various cracks in them, and I don't think these were made by dropping things. When I tap a mallet on them it does sound a bit hollow.I would have thought the substrate was pretty solid. Could it be the addy wasn't laid properly.
Any dips in the tiles I would SLC. Would it be safe to tile on top of these or would you lift all the tiles and have a look at the ply.

 

[Andy Allen]

Why has ply been put on top of a concrete substrate...??

 

[Keith]

Hi Andy. I don't know because it was done previously. I have only just seen it this morning with a view to pricing.

 

[Andy Allen]

Definitely wouldn't tile on top.

I would rip it all up including the ply, prep the concrete substrate and tile on that....

If the floor needs building up 10mm for some reason, I would stick down 10mm insulation boards and then tile...

 

[Keith]

Thanks for your reply Andy. Have just been onto Bal Tech and they advised ripping it up. Also as you pointed out the 10mm can be substituted with insulation boards. That is deffo what I will price for.

 

[Concrete guy]

My best guess for the reason the ply is there (without visiting the place of course) is you might find it's part concrete screed part floorboards.

9mm or 12mm ply over the whole thing to level it all used to be par for the course two decades ago.