Huge problem and sleepsless night ,opinions please

[pdmbathrooms]

Kinda new here, so hi to you all. I have been using Tops tiles flexi rapid set for years now and never had a problem ( i get a lot of work from them ,scratch each others backs ) Recent job was a 30 sq meter extension on a wooden floor with 600x600 porcelain floor tiles, builder had laid 6 mm ply and was told to bolt obviously, i was reliably informed he then relayed the floor with 18 mm ply, he then laid a electric heat mat and screeded over the top of it, made an arse of the screed lots of high and low points, anyway i then primed the screed with Bal SBR and tiled it which all turned out well.
Two months down the line and about half the tiles have popped in the central area , the tiles round the edges and those that are in the boundaries of the original building are all fine, the ones in the centre of the new extension have all popped. i Lifted a couple and could immediately tell that the ply had not been primed before screeding which the builder has now confirmed , i could lay the blame on this however when i lifted more tiles and i have noticed that while the tile adhesive has obviously been in contact with the tile (the whole area under the tile has the imprint of the back of the tile on it) ,the tile has lifted up clean with very little adhesive attached to it, any ideas why this would happen ? ive been tiling for nearly 15 years and never had a floor pop to date . When i chip away at the screed the ply underneath is very clean and intact. I'm not looking to apportion blame here but would really appreciate genuine thoughts and opinions on what has went wrong and suggestions on how to fix , thanks in advance guys/gals

 

[Qwerty]

Problem lies with UFH onto ply then levelled over. This will cause the wood to expand and contract and pop your tiles. Full rip out only real solution here

 

[pdmbathrooms]

Thanks for that , any thoughts on why the tiles lift clean ?also any time I have layed heating ive had to paint\prime the substrate with a primer supplied with the heating matt

 

[AliGage]

Take it we didnt back butter the tiles then?
Was it rapid set used? The adhesive is well bonded to the floor? What UFH was used?

 

[White Room]

How long was the ufh left before it was switched on...

 

[AliGage]

I take it the client was using the UFH until the point you were called in

 

[gamma38]

Loads of problems here i reckon, it did start with the prep but a bit of thermal shock sounds likely as well. As plan tec says though, timber substrate heated will expand and move a lot compared to what's on top of it, it was destined to fail really.

 

[radim]

i would bet money on thermal shock....

 

[Time's Ran Out]

Ok - so the Op knows - what causes thermal shock and how is it identified?

 

[pdmbathrooms]

Really appreciate all the input guys ,I used a flexi rapidset ,ufh was only installed two days before I tiled , not sure which brand to be honest , thr adhesive I have used has bonded well to the screed he put down but the screed itself lifts fairly easily off the ply