New marble tiles layed in the bathroom with underfloor heating - why is the grout cracking after 3 months?

I am assuming that the room is floorborded....if so they should firstly be screwed down in a grid pattern - not nailed. The correct sub floor must be used and firmly fixed, appropriate flexi-floor tile adhesive shold be used & also flexi - grout. Any minute movement within the floor will cause hairline fractures in the grout especially when the heat is turned on.

My suspicion is that most of the correct materials have been used but that ordinary floor grout has been used, though I would have thought it would have cracked almost immediately rather that after 3 months, so the problem may be movement If this is the case it will always crack so you will need to rake out the faulty grout & re grout with flexi grout. & try to eliminate any movement that you can detect IF YOU DO RAKE OUT THE BAD GROUT TAKE EXTREME CARE NOT TO DAMAGE THE HEATING MAT UNDER THE TILES! More.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.