Cracked conglomerate.

[alexander]

As far as Im aware you only need to prime the boards when installing ufh, I was told by the nmp rep that priming is more likely to reduce the bond of the adhesive. cement boards are impervious to water as the are.

Im only asking to try to work out what has gone wrong with your floor.

Is it possible that there were voids left under the tiles? If not then it may have been a void under the cement boards?

Hey thanks for taking an interest (everybody)
I am 100% sure no voids between tile and board, as for board and floor it seems unlikely as i dont remember the floor being that un-even and like i said i overkilled the screws and glue. I have being using nomoreply for about three years, i started with the 6mm, and swapped to the 9mm when local stockists recommended the 9mm for bigger areas such as kitchens a couple of years ago and have been using 9mm ever since. Personally i hate the stuff to work with, i just use it because i liked its princables. I have tiled 100s of metres over this board and i pray this is my only problem.