Gcol
Under the polyester foam is concrete, but the foam is 4 in. thick and filing in the whole room seems like a big job, as we are hoping to do this in the next few days.
Is there no way of putting the tiles on the board, as the movement isn't so large you can see it, just you can feel it...
I'm after a bit of advice. My kitchen floor is chip board floating on polyester foam (used as the insulator) which means there is a little movement in the floor. I have been told that as long as I use a flexible bedding agent and flexible groat I should be able to lay granite tiles without them...