I think I may have mentioned this in passing at some point a while ago, but I'm now revisiting this issue.   A few months after we moved in, I noticed that we had vertical cracks through the marble tiles in our ensuite.  Shortly afterwards, I noticed that in fact there were vertical cracks in all of the bathrooms.  The cracks follow the lines of the board joints underneath.  We have Hardibacker or some other sort of tile backer board in these areas.      I initially wondered