Bathroom Floor Disaster

My boyfriend has just laid granite tiles on our bathroom floor, but the tiles have started to raise up into a peak along two of the grout lines, which has caused major slopes in the floor. None of the tiles or grout have cracked as yet, the tiles haven't become loose and there's no sign of movement in them, although the floor has started to creak. Does anyone know what could have caused this to happen?

The tiles were laid over 6mm ply, which was nailed to the floor boards and the tiles were fixed with flexible adhesive.

I've read that the ply should be 12mm thick and water resistant and resin bonded, is this true?
I've also heard that the ply should be screwed down, not nailed down, is this also true?

We've now got to take the tiles up and relay them, so any advice would be gratefully received.

Thank you in advance.

screws do not allow movement, nails can "work loose"

Hi Scarlett,
the plywood should be w.b.p. construction / exterior grade and must be nailed at 100mm centres (i.e. the space between any two nails shouldn't be more than 100mm or 4 ", using 1inch annular or serrated ringnails (stiletto).
As long is the subfloor isn't rotting then the serrated nails will not work loose.

Check that the existing floor is sound - screw it down if there is any sign of movement (creaking boards etc.)

use 12mm WBP ply

Let the ply 'condition' in the room for a least 48 hour.

When you fix the ply down use screws (check for pipes and cables!)

Screw the ply down in a pyramid pattern