so do I leave this 18 mm down or get 9 mm put down instead then, with the tiles and adhesive im going to have a 25 mm increase in height from landing to bathroom, the floor was in ok condition , the centre area was fine the sides where it had been taken up loads for rewiring and plumbing wasn't so great but then the sides near airing cupboard wouldn't really be walked on , I can only ask her for like to like replacement , that's the problem , is back board dearer