I have laid some large (600 x 600) porcelain tiles on my kitchen floor over underfloor heating and 18mm chipboard, using the suitable adhesive and grout. I've noticed that close to half of the tiles are loose and 'bouncing', mainly in the corners producing a 'hollow' sound underneath. I know the chipboard was firmly glued and screwed down and that this isn't causing the problem (the loose tiles are scattered about the kitchen).
My first thought was to take them up and replace them. But i'm worried about damaging the UFH (its electric). It's also close to half the floor area (total area 18sq metres). So my second thought was to tile again over the top of the porcelain tiles. My questions are: Can i tile over porcelain? Which adhesive do i use? Do i need to prime the tiles with anything first? what about the 'loose' tiles underneath?
Many thanks in advance of any help,