Help!: Cracked tles and UFH

[kampc8856]

Has anyone had a problem with porcelain tiles fracturing along the join lines of the underlying plywood. A warmfloor solution heating mat was also installed. The floorboards were sound and I used 18mm ply, screwed at 6 inch intervals. A good job I thought but still the tiles cracked. Is it possible that the plywood expanded when heated, causing raised edges on the ply, resulting in the tiles cracking. I have heard that a DITRA uncoupling membrane can be used to prevent the transfer of stresses to the tiles. Any view, thoughts, solutions would be much appreciated.

[Dave]

Did you leave expansion gaps between the boards and around the perimeter of the room..?

[kampc8856]

No not between the boards, was this the mistake. If yes, what sort of gaps should I leave. Do you leave gaps between boards only when using underfloor heating mat or always

[Dave]

The boards can expand and contract and wood against wood can cause it to bulge...leaving a small 2 to 3mm gap gives it room if needed....but sometimes it can happen anyway thats where a product like ditra as you said comes in handy...

[whitebeam]

It's also a good idea to put the ply at right angles to the floor boards so no two joints follow threw to the surface

[Little John]

leaving an expantion joint in between the boards is always a good idea, what i was taught was to tape along the joints with gaffer tape so any movement happens under the tape and shouldnt crack the tile ,getting the tiles up when theres under floor heating is another matter, its going to b very difficult not to break a wire, its never happend to me, and i dont know how well there fixed but would suggest lifting them carefully!!