Lay decoupling mat under porcelain tiles?

5 Nov 2009
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United Kingdom
I am having an extension built where the floor is going to contain a water, underfloor heating system. The UFH pipes are going to be bedded into concrete screed with porcelain tiles laid on top.

I went to a tile shop recently looking for porcelain tiles and the very knowledgeable shop assistant strongly recommended that a decoupling mat such as is laid under the porcelain tiles to prevent any chance of the tiles cracking due to the UFH.

Would it make sense to lay this decoupling matting? Would it slow down/reduce the heat output of the UFH?
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I used decoupling (Ditra 25) for some outside porcelain tiles on a concrete raft because I expect the concrete to move slightly with temperature. So far so good. I doubt it would make much difference to UFH. The mat is quite thin itself and in effect what happens is that the tiles are glued down to little pockets of wedge shaped adhesive enclosed in the mat structure - no thicker than normal tile cement bedding. These little pockets can move about slightly as they are separated by little air pockets within the mat structure.

The mat itself has a thin fabric backing which is used to glue the mat to the floor material keeping the internal air pockets free.
Thanks @mrrusty. I am also looking to have the decoupling matting laid on a concrete raft for an outside patio and then porcelain tiles laid on top. Did you have any movement joints on your outside patio?
Yes, use a decoupling mat, for UFH with tiles. You want notice any slow down of the heat.
I also used Ditra, it's not cheap, but I had no hassle installing my own.

A couple of years later, I have noticed there has been some expansion/contraction at the edges of walls (1mm or so), so the floor does move when on/off.
I've had no tiles loose/pop off.
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No, mine is only about 4 x 3.5 so no joints

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