Floor part heated

[Old Mod]

Yes that is something I can't understand either. If there is no movement joint in the substrate what is the point of one in the tiling unless the tile adhesive layer allows some shear movement of the tiling relative to the substrate (which most don't).
If a decoupling layer is used then this allows the shear movement. There must be a gap in the decoupling mat at the movement joint line so that each tiled area can move (expand/contract/vibrate) independently.

So are u saying if there is no expansion in the substrate, but one in the decoupler this will work as an expansion joint in the tiling?
Sorry if I've miss read it.

 

[SJPurdy]

Yes that is my understanding.
If you have a continuous flat substrate and you just tile it leaving a gap to be filled with backer and flexible sealant then I don't see how that can work.
However, if you have the same substrate and you fix a good decoupling membrane to it, leaving a gap where you want the movement joint; you then tile it leaving a gap above the gap in the decoupling membrane which is again filled with foam backer and flexible sealant then I can see that will work to reduce lateral stresses in the tiling.

 

[Old Mod]

Yes that is my understanding.
If you have a continuous flat substrate and you just tile it leaving a gap to be filled with backer and flexible sealant then I don't see how that can work.
However, if you have the same substrate and you fix a good decoupling membrane to it, leaving a gap where you want the movement joint; you then tile it leaving a gap above the gap in the decoupling membrane which is again filled with foam backer and flexible sealant then I can see that will work to reduce lateral stresses in the tiling.

Hmmmm ...... Don't see it I'm afraid.
The tiling isn't the problem tho is it.
It's the substrate that moves not the tiling. If the tiling was the only thing moving, then 100% no problem, I get it.
But the problem is it's the substrate that moves! It has to have some where to go!
The tiling only moves as a result of the movement in the substrate. So if the substrate has no where to go, it goes up!
Not being difficult @SJPurdy just don't see the physics in it.
And if the substrate was to shrink then the decoupler would cope anyway. (Apparently haha)
I do believe u believe what you're saying.....almost! Haha
Feels like there's a little doubt for you too!

 

[SJPurdy]

I believe what I'm saying is true for coping with movement in the tiling where the substrate is stable. for example where there is a screed greater than 8m across so that movement joint is required in the tiled layer to meet BS for a heated floor. I actually came into the post to explain that decoupling alone was not a substitute for a movement joint.
But oops I've just gone back to the original post. obviously if laying a new timber floor then there has to be gaps between the boards in places like doorways to accommodate expansion/movement in the timber floor itself and prevent bowing up of the floor.
But then if a de-coupleing membrane is fitted over the plywood then this should allow a small amount of expansion/contraction in both the tiles and substrate otherwise what is it for?

 

[Old Mod]

I don't disagree that movement joints are necessary, on the contrary, I believe that they are.
I'm just struggling in my head with the concept of there not being one in the substrate that one in the tiling alone will work.
I appreciate the effort u've gone thro to convince me too! Haha
And for that I thank u SJ
I know I'm also playing devils advocate to a certain extent

I actually came into the post to explain that decoupling alone was not a substitute for a movement joint.

And I only came in to give my solution to height issues! Haha
I did twinge when u posted straight after I don't mind saying, felt like your post was a little pointed! (At me )
Maybe thinking that I thought decoupling was enough.

But we now know that's not true don't we!
Good discussion tho @SJPurdy
What is ur name anyway?

 

[SJPurdy]

So back to the original post:
construct rigid stable18mm ply floor primed, gapped, screw 300c/c etc.
cover with 10mm insul board (eg Wedi fixed to their spec) in tiled rooms and 12mm in hallway; idealy with a 6mm gap between them in the doorways aligned with where there will be a joint in the tiles.
Fit undertile heating wires and cover all with SLC but preventing it filling the 6mm gap.
Cover with ditra mat again leaving the 6mm gap (I think this is optional but it should give better heat distribution and more uncoupling than the insul board alone).
Tile, grout and then fill the 6mm gap with foam and matching sealant.
If it makes it easier cut out the gap in all the layers when you have tiled up to it rather than trying to guesss where it will be.
This way you have the appearance of a continuous floor with the doorway joint a bit wider (6mm rather than the normal 3mm).

 

[SJPurdy]

I did twinge when u posted straight after I don't mind saying, felt like your post was a little pointed! (At me )

I just happened to be on site at that point. I aim to help or add to discussions constructively so promise it wasn't trying to be "pointed"!!
Diners ready now so have to go but I'll be back.
Steve