Floor part heated

[Old Mod]

Maybe I'm reading this wrong or have missed something.
Why don't u use 6mm boards under heating then outside of heated area use 10mm back fill lower areas with SLC. Then the floor is decoupled too.

 

[aaronhaywood]

Yep that's what I'm gunna do, dident realise you can get the boards in different thickness - will order from tilefix direct

 

[SJPurdy]

decoupling works in conjunction with movement joints, it is not a substitute for them.
Consider a room that is heated so that the floor area expands slightly, the decoupling allows the floor surface (tiles and adhesive) to expand at a different rate to the substrate (still colder under the insulation) without putting a massive shear stress on the adhesive. This expansion is taken up by the compression of the perimeter movement joint (even if its only an air gap covered by skirting). It is also taken up by the compression of the movement joint strip in the doorway otherwise all the tiling in the hallway/next room would be pushed away from the heated room.

 

[Old Mod]

decoupling works in conjunction with movement joints, it is not a substitute for them.
Consider a room that is heated so that the floor area expands slightly, the decoupling allows the floor surface (tiles and adhesive) to expand at a different rate to the substrate (still colder under the insulation) without putting a massive shear stress on the adhesive. This expansion is taken up by the compression of the perimeter movement joint (even if its only an air gap covered by skirting). It is also taken up by the compression of the movement joint strip in the doorway otherwise all the tiling in the hallway/next room would be pushed away from the heated room.

That all makes perfect sense and is a great explanation!
I avoided the subject of movement joints for one very good reason.
I was always taught that if there is no expansion joint in the substrate then inserting one in the tiling is pretty much a waste of time, and that is the rule I have always followed.
Now many years on, I still struggle with the concept that if you put an expansion joint in tiling, (without there being one beneath) that it will be of any benefit.
The way I reason this is, if for example the substrate is continuous through a doorway for instance (seems to be relevant) and you have expansion from a warm room to a cooler one, the warm room cannot push toward the cooler one because there is no expansion.
So which way will it go? To my mind there is only one way it can go, up!
And no expansion joint in the tiles is going to stop the floor from effectively 'blowing'
For it to work effectively surely there must be one in the substrate too?
I'd really like to know the answer to this because it's baffled me for quite a while now.
Thanks.

 

[SJPurdy]

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.

 

[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?🙂