Do I need an uncoupling layer over new screed?

[Ajax123]

Hi Ajax

I'm always interested in your posts and read them front to back and again to digest them in detail

Are you saying that if a standard screed was used in this case, say 75mm deep or deeper then as long as you left long enough time to fully dry out and commisioned the UFH properly prior to laying tiles, then a quality polymer modified ady - ie one part flex or standard high resin ady with polymer additive would be all that was needed and no need to use uncoupling mem. ?

I like the advice about good practice - -sound !

Essentially yes. I will qualify that though by saying a correctly used uncoupling membrane can add to the robustness of the system. they are also usefull where softer natural stone tiles are used as these are not capable of resisting som much expansion and contraction as say porcelaine or even ceramic.

 

[giz]

Thanks for all the feedback, thoughts and detail - plenty to consider.
On the plus side, none of this is in place yet so there is time to make adjustments - def worth signing in.
Regarding the depth of screed - this started at 75mm but was adjusted by the architect to make the insulation numbers work with the rest of the build - more celotex less screed.
I've read through all this and also had a conversation with my builder. Taking everything on board we will reduce the thickness of the insulation to 100mm (hidden from the inspectors!) and increase the screed depth to around 70-75mm.
The final level will be driven by the track for the patio doors - build it up and stop at the right level
Interesting comments about the expansion joints too. Is this a by product of the size of area? 9.4m x 3.3m = 31 sq-m (missed the typo in OP).
Will a joint be necessary or just a precaution? I also presume you're talking about a joint in the screed not the tiling, wouldn't a gap be left between the tiles and the walls for this purpose?
And if it makes a difference it's a north facing room.
Given the use of a rapid set screed how long do you think I should leave it before comissioning the UFH and tiling - 28 days as reccomended by Ajax?
I want it to last and work well but I'd also like to have a kitchen sometime in early November - screed can't go down before October 9th.
However, if it needs to wait 4 weeks then so be it.

Thanks

 

[Richard Edwards]

Hi Giz

Sounds like you heading down the right road now and as long at the screed quality is up to stratch and the tiling is to the correct standards you should be ok.

Re - expansion joints, My thoughts are that as the area is only 3.3m wide but nearly 10m long then the issue is the length. It is on the cusp of a joint required. The very broad yardstick amongst the established tilers on here is a 50m2 limit - this assumes its roughly square. Your area is long and narrow and that increases the risk. If you decide to add an expansion joint - say half length, then this must be a full depth 'break' in the screed of approx 10-15mm wide. [Normally made by the screed layer with a piece of polystyrene foam / wood etc and extracted when fully cured.] An expansion joint in the tiled surface must be sited directly over the screed break. Use of a proper expansion joint - Shulter / Butec / Others with grout matching coloured Silicon rubber insert 10mm will be the best - ribs in stainless steel / aluminium [Kitchen is better in SS if your pocket will allow].

Ajax reading this post is the best guy to advise on time of curing / drying before commisioning although the longer the better. I advise my clients 8 weeks and bring the UFH up 5 deg / day and run it at normal op temp for 24 hours and then allow it to cool to room temp - Then we arrive and make a start on the tiling work.

Hope this helps