Do I need an uncoupling layer over new screed?

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G

giz

Hi folks,

I'm looking for some thoughts on an uncoupling layer - the posts already here have helped a bit but....

We're looking at laying a new kitchen / family area thats about 34sq-m if size matters.

Here's the detail:
Slurried Block and beam base over a void.
125mm Celotex insulation
Wet UFH in a 50mm rapid set screed.
PEI 5 grade Porcelain tiles on a flexible adhesive.

I want to tile within 2 weeks of the screed going down and I'd like to know if I need to use an uncoupling layer such as Schluter DITRA (is there a cheaper equivalent that's as good)?

Advice and tips gratefully received.
Thanks
 
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A

Ajax123

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giz said:
Hi folks,

I'm looking for some thoughts on an uncoupling layer - the posts already here have helped a bit but....

We're looking at laying a new kitchen / family area thats about 34sq-m if size matters.

Here's the detail:
Slurried Block and beam base over a void.
125mm Celotex insulation
Wet UFH in a 50mm rapid set screed.
PEI 5 grade Porcelain tiles on a flexible adhesive.

I want to tile within 2 weeks of the screed going down and I'd like to know if I need to use an uncoupling layer such as Schluter DITRA (is there a cheaper equivalent that's as good)?

Advice and tips gratefully received.
Thanks
Click to expand...

I would say an uncoupling membrane is essential in this one. This is mainly due to the screed not being deep enough to do its job. Sand cement screeds should be laid at a minimum depth of 65mm when floating. This by virtue of it being on Celotex is floating. The thinner the screed is the more propensity it has to curl and the weaker it is in tension so the more it is likely to crack.

Rapid drying screeds are simply sand cement screeds with an additive which is usually a plasticiser which reduces the water content. To hydrate fully cement needs less water than you might think and a good deal of the water in traditional screeds is there for workability purposes. Typically a rapid drying screed will fry at around 25mm per week however they should be cured first for 7 days following installation. This is more important in these types of screeds because the loss of water when there is already not much ion the period during which the chemical reaction is at its fiercest can lead to drying shrinkage cracking. This may not be immediately visible and often manifests itself more when the heating is commisioned.

The underfloor heating MUST be comissioned prior to installing any type of covering on any type of deep screed. An uncoupling membrane and a green screed adhesive do NOT replace the need for good practice.

Regardless of the type of sand cement screed the underfloor heating should not be comissioned for a period of 28 days following installation. this is based on the rate of gain of strength associated with screeds and testing etc. which are carried out and therefore declared at 28 days age. Comissioning sooner than this could exacerbate cracking and will almost certainly lead to cracking especially given the shallow depth. It is not always about moisture I'm afraid.

Sorry to say but tiling this screed after just 2 weeks is foolhardy and fraught with risk and would force you to operate outside of national standards. Not what you wanted to hear I know but standards are standards.
 
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R

Richard Edwards

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 !
 
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D

DHTiling

Too add to above post.. ( Ajax 123)Schluter do say that their membrane can be used on an Un-commissioned screed floor.. The heating needs to be slowly turned up and the membrane will cope with screed shrinkage and expansion.. I got this verbally direct from Schluter technical..

So we have a few membranes that work in the same way.. so will work for all then,, but expansion joints must be used in respect to BS5385..
 
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Ajax123

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Dave said:
Too add to above post.. ( Ajax 123)Schluter do say that their membrane can be used on an Un-commissioned screed floor.. The heating needs to be slowly turned up and the membrane will cope with screed shrinkage and expansion.. I got this verbally direct from Schluter technical..

So we have a few membranes that work in the same way.. so will work for all then,, but expansion joints must be used in respect to BS5385..
Click to expand...


You are right Dave about Schluter and if the screed was designed and installed correctly I would have fewer concerns. In this case it seems it hasn't. I'd bet my beer that it has no expansion joints in it either.....
 
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Ajax123

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Richard Edwards said:
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 !
Click to expand...


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.
 
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G

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
 
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R

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 silicone 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
 
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