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
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.
