Do I need an uncoupling membrane? - Tile Advice

[Stevoe]

Hi everyone, I am in the middle building an extension, the outer shell is up and most of the windows are fitted. I have laid 75mm insulation on the concrete then 300 micron dpm then warm water ufh pipes (holding pressure nicely). I am now ready to flood the place with a self levelling liquid anhydrite screed to a depth of 65mm. The area totals 60 square meters and is four rooms, kitchen (former garage) work room (nearly a new garage) utility room and downstairs loo. The biggest room is 22 square meters and I will be fitting porcelain tiles throughout all 4 areas. My question is should I fit an uncoupling membrane (homelux, Ditri) over the screed before tiling or will flexible grout be enough, my understanding is that the screed expands and contracts at a different rate to the tiles but as it is a water system the movement won't be too much. The UFH manufacturer says the membrane is optional, Topps Tiles salesman says I must fit it and and an independent local tile merchant says flexible grout will be more than good enough. I don't want to spend £400+ on matting + grouts if it isn't needed. Any avice would be appreciated.

 

[Time's Ran Out]

Stevoe - I've yet to see a screed that has not cracked with UFH ad the question is whether the flexible adhesive will be enough for the movement especially with the strength of porcelain tiles.
You will obviously test the UFH as advised by the manufacturer and allow to cool down/switch off.
IMO it would be the perfect time to ensure a stress free floor by using the de-coupling membrane and although an initial expense will prove to be money well spent.

Timeless John.

 

[faithhealer]

I would say use a membrane but wait until the lads get in from work tonight and see what everyones opinion is.
Give Wet Decs a call today, if you need it it'll be cheaper than topps! (mention the forum)
Wetdecs wet room supplier, wet room shower tray, tanking. waterproofing from leading wetroom company.

 

[larmar]

I would go for the matting worth it in the long run ,no probs ,still use your flexi addy and grout

 

[Stevoe]

Thanks, what a swift response, Matting it is then, Topps want me to use Homelux, Nu-Heat (UFH) suggest Schluter Ditra, what adhesives would you suggest.

 

[grumpygrouter]

If it was me, I would invest in the membrane. Do it right now and save the potential grief. Take note that Anhydrite needs to be prepped properly if you are using a cement based adhesive. Latence needs removing and it needs to be correctly primed.

Oh and it would be Durabase or Ditra for me. Do homelux do a decoupling membrane?

 

[Stevoe]

Maybe the Topps chap was talking ballcocks, will look for some detra.

 

[larmar]

Ditra mat for me

 

[grumpygrouter]

Maybe the Topps chap was talking ballcocks, will look for some detra.

Send Tony at Wetdecs our sponsors a PM. He does Durabase which is similar to Ditra and he will post it out to you!!

 

[Ajax123]

I don't think you will have much choice but to use a decoupling mat. Who drew up your floor spec? It appears to be defficient by one DPM and by the sound of it it is missing some joints as well. If you stick your tiles straight down I think they will fail.

The spec should inlcude 2 polythene membranes. One, DPM grade (1200g), over the concrete base and under the insulation the second, minimum 500g, over the insulation as a slip membrane. Problem with your set up is that over time the concrete substrate will give up its construction moisture very slowly. This will rise through the floor as it can't go downwards because presumably there is a DPM there. A DPM on top of the concrete will trap this moisture which will, as it is cement based do no harm whatsoever. The moisture will over a long period be used up in the hydration reaction with the cement and the rate at which it does penetrate the DPM will not be fast enough to cause a problem. with no DPM it rises through the insulation layer and hits what was a DPM but is now, thanks to the numerous penetrations by the underfloor heating clips, a seive. This moisture holds the potential to disrupt the adhesion of the tiles.

Additionally there should be joints in the screed running across door thresholds and also to separate any independently controlled heating zones. Bay sizes in this instance fall well within the maximum sizes recomended either by the anhydrite manufacturers or by the British Standards so at least that is not of concern.

At 60m2 the level of thermal expansion is likely to be negligible in the screed.
It is fair to say however that the tiles and adhesive will expand and contract at a different rate thus presenting a potential mechanical stress at the interface. If this interface is weakened by the presence of moisture your tiles will pop.

If he was saying that you need a decoupling mat just because it is anhydrite, then the chap from Topps Tiles was talking "Ballcocks". A decoupling mat is not essential on anhydrite screed. Provided it is designed and installed correctly and you use the right materials over the top it is not a problem sticking directly to it
However if he is more astute than I give him credit for and he was referring to the specification then he was absolutely right.

The adhesive over the decoupling mat is largely irrelevent to the screed type as it is completely divorced from it. A flexi adhesive will be suitable. It does not generally matter if there is some localised delamination under the decoupling mat but obviously it should be stuck down fully so I would be looking at a water dispersible epoxy primer and a flexible cement based addy in this particular case.

Also your screed is deeper than it needed to be. In most underfloor heating systems you only need 30mm cover to the pipes hence 50mm nominal is usually sufficient. The extra 15mm equates to 30days extra drying time.

Don't forget that you need to comission the underfloor heating system before you start to tile. It is not really likely that it will crack but if it is going to do so it will do it at this point. You don't want your tiles in place if it does crack. It is most likely to go at door thresholds hence requirement for joints.

Homelux, ditra, g-mat and durabase all perform exactly the same function. use the one which offers you the best value for money. Topps are presumably not offering you a guarantee if you use Homelux in preference to other mats.

As Grumpy says PM wetdecs cos he supplies several different ones.

 

[Stevoe]

Wow Ajax123, very comprehensive, clear and unambiguous, thank you. I have had great difficulty in getting anhydrite screed in Lincolnshire, the only company that I could find did not want to supply it as my builder is not an registered liquid screeder (they said it was a government requirement?) and probably because I am doing it myself too. So I consulting Nu-Heat (UFH supplier) they agreed that it would not make much difference to put a normal cement screed down though the liquid would be better, so the concrete supplyer is supplying normal screed (stuff with fibres in) the UFH supplier said I should use a minimum of 65mm too and that is achievable, the UFH pipes rest in clip-tracks that are stuck down, there are some staple type clips too but not that many, they do puncture the dpm that I placed ontop of the insulation but sit flush. The kitchen was a garage so the concrete there is 15 years old, we know there is dpm and insulation already, I have put further insulation and a further dpm before the cliptracks and pipes, I will have to take the risk of tiles popping the risk seems slight from what you have said. The other three areas are newly concreted (December '08), there is a dpm 1200g Visqueen beneath the concrete, again I am going to have to take the risk of moisture entrapment and because the risk is slight and because everything is ready, the screed arrives thursday, there is no time for re-work.

After your observations maybe the Topps chap was looking at the Homelux as a vapour barrier more than a decoupler, he showed me a cardboard box lined with Homelux filled with water (even had a duck in it) to show how good it was.

Excuse my ignorance but what is "addy" I assume it is a term for adhesive/cement and does this mean I have to prime the screed too. If so my understanding is that I need to prime, then flexible cement (I have Ultra Pro-Flex SP for one of the areas supplied with the tiles) then matting, flexible cement then tile and grout. This would be after the screed is dry and the comissioning process has been completed of course.

 

[Ajax123]

I have had great difficulty in getting anhydrite screed in Lincolnshire, the only company that I could find did not want to supply it as my builder is not an registered liquid screeder (they said it was a government requirement?)
After your observations maybe the Topps chap was looking at the Homelux as a vapour barrier more than a decoupler, he showed me a cardboard box lined with Homelux filled with water (even had a duck in it) to show how good it was.

Excuse my ignorance but what is "addy" I assume it is a term for adhesive/cement and does this mean I have to prime the screed too. If so my understanding is that I need to prime, then flexible cement (I have Ultra Pro-Flex SP for one of the areas supplied with the tiles) then matting, flexible cement then tile and grout. This would be after the screed is dry and the comissioning process has been completed of course.

It sounds to me like you have been given quite a bit of bull**it on this one. If you will forgive the term of phrase
There is no such thing as a "government requirement" that you have to be a registered installer with anhydrite. That is complete and utter nonsense. If you need some contacts for installers in your area I will be very pleased to talk to you. Send me a private message or ring me at work on 07545 932723. Lincoln is not over run with installers but there are plenty about and they are as desperate for work as tilers at present so am sure would be grateful for the lead. Also the more established screeders tend to travel. I know of 2 in Lincoln, 1 in Boston, one in Scunthorpe, several in Nottingham and Mansfield so there is no shortage.

Obviously if you cannot turn back from sand cement then the decision is made but obviously for future reference.....

Anhydrite is available from Tarmac as Truflow and from Lafarge as Agilia Gyvlon. It is true to say that they will only sell it to people who have been on a training course at present (which incidentally I run) as it is important that the people that install these things know what they should be doing. It is equally fair to say however that they are both resonably comfortable in training anyone who is interested although they always have to balance demand and numbers and also have to take into account the cost versus benefit. If you are doing this as a one off I would think you would be better to use a previously trained installer anyway. You don't want it as a practice ground. Lafarge run an approved user scheme specifically for builders and developers who do not wish to go outside of their business to sub contract. It would be unusual for a self builder to take this on themself but it is by no means unheard of and some have actually gone into business as a result of training.

65mm sand cement on insulation and underflor heating will crack so you need tom make sure you joint it adequately to control the cracking. The advice of the NHBC is that is should be split into 15m2 bays or room size whichever is smaller with an aspect ratio no more than 2:1. There should be full thickness isolation joints across door thresholds and between areas of differential temperature e.g. if your lounge is split into two separately controlled heating zones it needs a joint or if you have a conservatory you will need to separate it from the lounge because of the level of thermal gain. Finally you need joints at areas where there are large changes to aspect ratio e.g. if you have large room that is semi split with a double door, or if you have a L shaped room there should be a joint. All of these joints are best placed into the screed during installation as they should be full depth and filled with compressible material such as etherfoam. They would normally need to be between 5 and 12mm wide. All joints should relfect through your tiled surface even if you use a decoupling mat. You can use the decoupling mat as a waterproofing barrier but any joints should be sealed (in the case of Ditra with a Kerdi Strip joint) It will also need to be reinforced. You will probably be best served using polyproylene fibres for this as the distribution through the mix is better than with d49 mesh. The fibres will usually be added at the batching plant for readymixed screeds.

I am obviously sorry you have been unable to use flowing on this one but hopefully if there is a next time you will consider it again.

Oh by the way - addy - short for adhesive. You will need to prime the surface of the screed before applying flexible adhesive. A good quality acrylic primer will be fine (not PVA) you then lay the matting and then tile on top of it again with flexible adhesive. Schluter have a video on their website of their stuff being laid if it is any help

 

[Stevoe]

Ajax123,

Many thanks for all of that very valuable information, the screed is bought and paid for through the suppliers now and the workforce is prepped and waiting, so I really don't want to stop that snowball, I do wish I had sought advice earlier as the self levelling stuff just seems to make more sense but as my very astute wife tells me "prior planning prevents pi*s poor performance" and it look as if I will hearing that again.:mad2:

I will take a little extra time and seek advice from the forum for the walk in shower (next project) but until then we are limbering up for a day of hard graft screeding.

Regards

 

[Ajax123]

Ajax123,

Many thanks for all of that very valuable information, the screed is bought and paid for through the suppliers now and the workforce is prepped and waiting, so I really don't want to stop that snowball, I do wish I had sought advice earlier as the self levelling stuff just seems to make more sense but as my very astute wife tells me "prior planning prevents pi*s poor performance" and it look as if I will hearing that again.:mad2:

I will take a little extra time and seek advice from the forum for the walk in shower (next project) but until then we are limbering up for a day of hard graft screeding.

Regards

Best of luck. Perhaps some pics posted of the finished job might be nice

 

[DHTiling]

The homelux mat is simply a tanking / anti fracture membrane and NOT an uncoupling membrane.