Tiling Anhydrite screed with wet UFH

[Richard Head]

Good morning, Gents

I'd really appreciate the advice of experienced members here. I've been doing a small barn conversion in Kent for myself for the past 7 years and it's still nowhere near complete. I think I probably have OCD and this is only getting worse.

The footprint of the building is rectangular and 15 x 5m. An anhydrite screed was pumped in over wet UFH in August 2014. The laitance was removed with an orbital floor sander about a week later and the finish is near perfect. Expansion strips were provided around the perimeter only. It has been subject to only light wear and tear since and got splattered with water when the place got plastered out a month ago. I'm pretty damn sure it's now dry as a bone.

There are some stud partitions which are built off the screed so the screed is effectively one big slab which can expand and contract freely. The UFH has only just been commissioned. I'm pleased to say it works perfectly and there is no sign of the screed cracking.

The entire screed is to be tiled with large format (800 x 800mm) porcelain tiles in a chessboard pattern. The tiles cost a small fortune and are now on site. I wish to have tight 3mm grout lines. There will be very few cuts.

Recognising that I'm out of my depth and comfort zone, I've had various local tilers look at the job. One didn't want to know having had a bad experience with this type of screed and suggested I lay carpet instead! Others took advice from their respective suppliers and came up with different fixing specifications and completely contradictory advice. Interestingly, none allowed for gypsum based adhesive. The quotes ranged from high to very high and none would offer watertight guarantees. Needless to say the cost of this going wrong would probably sink the whole project - and me - so failure is not an option!

So, I began doing my own research and amongst other resources discovered this forum. However the sorry truth now is that I'm probably more confused than I've ever been. A little knowledge is a dangerous thing.

My wish is to keep the fixing specification as simple as possible and the simplest I can think of is as follows:

1) Clean off screed and prime with Tilemaster PrimePlus
2) Fix tiles using Tilemaster Anhyfix with 10mm x 10mm notch trowel after back buttering each tile
3) Grout using TileMaster Grout 3000 despite this being cement based (it will stop any free water getting into the bed)
4) Apply TileMaster Silicon 3000 at perimeter.

You'll see that this does not allow for a decoupling or anti-fracture layer. My worry is that this would only introduce weakness and possible new modes of failure. It would also add time and cost and increase the depth of the build up. My simple logic is that if the tiles and screed are all bonded as one, then they will just expand and contract as one? Won't they?

I'd be very grateful for your thoughts and advice on what I'm proposing. There's also a nice job in deepest Kent for someone willing to take it on.

Cheers,

Jonathan

 

[Flintstone]

Jonathan.

It sounds like the plan you have is pretty much correct. Is the screed contaminated and dirty after people have been working over it ?

These screeds expand and contract very little, even with the underfloor heating, so no need the uncoupling, although an expansion joint should be put in half way across the 15m length.

What methods did the other tilers come up with?

 

[Richard Head]

Thank you - some early comfort!

Its become a bit dirty with splashes of coffee and mud from dirty boots etc. I think one pass with an orbital sander and it will polish up as new. Certainly it remains perfectly flat.

The tension amongst the tillers was around what sort of decoupling to use (if any) and how to stop cement based adhesives which they all wanted to use from reacting with the anhydrite by the use of primers or membrane.

Jonathan

 

[Flintstone]

You can use a cement based adhesive, but it requires specific priming to create a barrier between the surface and the adhesive basically. Whilst there is products like anhyfix on the market, there’s no reason to use cement based Adhesives any more.

 

[TheAofT]

Hi Jonathan,
You've actually answered your question and you're in right place to gain some reassurance.
I'm completely agree with localtiler regarding no need to go with cement based route because there are so many specially designed products for that particular screed.
Where about in Kent are you?

 

[Ajax123]

800x800 porcelain...15mx5m...3mm grout joints...heated screed...no movement joints...failure waiting to happen.

 

[Richard Head]

oh - and tonight was going so well!

 

[Richard Head]

My thanks to all who have taken the trouble to reply. In response to AofT, the site is near Sevenoaks in Kent - just off the M20.

Ajax, I've gathered that you are the forum expert here when it comes to this type of screed and I was really hoping you would be able to provide a thumbs up. Alas, it was not to be. I might add that a 'disaster waiting to happen' is perfectly acceptable to me - provided it never happens!

I'd really like to try to understand where you think the risk lies and what I could do to mitigate this. As I said, failure is not an option I can even begin to contemplate here - I think I'd top myself.

I attach a photo of the screed taken just after it was laid and would add some further comments which may be relevent:

1) The screed is a Gyvlon screed and the brand is Thermio+. The minimum cover over the UFH pipes specified by the maker was 25mm. More like 30-35mm was actually laid giving a total screed thickness of about 50mm. One of its key selling points was it's relatively low coefficient of thermal expansion.

2) The design flow temperature for the UFH is 40 degrees C. There are 5 heating loops installed but they are all controlled as one zone. So there shouldn't be any material temperature difference across the screed.

3) The sub base is completely over engineered and probably bomb proof. It comprises 150mm compacted MOT, 100mm concrete, DPM and 100mm celotex boards. No structural movement joints were deemed necessary by the screed manufacturer except at the perimeter, so none were carried up through the screed.

4) I'm aware that Tilemaster recommend movement joints for every 40m2 and I'm looking at a floor area almost twice this. However I don't think these would help unless they were carried through the screed below which wouldn't be the case.

5) I said that "My simple logic is that if the tiles and screed are all bonded as one, then they will just expand and contract as one? Won't they?" I might have added that I'd hope that any differential movement which did occur could be accommodated within the grout lines.

6) I understand that Tilemaster make a latex additive that can be added to make the adhesive and grout more flexible. I wonder if using this in the mix would help at all.

Once again, I'd really appreciate your thoughts on where the risk lies and how this can best be mitigated.

Regards,

Jonathan

 

[Ajax123]

As you have used Thermio+ it would be inappropriate for me to comment on the screed itself as it is not one of my companys products. The main risk however lies in the assumption that the tiles, adhesive and screed will act compositely. They will not. The tiles being a different material will move differently to the adhesive and the screed. The adhesive is mechanically bonded to the primer and that to the screed. The tiles in turn are mechanically bonded to the adhesive. there will be significant stresses on these interfaces. In order to make it work in my opinion you would need joints in the tile face to allow for lateral movement and bigger grout joints to prevent upward tension and tenting which is the biggest cause of failure where I have seen very large format tiles such as these used.
your idea of 3mm grout joints is simply not sufficient in my opinion and the maximum bay length for porcelain on heated screed is about 8m, although with this size of tile I would probably recommend smaller... probably 5m bays.

Use of Tilemaster prime plus and anhyfix is appropriate to this type of screed.

 

[TheAofT]

Hi Ajax,
Is decoupling membrane any help to reduce upward tension and leave 3mm grout joint with 5m bays divided by movement joint?