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Discuss Tiling Anhydrite screed with wet UFH in the Adhesive and Grout Forum area at TilersForums.co.uk.

  1. Richard Head

    Richard Head Active Member

    Location:
    Kent
    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 Silicone 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
     
  2. Localtiler

    Localtiler Professional Tiler Top Contributor

    Location:
    Yorkshire
    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?
     
  3. Richard Head

    Richard Head Active Member

    Location:
    Kent
    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
     
  4. Localtiler

    Localtiler Professional Tiler Top Contributor

    Location:
    Yorkshire
    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.
     
  5. TheAofT

    TheAofT Active Member

    Location:
    North London
    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?
     
  6. Ajax123

    Ajax123 Tilers Forums Pro - Screed Advisor Trusted Advisor Professional Tiler

    Location:
    Lincolnshire
    800x800 porcelain...15mx5m...3mm grout joints...heated screed...no movement joints...failure waiting to happen.
     
    • Disagree Disagree x 1
  7. Richard Head

    Richard Head Active Member

    Location:
    Kent
    oh - and tonight was going so well!
     
    Last edited by a moderator: Nov 10, 2017
  8. Richard Head

    Richard Head Active Member

    Location:
    Kent
    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

    screed 070.jpg
     
  9. Ajax123

    Ajax123 Tilers Forums Pro - Screed Advisor Trusted Advisor Professional Tiler

    Location:
    Lincolnshire
    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.
     
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  10. TheAofT

    TheAofT Active Member

    Location:
    North London
    Hi Ajax,
    Is decoupling membrane any help to reduce upward tension and leave 3mm grout joint with 5m bays divided by movement joint?
     
  11. Ajax123

    Ajax123 Tilers Forums Pro - Screed Advisor Trusted Advisor Professional Tiler

    Location:
    Lincolnshire
    Youd need to confirm that with schluter but I don' think so as ditra is only for relief of lateral stress as far as I know.
     
  12. TheAofT

    TheAofT Active Member

    Location:
    North London
    I did, actually.
    Thank you for your advice.
    What size of grout line would you suggest to accommodate all the tension?
     
  13. Ajax123

    Ajax123 Tilers Forums Pro - Screed Advisor Trusted Advisor Professional Tiler

    Location:
    Lincolnshire
    Closest I can get is 1000x800 porcelain tiles on a job I was involved with last year that failed due to renting. They had a 3mm joint...hence my concerns here. They increased the grout joint to 5mm and as far as I am aware the job is still intact.

    Additionally I would put a movement joint at 5m to split the floor into 5mx5m tile bays.
     
  14. Ajax123

    Ajax123 Tilers Forums Pro - Screed Advisor Trusted Advisor Professional Tiler

    Location:
    Lincolnshire
    What did they say out of interest
     
  15. TheAofT

    TheAofT Active Member

    Location:
    North London
    Mainly it designed for uncoupling or separating the floor covering from the substrate and prevents the transfer of stresses to the tiled surface. In this way, a deformation or crack caused by shrinkage in the substrate is neutralized. The mat bridges the cracks and does not allow the transfer of this energy to the tiled surface.

    Also, mentioned load bearing term as it is important that the cavities in the mat are completely filled. The column-like structure which becomes filled with adhesive provides a sound base which allows applied dynamic and static loads to be transmitted directly to the substrate and makes the mat highly load resistant.

    Basically, accommodates movement, suppresses cracks in screeds and prevents moisture penetration to the substrate
     
  16. hmtiling

    hmtiling Trusted Advisor Professional Tiler JOTM Winner

    Location:
    Isle of Wight
    3mm grout joints, 1 expansion joint(giving sub 40m2 bays) and a decoupler(thin if sub 6mm tile) will be fine in my opinion. Not an expert on these screeds, as Ajax is, but I thought these gypsum screeds had less movement than traditional screeds?
    I'd happily fix decoupler with anyhyfix, switch to cement based on top, incorporate a soft joint(silicone joint as opposed to unsightly expansion joint) and sleep well at night.
     
    • Agree Agree x 3
  17. 3_fall

    3_fall Administrator. Staff Member Trusted Advisor Professional Tiler JOTM Winner Top Contributor

    Location:
    SW London
    Must admit, I wouldn’t have thought twice either @hmtiling@hmtiling, and with the backing of the adhesive company I use, use cement based adhesive. They have no problem specking their products on anhydride.
    In fact just completed a 50m2 floor with 1800 x 900 that was initially a failure due to incorrect prep.
    It has expansion joints at the two doorways.
    It also has ufh which was initialised in the correct manor and is cranked up fairly high.
    It’s a good install and I have complete confidence in it.
    Personally I wouldn’t have dreamt of using 5mm joints, 3mm would have been my choice.
    I’m certainly not trying to butt heads with you Alan, I’m just saying what I’d of been happy with.
    If it is a thin tile, then as Harry says, definitely an anti fracture mat too.
    And if water proofing was necessary, I have complete confidence in a waterproof one that I use also.
    Obviously expansion joints in this situation are a given, agin tho, I’d of only gone for one.
    That would conform to all the relevant information that I possess, so if things in the gypsum screed world have changed, then we as a forum, need updating.
     
  18. Ajax123

    Ajax123 Tilers Forums Pro - Screed Advisor Trusted Advisor Professional Tiler

    Location:
    Lincolnshire
    No head butting going on here... I accept that from a technical perspective you could use 1 joint. In ust think 2, evenly spaced, making 5mx5m bays would make it more robust. My real concern though is the 3mm grout joint. I'e just done some calcs based on the coefficient of linear thermal expansion of porcelain, wich is about 4x that of the screed. Each tile based on a change of temperature from 5degrees up to 25degrees (I.e a cold floor up to an operational surface temp) would expand by 0.64mm so about 0.3mm in each direction. That is 10% of the grout joint width at 3mm which will place huge stress on the joint. I don' know the compressive strengths or coefficient of linear compaction for grout so csnnot say if it will accomodate that level of movement. It does worry me though...

    I guess, having a slightly paranoid streak I just don' want the floor to fail and the screed to get the blame as it often does...
     
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  19. jcrtiling

    jcrtiling Professional Tiler TTA Member Top Contributor

    Location:
    Salisbury
    As regards adhesives I had to take a tile of a alpha hemihydrate the other day . Fixed with cement adhesive . Trust me the adhesive didn't want to leave the substrate .
     
  20. 3_fall

    3_fall Administrator. Staff Member Trusted Advisor Professional Tiler JOTM Winner Top Contributor

    Location:
    SW London
    Totally get the paranoia Alan, non of us want want our product blamed, including workmanship.
    Reading your data certainly makes you ponder the possible outcome.
    Some thin tile companies and Mapei are promoting 1mm joints, I heard it myself only the other night at a seminar. Granted it was on walls, and I appreciate they’re not subject to same direct heat sources generally, (heating walls is becoming more popular, and they’re being used as a primary heat source) but the tiles will still expand, will they not?
     
  21. TheAofT

    TheAofT Active Member

    Location:
    North London
    Hi Alan,
    I've found some info (picture attached) on the compressive strength for Mapei ultra colour grout as an example.
    Unfortunately, couldn't find any data on BAL, Weber, Ultra, and Nicobond to compare but pretty sure they all in the same category.
    In you opinion....
    Would the 3mm joint filled with Mapei accommodate that level of movement.

    Screenshot-2017-11-11 2801-ultracolorplus-uk pdf.png
     
  22. 3_fall

    3_fall Administrator. Staff Member Trusted Advisor Professional Tiler JOTM Winner Top Contributor

    Location:
    SW London
    So I’ve had time to sit, read and digest your post now Alan, and these are my thoughts on it.
    Most of the time we fit tiles at room temperature, typically around 18 degrees, so as long as the tiles have acclimatised, they too are going to be around that temperature.
    So given that the temperature of the tiles is increased to the 25 degrees that you mentioned, as a comfortable underfoot temperature, the actual temperature range is from 18/19 to 25 degrees, in fact only an increase of around 6/7 degrees, how would that effect your calculations?
    Because if they do start at around room temperature, and the floor gets colder the tiles will already be in an expansive state, so therefor it would follow that they would contract, making the joint wider effectively.
    I appreciate I’m kinda splitting hairs a little but it seems that everything is balanced on a knifes edge these days, so it may well come down to being a little pedantic.
    Sod all would get done otherwise. :D
     
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  23. jcrtiling

    jcrtiling Professional Tiler TTA Member Top Contributor

    Location:
    Salisbury
    Really good points raised there but I am not trying to be smart or too pedantic . But most wet floor systems I have worked on have had air stats so to get the room into that comfort zone temperature spread does anybody know what temp the floor slab would be run at .
     
  24. 3_fall

    3_fall Administrator. Staff Member Trusted Advisor Professional Tiler JOTM Winner Top Contributor

    Location:
    SW London
    Alan stated above that an operational surface temperature of 25 degrees.
     
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  25. Ajax123

    Ajax123 Tilers Forums Pro - Screed Advisor Trusted Advisor Professional Tiler

    Location:
    Lincolnshire
    I calculated based on an uplift of 20 degrees from 5 to 25 so you are quite correct that a smaller uplift would lead to less expansion. It would be approximately a third so 0.1mm in any direction. I'm not absolutely certain how this translates to a compressive force so need to think on it more.
     
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  26. jcrtiling

    jcrtiling Professional Tiler TTA Member Top Contributor

    Location:
    Salisbury
    Interesting that i would of thought that porcelain was dimensionally more stable than the screed . Everyday is a skool day
     
  27. Localtiler

    Localtiler Professional Tiler Top Contributor

    Location:
    Yorkshire
    Does a movement joint compensate for tighter grout joints? Is a smaller joint more acceptable with a smaller tile - more joints and a larger joint with large tiles ?
     
  28. Ajax123

    Ajax123 Tilers Forums Pro - Screed Advisor Trusted Advisor Professional Tiler

    Location:
    Lincolnshire
    Yes a movement joint would mitigate some of the risk as it would help alleviate stress by allowing free movement. Yes smaller tiles allows thinner joints as the movement per tile is less.
     
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  29. Ajax123

    Ajax123 Tilers Forums Pro - Screed Advisor Trusted Advisor Professional Tiler

    Location:
    Lincolnshire
    Porcelain has a coefficient of linear thermal expansion of 4 whereas anhydrite is 0.12 both x10 to -6 m/mK based on the engineering values I have. Sand cement is much higher than anhydrite hence the need for fewer joints in anhydrite screeds...
     
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  30. Richard Head

    Richard Head Active Member

    Location:
    Kent
    Thanks again to everybody for their input. I'm glad my post has prompted a debate which I've found really helpful in understanding the issues.

    I am hugely relieved that a decoupling layer is not required - this was going to cause me the greatest headache of all and even more costs. I am perfectly relaxed about putting in extra hard or soft expansion joints and realise now that it doesn't matter that these aren't structural joints carried through the screed. The plan is to go for two, dividing the floor into 3 x 5x5m bays, as Ajax suggested.

    I am inclined to stick to 3mm grout lines for aesthetic reasons and because the tiles are rectified (10mm thick). I had a long and very helpful chat with a technical guy at Tilemaster who was happy with this provided sufficient expansion joints were provided. I might incorporate additional soft joints if I start having sleepless nights worrying about this.

    Ajax, I wonder in the failure case you mentioned whether expansion joints had been provided? and who picked up the tab for the remedial work. I'm guessing it would have been a big number.

    In a similar vein, I wonder whether any supplier of materials would provide a watertight guarantee provided they were happy with the specification. Of course, if the job really did go belly up, the first port of call would be the tiler who did the work but I suspect that wouldn't get me very far. In my experience, the customer always end up paying when things go wrong since it's always deemed to be his fault!

    Thanks again to one and all

    Cheers, Jonathan
     
  31. callatiler

    callatiler Professional Tiler JOTM Winner Top Contributor

    Location:
    Aberdeen
    This thread and others prove that there is still not a definitive code of practice regarding materials and installation onto this screeds and seems to be getting more and more confusing the more it is debated. I’ve only done a couple of them around 50 metres each and BAL came and spec”d everything and installed to the letter as he told me to. Never heard back 2 years and 3 years later so that’s good enough for me and would stick to what I know.
     
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  32. Ajax123

    Ajax123 Tilers Forums Pro - Screed Advisor Trusted Advisor Professional Tiler

    Location:
    Lincolnshire
    My comments are nothing to do with the screed. They would be the same regardless of screed type.
     
  33. Ajax123

    Ajax123 Tilers Forums Pro - Screed Advisor Trusted Advisor Professional Tiler

    Location:
    Lincolnshire
    It was several 10s of thousands of pounds. No movement joints were incorporated into the floor which was about 8mx15 from memory. The builder picked up the initial tab through insurance. He I would think then chased the timer but ultimately its the disruption to the client that was the worst. Always difficult to see things go wrong when someone sinks their heart soul and life saviings into a project.

    Suppliers wont guarantee workmanship or design unless they are directly involved. My company is a supplier. We can guarantee out material and that it will perform in the way we say if it is used in the way we recommend. We provide a huge wealth of information to back up what we claim and so that people can do it right. Sadly there are so many people who "know better"...
     
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  34. Richard Head

    Richard Head Active Member

    Location:
    Kent
    Thanks Ajax and everyone else that has contributed.

    I think we have nailed where the risk actually lies in this sort of job.

    I'm going to have a go at laying a couple of tiles myself to see how I get on. I have just bought the materials and a shiny new notch trowel so what can possibly go wrong?!

    Fortunately I am not under any time pressure and can afford to lose a few tiles if it all goes wrong and I have to rip them up.

    Of course the definitive test will come when the whole floor has been laid and grouted in and its been through a few heat cycles but I'm hoping any problems will manifest themselves before then.

    I have a great deal of respect for the many here who are making a living out of doing this work, doubtless under huge time and cost pressures. Hats off to them - I know I couldn't do it.

    Cheers, Jonathan
     
  35. Localtiler

    Localtiler Professional Tiler Top Contributor

    Location:
    Yorkshire
    Laying a few tiles isn’t gonna be the problem, is it dick? It’s when the whole floor is laid without the correct methods that the problems start
     
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    Last edited: Nov 13, 2017
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