Help!: Hyflow screed / underfloor heating/ limestone ....... URGENT HELP BEFORE THURS 23rd

[njtkc]

NEED SOME URGENT ADVICE.
we are building a house and hAve planned to have HYFLOW LIQUID screed on underfloor heating which is due on thurs 23rd sept....../ the finished floor planned is limestone.
We have just spoken to a tiler and he has experienced issues with this gypsum based screed. His concerns: bonding of the limestone as this is ground with a plentiful amount of water this may cause the limestone to lift as the hyflow does not like too much water.
He has experienced water on this screed after porcelain tiles had been laid where there was a leak of about half inch of water covering the finished floor and all the finIshed floor covering had lifted. the consequence was having it relaid at the expense of a lengthy insurance claim. AlSO he has had negative feedback from other tilers.
WHAT VIEWS HAVE YOU ALL GOT........... on the screed with limestone or otherwise.
thank you for any help

[Ken Bruty]

Firstly, welcome to the forum njtkc,

Bardon highflow screed is designed to take all modern floorcoverings, over ufh, and, provided that the correct prepping has taken place and the right products have been used for tiling onto a Gypsum based screed, there should be absolutely no problems at all with it. That incident mentioned to do with the insurance claim was not to do with the type of screed, more likely a freak fault with one of the materials or fixer error imo Also, limestone should not be fixed directly to an underfloor heated screed, gypsum or otherwise, it's asking for trouble from my personal experience, a decoupling membrane should be used.

[Ajax123]

There are lots of scare stories about tiling onto gypsum based screeds......beleive me I hear them all the time. However as Ken says if things are done correctly then there is no reason to worry. Bardons are using an "old" anhydrite formulation in some of their plants and hemi hydrate in others. It would be worth finding out which they intend to supply in this instance as the prep would be slightly different for each. Both will need to be sanded but the anhydrite would need to be sanded to remove the friable skin that appears on the surface whereas the hemi hydrate tends to be skin free and only needs sanding to clean it up just before tiling.

Make certain that the underfloor heating has been comissioned prior to the tiles going down.

Make certain the screed is dry to below 75% relative humidity if you intend to use a cement based adhesive but I would seriously consider using a gypsum based adhesive due to it's chemical compatibility. The benefit of using Gypsum is that the level of thermal expansion and contraction is the same as the screed and the limestone also being a natural stone is likely to be similar. This reduces the risk of mechanical delamination from the screed. Alternatively you could use uncoupling to do the same thing. BAsed on limestones propensity to be soft it could crack due to thermal action so uncoupling is a realistic option.

One really important bit that people often miss out is the requirement for joints in the screed. These shoudl be placed at the very minimum through all door thresholds and between independently controlled heating zones. These should be full depth isolation joints placed prior to screeding using preformed joint formers. Additional joints may be needed in long thin areas (aspect ratio should not exceed 8:1) and where there are abrupt changes to aspect ratio e.g. L shaped rooms.

You are unlikely to need bay joints as this type of screed can be laid in single zone bays of up to 300m2.

95% of the issues I see are due to application errors such as failing to comissione the underfloor heating, not enough primer, incorrect bay sizes, no joints in the tile face. Using either Gypsum adhesive or uncoupling will help install robustness against all of these issues significantly reducing the risks.

[njtkc]

Thankyou very much to you both for your detailed replies it has made me feel much better.
I think my tilers concern is the amount of water it can take after it has dryed, as he explained that the limestone we have talked about is not pre-prepared and will need grinding with a machine that generates large amounts of water. We want the effect of a smooth floor like they do with marble in places like spain. The limestone being very porous absorbs the water. The insurance claim he had to fix (he didn't lay the original floor) they had all lifted from due to a water leak under the sink where they had about 1/2 inch of water on them for a very limited time (a few hours) and according to the builder the correct adhesives had been used, the whole floor had to be dug up and redone from scratch? This is also a neighbour of ours so as you imagine we are a bit scarred of spending all this money and we want it to last.
I am not sure what uncoupling is?
Any other comments appreciated.

[Ken Bruty]

The uncoupling is a mat (plastic and felt) which is adhered directly to the screed, and allows the subfloor to move underneath without affecting the tiles adhered directly on top of the uncoupling matting, e.g if the screed were to crack at any point the matting would take up the movement and leave the tiles unaffected on top. I hope that clears that up for you. It's a particularly important process with natural stone floors on ufh because stone does not like any kind of lateral or vertical movement, particularly soft stones like limestone, travertine and marble for example. I have seen and dealt with many problems associated with installations without uncoupling matting, and it isn't pretty.

[ozboz]

De coupling is basically a membrane between the substrate and the selected cladding, in your case Limestone on screed, the de coupling membrane is adhered ot the screed, and then the limestone is adhered ot the membrane, I am not to sure if we should nominate adhesives here , but there are adhesives and adhesives, also there are different de coupling systems, the one I prefer is economically advantageous and there is no difference in quality, I recently used it on an Architects offices at Canary Wharf on a raised access floor. If you ring Tech services ( 01215086970 ) they will give you the exact spec for what your req'ts are. Also , heres something to consider, there are companies that will do the ' grinding' as you call it with machines that are used dry and are totally dustless, totally more suitable. especially in domestic situations. I see them quite regularly doing this and does give an excellent finish. They will finish the floor either highly polished, or as most limestone floors they will hone it, but you will need to contact them, I can get you a number if you want, any questions I am always available on 07824365234 Ok Mate Steve aka ozboz

[Ajax123]

Thankyou very much to you both for your detailed replies it has made me feel much better.
I think my tilers concern is the amount of water it can take after it has dryed, as he explained that the limestone we have talked about is not pre-prepared and will need grinding with a machine that generates large amounts of water. We want the effect of a smooth floor like they do with marble in places like spain. The limestone being very porous absorbs the water. The insurance claim he had to fix (he didn't lay the original floor) they had all lifted from due to a water leak under the sink where they had about 1/2 inch of water on them for a very limited time (a few hours) and according to the builder the correct adhesives had been used, the whole floor had to be dug up and redone from scratch? This is also a neighbour of ours so as you imagine we are a bit scarred of spending all this money and we want it to last.
I am not sure what uncoupling is?
Any other comments appreciated.

This in my opinion is falacy. The limestone would not have absorbed enough water in this time to cause a wide spread delamination event. It is likely that this is a folk tale which has been exaggerated out of proportion. It is probably that the screed has been easy to blame - this happens a lot. Even if the tiles did delaminate this would in no way justify taking the whole floor up. These screeds are perfectly ok to absorb water provided they are subsequently dried. I have studies on behalf of most of the major suppliers and from independent universites which have been carried out on these screeds over the last 60 or so years to determine what happens when they get wet. The answer is very simple - they get wet.

Once it has hardened, if a calcium sulphate screed is subsequently fully saturated by ingress of water it will have an impact on the mechanical strength of the material such that it will reduce in strength to that which it acheived at approximately 3 days. That will equate to approximately the 65 to 75% of it's optimum strength. Let us say it loses a third of its strength. In terms of compressive strength this means it will revert from around 30Newtons to around 20Newtons. A semi dry 4:1 sand cement screed which is hand compacted will achieve around 10Newtons. If it mechanically compacted it may acheive 20Newtons (althoguh I acknowledge that some will acheive higher strengths). The point is even a fully saturated calcium sulphate screed will still be strogner in both compression and flex than a sand cement screed which is 28 days old. The issue of course is that the floor coverings will potentially fail if the screed is saturated.

If you are to tile this screed it would be well worth a call to Creative Impressions about their adhesives. I know they are working on susceptibility of the bond strength to moisture the idea being to offer a gypsum based adhesive which will give a permanent bond regardless of water ingress. Not sure where they are with this at the moment.

It is far more important to ensure that the design is optimised to ensure that the benefis of using the screed can be exploited without falling into the problems. This means double membrane systems using thin screed sections. for example a flooor build up should be something like

Concrete sub floor under 1200g polythene membrane under insulation (depth adjusted to maintain minimum scree depth) under minimum 500g polythene membrane under 15mm underfloor heating pipe under minimum 45mm (Nominal 50mm) screed.

Uncoupling is optional but as it is a natural stone I would not discourage it at all.

[njtkc]

Brilliant Ajax 123 thank you so very very much to everyone that has answered. I am feeling much happier about this now, our floor structure is DPM, then 150mm concrete, 80mm insulation, 10mm perimeter strip, 500g polythene, 50mm hyflow. So just have to make a decision on exactly what to put on top.

[Ajax123]

Brilliant Ajax 123 thank you so very very much to everyone that has answered. I am feeling much happier about this now, our floor structure is DPM, then 150mm concrete, 80mm insulation, 10mm perimeter strip, 500g polythene, 50mm hyflow. So just have to make a decision on exactly what to put on top.

If there is no DPM on top of the concrete and you have no facility now ot put one in then I would strongly recomend either GYpsum adhesive or uncoupling. The issue here is that new concrete takes ages to dry out. Whilst the surface of the screed is open the screed will appear to be drying. When you then cover it with tiles the action of the heating will over a period of time equillibrate the oisture throughout the system. This can take quite a long time and you will not see an issue for quite a while but if no DPM is present then I can almost guarantee that at some point in the future you will see some issues. This DPM is really really important in heated screeds. People usually say "well there is a membrane on top of the insulation". This is firstly only 500 gauge and there fore not a DPM but a vapour barrier which will allow the migration of moisture. Additionally it is punctured at regular intervals by underfloor heating clips so is no longer a DPM but a seive.......The moisture will NOT cause a problem with the screed but it may well interfere with the bond between the screed and the tiles unless one or even both of the above precautions are taken.

If on the other hand you have the option to put in a DPM under the insulation and on top of the concrete I would urge you to do so.

[njtkc]

The concrete was laid over 14mths ago and the building has been watertight for 7mths. the DPM is under the concrete and due to the length of time I am assuming and hoping that it is thoroughly dry. The floor had to be finished last night ready for the pipes today and the screed tommorrow. We won't be laying any floor covering until at least January so it will have a lot of natural time to dry. Thank you for all your advice, I just wished i found this site a little earlier then I may have been able to make some changes.

[njtkc]

The Hyflow has been successfully pumped in on Friday we are waiting until tommorrow, Sunday before we go in, but it seemed to go smoothly so lets hope the rest does too.