Natural Stone Limestone over Gyvlon Thermio screed with wet UFH questions - Tile Advice

[Graham Horne]

I was impressed with some of the technical understanding in this forum so wanted to put my question here. The plan is to put limestone tiles into my kitchen. The project is a large renovation and I dont have some of the major pressures as we do not live in this house during the project. Some background: This kitchen area 3.5m x 6.6m (23m2). The subfloor was concrete (old) over which a poly water barrier was laid then 20mm of kingspan insulation, then another layer of poly water barrier, then UHF pipes stapled to the insulation in 2 circuits (1 zone) fitted (spiral in and spiral out patern) 150mm apart. There is an expansion barrier fitted around the perimeter, then 40-50mm of Gyvlon Thermio screed was pumped into the area (the whole of the ground floor also). The screed was poured in early July and the house was then completely closed up for 7 days with no foot traffic, then the house then was opened daily for the rest of the summer. The laitance was not removed at this stage.

The screed sat in that condition for 80 days before the UFH boiler was turned on. The lowest temperature on the manifold mixing valve is 35C and the UFH run for 4 weeks with constant 35C water through the circuits with the house air temperature at 28C and then the thermostats were commissioned. Each zone has a thermostat probe in the screed ensuring the screed never goes above 27C and the air temperature is set to 19C. The use of low temperature UHF water temperature (35C) was the plan hence very close pipes in the install. During this final phase the with thermostats enabled the water was raised slowly to 45C and slowly back down as well over a period of 2 weeks. I think my substrate is in good condition and dry. I have tried the stick plastic sheet down trick with no sign of moisture.

We are now ready for floor coverings and have chosen large limestone tiles for the kitchen area as mentioned. Here come my questions. The laitance now must come off and it was my plan to do that with a large hand held belt sander with a 40 or 60 grit belt attached to proper dust extraction. Will the belt sander be sufficient? I need to prepare that in advance for the tiler.

Many recommend the use of a decoupling layer. The tile company (Stratford Tiles) from where I purchased this limestone have a lot of experience with the product (they import it) and say the decoupling layer is not a must but is belt and braces so it is a risk call. My substrate looks to be good and it seems like it is commissioned pretty well and there are no cracks in the floor. The decoupling layer is a lot of extra cost (product+adhesive+fitting) so is it really required?

I understand the need for the primer and that will be done properly and I think that must be one suitable for the adhesive chosen. That is where the next variation of opinion comes. Some say a cementitious adhesive whereas other say a flexible adhesive. The tile supplier recommends an S2 type adhesive. I have watched BAL's youtube moisture test and recommendations for a similar screed and they recommend their 'Stone and Tile PTB' which is a flexible pourable thick bed adhesive. What recommendations should I follow?

Thanks in advance for your ideas and help.

 

[Dumbo]

Personally I would get a membrane down In
Your kitchen about £ 405supplied and fitted inc adhesive .
With the membrane I would use s2 type is not recommended so would use an s1 type .
I would membrane as £405 compared to doing twice is cheap .
Cementitious adhesive can be flexible , I think your are confused between cementitious and gypsum based.
You can stick the membrane down with gypsum based and then use cementitious on top or make sure your floor is properly primed and use cementitious under membrane as well , I have done this many times without any issue even in my own house on a knauff alpha hemi hydrate screed

 

[Tile Fix Direct]

Alternatively use BAL Flexbone2Easy loose lay de-couplingmembrane to save the stick down costs and some time. Comes with 25 year guarantee.Make sure you put it right way up!

BAL Flexbone 2Easy Floating Uncoupling Membrane Matting (per m²)

Shop BAL Flexbone 2Easy Decoupling Membrane sold per Linear Metre at Tile Fix Direct. See our Range of Uncoupling Layers & Decoupling Solutions Today. Free Delivery Over £375ex

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[Ajax123]

Your tiling company is incorrect in saying that the uncoupling membrane is not a must but is a belt and braces. It is actually an essential part of the system with this type if tile regardless of the screed type. The British stone federation say that for natural stone on heated screed it is necessary to uncouple. In all honesty given the cost of the screed and the cost of the tiles I dont think there is a cogent argument not to use uncoupling for the sake of how little it costs in comparison.

Just using a plastic sheet stuck down is NOT a suitably accurate moisture test. You should carry out either a proper hygrometer test or better yet a carbide bomb test to ensure the screed is suitably dry. Given what you have done sor far it is highly likely that it is indeed dry but why risk it for the sake of doing a quick test. If your tile supplier cant do this (many adhesive manufacturers can test for moisture) then it costs about £30 quid on Ebay to buy a hair hygrometer to do a one off test or about £100 to buy a digital hygrometer which you could then sell on.

After such a lot of work it would be a real shame to spoil the floor for the sake of doing the technically correct testing.

In terms of preparation the screed, as with ALL screeds regardless of type, should be mechanically abraded ideally using a suitable rotary floor sander to remove any surface contamination. This will also remove any laitance remaining in place. Whilst they do some great systems I personally would not go down the BAL route for sticking down the uncoupling membrane as they only use cement based adhesives. These are not chemically compatible with the screed and so present a risk. I would use a gypsum based tile adhesive such as Anhyfix from tile master or something similar. You can use whatever you like on top of the membrane. Another good alternative worth considering especially if BAL is your preferred supplier is their flexbone membrane. As no preparation or sub membrane adhesive would be required.

 

[Graham Horne]

Thanks so much for your replies. The tiler is first priming the floor and then putting down a decoupling layer and then the limestone. You are correct the risk is too great for the difference in cost. There is also the added height of the layers. I know it's not much but it has to be considered.