Electric Underfloor Screed & Tile Expansion Joints?

[DHTiling]

no probs..

 

[Ajax123]

Hi Dingers

Screed looks quite stiff whch indicates Alpha Hemi Hydrate - there are a few of these delivery systems in the UK all working with anhydrite mainly because it is easier to lay because it isn't so stiff. One good thing about these delivery systems is that they use less water in batching than readymix operations because it is used immediately on bnatching rather than having to travel from a batching plant. The insulation looks like a routed styro foam. One interesting aspect is that there is no DPM under the screed which is fairly typical in France and in Holland. The DPM is a must in the UK. You will IMO be far better served using a Gypsum Based adhesive because it will allow the tiled surface to "breath" so that any residual construction moisture does not cause a delamination event. Many of these adhesives (I say many on the basis that there are 3 in the UK but more in France) can be laid when the screed is still relatively wet (usually 85% RH). Make sure the underfloor heating is commissioned first though.

 

[dingers]

Hi Ajax123,

There is a DPM within the slab, I presume this is not the one you refer to though.
Are you able to mention any proprietry brands of adhesives on the forum and there availability?

dingers

 

[Ajax123]

Hi Ajax123,

There is a DPM within the slab, I presume this is not the one you refer to though.
Are you able to mention any proprietry brands of adhesives on the forum and there availability?

dingers

No. DPM on top of slab is important. Concrete is notorious for giving up its moisture very slowly. As there is a DPM under the slab this moisture can only go upwards. It does not mind the moisture in the slab cos the cement will eat it up eventually so putting a DPM under the screed on top of the slab is not a problem. Most addys in the uK are cement based so are very sensitive to this moisture especially as it can cause sulphate migration which can cause ot to delaminate - Ettringite reaction. Up till about 4 years ago this was a common issue in France as it contines to be in the UK. If this happens the addy comes off with the tiles and they become useless.
If you use calcium sulphate addy there is no chemical reaction so you reduce the risks of delamination. The theory is also that the screed can breath through the grout joints thus allowing mmoisture to pass without disrupting the tiles. What you have here in pretty standard practice in Europe and the fact that they use calcium sulphate addy more than we do and dont have issues hence my advice.
Styrofoam is also pretty impervious to moisture so you have a pseudo DPM although probably not a efficient as a 1200g polythene shhet so I think with CS addy you will have success.
Only 1 minor concern is that you would be wise to ensure the screed is plenty dry enough before tiling - too much moisture can cause a failure of the bond between the tile and the adhesive thus popping the tile. I suppose in consolation if this happens at least the tiles are re-useable. I guess though you will be best to make sure it is dry so it doesn't happen.

There are three that I know of in the UK - Benfergyps from Benfer available from quite a few stockists in the UK - I have some names thanks to other forum members but it depends where in the country you are.
Kerrakoll do one but I don't yet know where it is available and the other is GBTA available from one company ion Preston called creative impressions - contact details available via the internet. I am searching for others in order to offer more choice but as yet unsuccessful.

 

[Dan]

Just bumping some threads in the underfloor heating forum. You can now find and search for both electric underfloor heating and water underfloor heating threads. By the way, if you feel some of the threads you want to be seen at the top of the forum need to be bumped back, for the next month or so from now, feel free to do that. Although it's not usually something we allow, whilst we're going through this process of changing over forum software, we see the need for this to happen.