Tiling Using a wet sand and cement screed

[AdamR]

Hi all,

First time poster but been on the site for a while.

Currently on a job where i due to start tiling 100m2 on a floor which currently has a concrete base then plastic piped underfloor heating installed on it. As agreed with the customer and out of choice i have requested that the screeder cover only the pipes and then the Riven slate (12mm average thickness) be layed straight onto a bed of sharp sand and cement with plasticiser and water in.

Now the dilema ive got is the plumber is playing whispering games with the customer and saying that it will not stick and the slate will come up.

Obviously i will be a putting a slurry onto the back of the tiles too, to ensure that it does bond and stick.

Am i cracking up and going mad or is slate actually not suitable for this?

Ive done lots of floors using this method, never had any problems and work with my old man whos been in the tiling game for 30+ years.

Basically im just double checking myself and thought id ask.

All replies much appreciated.

Thanks.

Adam

[timeless john]

Adam - The method you are advocating using would as you know be totally acceptable if your screed was just to the concrete sub floor. My only area of concern is the UFH and when this is commissioned how will you be able to tell the amount of shrinkage taking place in your semi-dry screed until it transfers to the slate surface. With the use of a flexible cement based adhesive onto a fully cured and UFH tested screed, you not only have the increased flexibility, the ability to build up the differentials in tile tolerances but also the bonding between tile and base.
How deep was your proposed screed to be and are you considering a mesh movement layer with this screed?

[AdamR]

Hi John,

Thanks for the quick reply. Much appreciated.

There is a metal Gridding system on the floor which i also forget to mention. This is all screwed down to the floor with each metal square being approximately 200mm from edge to edge.

I was going to use Portland cement with sharp sand and then the plasticiser in this to give it a bit of body and the slurry to then be made from cement and water and effectively painted onto the back of the tiles as laid.

The screeding to cover the pipes is being done by a professional company and i have already instructed the customer that it should be a suitable for UFH.

My screed that i will be laying on will be between 12-20mm thick and the screed for the UFH will be from my estimations approx 22mm thick.

[timeless john]

As I mentioned your materials and method of laying is fine but the heat from the UFH will crack your screed and at 20mm maximum it's too thin and will crumble IMHO.
I'd consider letting the professional screed co. provide you with a finished floor, get the UFH on and tested, supply and lay Devi uncoupling membrane (not 100% required but recommended) and then a thickbed flexible adhesive - which if you collate your slate correctly will allow a minimum bed to ensure solid bed fixing. There are adhesives out there just now that are about the cost of sand/cement per metre but the saving in time makes it a cost effective method of fixing.
Good luck and I know you are going to enjoy it - Slate

[AdamR]

Thanks.

Slate almost as big a bain as Black BalFlex grout.

[timeless john]

1315243337229.jpg Love it.

[AdamR]

Headache material to me.

This is a bit more up my street..............
IMG00070-20111013-1335.jpg

[timeless john]

As I mentioned your materials and method of laying is fine but the heat from the UFH will crack your screed and at 20mm maximum it's too thin and will crumble IMHO.
I'd consider letting the professional screed co. provide you with a finished floor, get the UFH on and tested, supply and lay Devi uncoupling membrane (not 100% required but recommended) and then a thickbed flexible adhesive - which if you collate your slate correctly will allow a minimum bed to ensure solid bed fixing. There are adhesives out there just now that are about the cost of sand/cement per metre but the saving in time makes it a cost effective method of fixing.
Good luck and I know you are going to enjoy it - Slate

This should have been DITRA or similar uncoupling membrane. Devi of course is a range of UFH mats.

[paul flight]

after screed has dried the prime screed them fix a decoupling matting ( ditra) fix slate with a thick bed flexi adhesive . then seal. then grout with flexi wide joint grout. seal again and polish
job done

[garythetiler]

Adam I have tiled many floors such as you described at the same bed depth the only difference is I used sbr bond through the sand and cement and sbr bond in the slurry the only job I have had to return to is one where the designer refused to allow an expansion joint and we had some tile crack which we removed and replaced adding the appropriate expansion joints