Ive just been to see a 50m2 floor to be tiled in mixed size limestone.
The area is a 7mx7m square room and the screed is Gyvlon with wet uf heating.

I was just going to use ditra mat but wondered if this would be enough, obviously expansion joints would be highly visible due to having to break the pattern.Any suggestions?? (think ive got screed paranoia!!!)

That room size is over the limit for sqr mtrge without a joint..

What you can do is install a cold joint and follow this along the central joint in the middle of the installation..

IMO i would always install an uncoupler on a heated screed...

On another note, how long has the screed been poured and has the heat been commissioned yet..?

Hi Dave, cheers for quick reply. What do you mean by a cold joint? un grouted and silicone?
The screed was only poured before christmas so still a while to go and alot of plastering to do plus commisioning.

Yep.. A cold joint is a matching silicone joint.. much neater on a patterned floor... If you need any bumff on Gyvlon drying etc i have some info you can have..

Ok cheers!
Just out of curiousity what primer do you use on gyvlon/anhydrite screeds? ive been using benferprim which seems ok but have recently used a mapei one called ecoprim i think, this was alot thicker and formed a skin when undiluted.

This is debatable.. what is best,, Epoxy primer is the best route as primers go but the way forward now is to use a gypsum based adhesive.. then there is no reaction between the screed and the cement in the adhesive..

nice one ...will check them out

I worked with a professional French tiler laying a very similar limestone opus floor on a wet u/f heating surface in my own house here in France. But please note I am very much an amateur, so I just offer this for what it is worth.

The two rooms were both around 42 m2 (6m x 7m). The floors have been laid with no expansion joints in the floor itself, just round the edges (where they will - eventually) be covered by skirting boards. Despite substantial summer/winter temperature differences over the last two years it has shown no signs of cracking anywhere. Perhaps it is a little early to tell.

Obviously there were no expansion joints in the anhydrite substrate.

There is a change in the pipeline to the British Standards that will say on heated screeds with natural stone you have to use an uncoupling membrane

Do you buy those at the chemist?