400 x 400 Tumbled Marble Tiles

[crownford]

I have bought some 55 square metres of 400 x 400 tumbled marble tiles, 50% are nero marquina and 50% are carrara white and will be laid throughout our hallway and dining room on top of wet underfloor heating which is covered (in different areas) by two different substrates, ply and cement based screed. To ensure that the rate of expansion differences between the substrates doesn't cause issues we intended to lay a dislocating membrane between the substrates and the tiles.
I provide these details for completeness but the essence of my problem is that I have had two tilers look at the job and both have different ideas as to how to approach it.
The first thinks that the dislocating membrane is unnecessary and that a good flexible adhesive will do the job well enough. He also says that the stone will require sealing with Lithofin stain stop prior to laying to prevent the grout staining or adhering to the tile face. (a real headache when you consider the amount of tiles).
The second thinks that the dislocating membrane is essential and that the tiles should be sealed with the Lithofin only prior to grouting after they are laid.(much easier of course)
Who is right? They both have good experience and both come recommended.
Many thanks in anticipation of your considered response.

 

[david campbell]

i would differ from both of them and use an expansion joint between the 2 subsrates as well as the membrane,i would seal before grouting and after

 

[merlecollins]

Second tiler mate

 

[The Legend; Phil Hobson RIP]

:welcome:Hi how thick is your ply and is it wpb? A decoupling membrane is always worth thinking about, also expansion joints where substraits meet. Deffo flex adhesive and grout. I would also go with lithofin

 

[The Legend; Phil Hobson RIP]

Sorry guys, my slow typing. One day I will get in 1st

 

[crownford]

Thanks for the rapid responses.
The ply is 18mm WPB and doesn't butt to the adjoining screed as the builder has left a 5mm expansion gap but hasn't installed an expansion joint as such.

 

[The Legend; Phil Hobson RIP]

I know someone will beat me to it, but I would sugest filling sub-floor joint with mastic. Then decoupling membrane divided on joint, then a tile expansion joint. you can get them in all colours

 

[The Legend; Phil Hobson RIP]

Wow I did it, my typing skills must be improving. Hope my answer makes sense

 

[crownford]

:yikes::yikes:Eeek, that will create a joint across the middle of the hallway. The tiles will be laid on the diagonal which will make the join even more unsightly. The reasons for the differing substrates are vaulted ceilings in the basement below running into conventionally joisted, lathe and plaster ceilinged rooms. I believed that the uncoupling membrane could deal with any minor lateral movement. Am I wrong or are you being belt and braces?

 

[The Legend; Phil Hobson RIP]

Sorry mate but you deffo need expansion where two different substraits meet. You can get all types of shades now