Unfilled Travertine?

[Old Mod]

20mm, that why it cost so much, you get twice as much stone compared to a 10mm one.

I'm not arguing I've only ever fitted the one natural trav! Haha

 

[Dhtiling]

Never had an issue with the thinset coming through, but I guess a very bad stone could be open enough for that to happen. I would use white as said earlier, and if you have a grout color already chosen, You could color your thinset in case you are a little sloppy on clean up and if it comes through a little.

 

[Rich Midge]

After you have fitted ufh I would use Ditra uncoupling membrane then
Lay them as normal paying special attention to back buttering the stone and filling ALL voids. Some my come though to the face, don't worry, just wash off when laid
Once laid clean them and allow to dry completely.
Give one coat of sealer, allow to dry.
Grout, filling all joints and holes in tile, wash off.
Leave to dry for a copy of days and seal again.

This is all you need to know! I couldn't tell you how much trav I've laid, we were getting through pallets and pallets of it every month a few years back and it's always laid to the exact method as detailed above.