Deffinately seal slate before grouting. What sort of slate is it? Tell me it’s not riven !
Thanks for the extra comments - still reading through them - the slate is polished / honed so is very flat and smooth (much smoother than roof slates). It was from fired earth so cost a fair bit but fortunately not too big an area!
Edit: I'll let you pros
fight it out about discuss what's best, but a couple more things that might be helpful to know is that I've already bought the underfloor heating mat - the wires on it are about 3mm thick.
Also, I'm not too worried about some of the heat going downwards, since the room below is the kitchen, and I've put some mineral wool insulation between the joists to help muffle sound which will insulate a bit too. There will also be a heated towel rail in the bathroom so the UFH is really to just take the edge off the coldness of the slate.
Thanks again for all the comments!
Edit 2: It looks like the height of the floor might not be an issue, since I have to build it up enough to meet the shower tray (20mm) plus 18mm WPB ply to support the shower tray from beneath (I can't put the 18mm WPB ply between the joists since there's an air duct in the way, so I'll have to put it on top of them, then the tray on top).