Limestone over floorboards & old concrete - Tile Advice

[chr15toffer]

I've got a kitchen/breakfast room that is now one room, one half floorboards and one half old, uneven concrete and looking to lay 500 x FL x 15 limestone over the whole lot. I've had one quote just come in where his suggested method is to prime the whole lot and overlay with 10mm tilebacker and then latex level the whole thing and tile on to that. My issue is that this is going to create a massive step as you come into the room: tilebacker adhesive (3mm?) + tilebacker (10mm) + latex (3mm?) + tile adhesive (4mm?) + tile (15mm) = 35mm! In my opinion, this is going to look awful, so I'm trying to find a different way. Do I really need 10mm tilebacker? Could I take up the floorboards and replace with 18/25mm ply, get him to level and tile on to that? Any suggestions greatly appreciated.
Chris

 

[Flintstone]

The correct way to deal with this is to have an expansion joint where the two different substrates meet, have no tile or board bridging that joint as there is a danger of the tiles cracking along that line. Usually a 6mm backer board is used over floor boards, and whatever backer board/insulation board thickness you require can be stuck down on the concrete site to make the levels suit, self levelling can be used if it's needed

 

[J Sid]

as above but take up floorboards and replace with 22mm hardiebacker board rather then ply

 

[chr15toffer]

Thanks, it's starting to make a bit more sense now. It seems like he wants to overboard the whole floor to address the concrete/floorboards joint, but I'm hearing that's a bad idea anyway.
To minimise the increase in floor height, I'd prefer to remove the floorboards, go with 22mm hardiebacker, level off from there and leave a joint where the two floors meet. What would you recommend for the expansion joint? Is it just a case of making sure no tiles straddle it and then use silicone rather than grout in the joint?
Really appreciate the advice.

 

[J Sid]

if you set out so the is a tile joint along the substrate joint that's good and colour match this joint with a neutral cure silicon. Ardex grout and silicone will work.
if setting out works different then cut tile at this point.