Advice on preparing to lay my slate tiles please

[Rf80]

Hi all, great forum - I have searched ate rough a lot of threads and found most of the advice I need, just a few things I am not sure of.

I am preparing to lay 600x600x10 calibrated slate tiles in our kitchen, dining room and utility. It's 50m2 in total. The existing floor is a concrete screened block and beam in the kitchen/dining room and a solid concrete base with 100mm insulation under 50mm screed in the utility. I plan to fit 6mm insulation boards, electric under floor mat, covered by SLC.

My questions are:
1. The kitchen already has a very solid porcelain tiled floor, can I board straight over this with the insulation boards or do I have to chisel it all up ( I have worked out that I can deal with the level difference with a double layer of the insulation boards in the dining room)?
2. The slate supplier has supplied a fast set 1 part adhesive ( flexi) for the tiles, can I use this for sticking down the boards?
3. I have got an acrylic levelling compound to cover the heating wire. The floor is already fairly level. I am assuming I can't walk about on the screed with spiked shoes in order to trowel out the SLC level so I have bought a spiked roller to help spread out the SLC as I go. I was planning on getting some mixing helpers in to keep the SLC coming as I poor it due to the size of the area, also i know from a small wetroom i did recently that covering the heating mat 'eats' the slc so am getting plenty of bags in ready! Obviously with a 600x600 tile i need to get it flat. Does this all sound ok?
4. I was planning on a 5mm tile spacer for the slate, would this be usual for this type of tile?
5. And finally what sized notched trowel should I use for the adhesive on the boards and tiles, 10 mm?

Sorry for the long list, but the materials for this job are expensive and I want to get it right!

Thanks for any advice you can give me, I have previously tiled a few floors and walls but this is my biggest job so far!

 

[Time's Ran Out]

All sounds ok to me but the only issue will be how 'very solid' the porcelain tiles are! And only you can ascertain that.

 

[Peter]

Personally, I'd be enclined to lift the existing tiles. Better not using a spacer either. Even though they're 'calibrated' you're better doing slate by eye as there will be a bit of sizing in them. Would do no harm putting an expansion joint in between the kitchen and dining room where possible.

 

[Ajax123]

What sortsof screed is it. If it is sand cementitious insulation 50mm is much too thin. Tis is asking for future troubles. The minimum depth required for floating is 65mm and the recomendation of the British standard and all reputable manufacturers is to aim for a design depth of 75mm. It should also be reinforced with either poly fibres or d49 steel mesh.

 

[White Room]

What sortsof screed is it. If it is sand cementitious insulation 50mm is much too thin. Tis is asking for future troubles. The minimum depth required for floating is 65mm and the recomendation of the British standard and all reputable manufacturers is to aim for a design depth of 75mm. It should also be reinforced with either poly fibres or d49 steel mesh.

What is that, not one I've heard of...or is it the standard steel for concrete.

 

[Rf80]

Ok, thanks for the advice so far.Not sure what type of screed is in the utility, I saw them laying it and it came off a concrete mixer truck which added the fibres as it poured. 50mm was a bit of an estimate as I cant really measure it now it's down - I guess it could be 65mm ( hopefully!).How does an expansion joint work, do I fill the gap between the tiles with mastic rather than grout?

 

[Ajax123]

D49 mesh is basically what people refer to as chicken wire but instead of being hexagonal it is a square mesh. Most people use pp fibres these days. It is 2.5 mm wire crossed at 100mm squares. I dont see much mesh these days but it is still a valid type of reinforcement for screeds.