Showerstone / Plasterboard / WBP Ply / Plastered Wall Tanking

[jimmycooker]

Hi,

I have a wetroom 1m x 5m.

There is a 1m x 1m marmox showerstone tray fixed directly to joists at one end and the rest of the room is floored in 18mm WBP ply.

There is one plastered block wall, the other walls are water resistant plasterboard with one wall of the shower area in 12mm WBP ply.

I have a couple of questions:

1.) The floor plywood is 4mm lower than the showerstone tray. What is the best way to raise this up 4mm so it is flush with the tray?

2.) How do you tank the showerstone tray? Do you just tape a tanking mat like kerdi ditra to the showerstone tray or do you tank with ditra all the way to the drain? Also, do i have to tape up the gap between tray and plywood with something?

3.) The shower has 3 sides (plastered block wall / Plasterboard / Plywood). The plywood is around 1m wide on one of the 5m walls so is joined up with 4m of plasterboard. Kerdi say you can use their matting if there is no joint between sheets. What about a ply / plasterboard joint?

4.) What are the best tanking materials can be used on plastered block wall / Plasterboard / Plywood / showerstone tray?

5.) Where is the best place to buy?

Thanks,
James.