New floor substrate

[Brindle]

Hi folks. I've a job to quote for, it's an extension, and the current surface is bison beams and blocks.
There's 200mm to make up before the tiles go down. Customer is wanting to know what to put down next. He's thinking of ending up with a 50mm layer of Marmox type insulation board, which will insulate his electric UFH well.
So he's got 150mm to make up.
Would it be an insulation board then a 2" screed; or would it be some thicker insulation, plus screwed down timber joists - then he could finish off with 25mm Ply, UFH straight onto that?
I did say as long as it's snooker table flat, and no movement that would be fine!

Any advice greatly appreciated!
Gareth

 

[david campbell]

with the bison beam and block there will be a fair bit of movement,most beams are over 3m in length(worked with these with my old man)
my preferred option would to crate a stud as you say with extra noggins and screw down 25mm ply