Its probbaly your best move unless you want to insulate the floor in which case maybe 2 layers of 6mm would give you a solid base.
Personaly I am a ply person and think weyroc & ply is ok to overheat which would make the previous post ok.
Existing weyroc
12mm ply
3mm Durabase CI mat
heating cable (not mat)
**Around 18mm to here** If you want to drop to 9mm ply then you will need to neat unibond and screw it to the weyroc
What limestone is it m8
tiler
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