in my case I went 25mm over the tops of the joists which gave me 35 to 40mm between the joists. i could do that as it was my own house but from a technically correct point of view it should have been another 10mm thicker. The joists themselves are original so are around 150 years old and are 4x2 inch at roughly 400mm centres (varies joist to joist) with a span into the bay window of just 2.2m.
They were here quite bouncy when I took the floorboards up so to add rigidity I used the sterling board as a "flat noggin"
The sterling board is screwed to 19x35mm tile batten which is in turn screwed to the sides of the joists. This actually adds very very significantly to the rigidity of the subfloor to remove as much bounce as possible. The undersides of the joist remain open so that the wood can "breathe".
The 500g polythene is there to act as a slip membrane and to "tank" the floor and the screed is Gypsol TimBRE screed which weighs in at 2tinnes per cubic meter so 20kg per square meter per 10mm depth.
It works really really well and because I used anhydrite it warms up very quickly..... Typically takes less than an hour to warm the floor. Keeps me toes nice and toasty....
