"Martin Bonner" wrote in message
ups.com...

On the other hand, if I glue and screw 18mm Stirling board on top that
helps quite a bit, and if I fit some thin ply underneath, that would
help even more.


I think I can confirm that from experience..
I have just built a shed and used 18mm ply glued to 25mm polystyrene glued
to 6mm WBP for the floor.
It is very stiff even before putting it onto the 4x2 joists 5 foot span.
Once its on the joists at 20" centres myself, my wife and my daughter can
jump up and down on it and it hardly moves.
I suspect it was a bit OTT.