On 21 Sep, 17:30, "Ian G" wrote:
The ply was glued and screwed to the top of the joists substituting for, but
perhaps a couple of millimetres less deep than the more conventional
chipboard. I think a difficulty in sandwiching between joists might be in
maintaining the strength at the joints. Presumably glue & screw battens to
the joists then glue & screw your inserts to these battens?
Now if you could somehow get access to the underside of the joists with
sheet plywood and complete the box, you are talking about a MAJOR increase
in stiffness ;-)

Kerching !! how about making up a timber box beams? A pair of new joists
say 65mm deep by 50mm wide. glue & screw 18mm ply top & bottom & slot into
inter joist space screwing through existing joists into new ones, or resting

Does anybody (eg Tony) know if Superbeam will model such a system?

I'm still working on my shed, and Superbeam tells me that the 2x3
rafters at 2' centres aren't good enough to hold a heavy green roof.
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.