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
onto existing dwarf walls. then replace your original flooring on top of the
lot.
Could be quite a soundboard you make though ;-)

Another alternative you may be able to consider is known as A "Flitch Beam"
I think, constructed by sandwiching a plate of steel between a pair of
timber joists and bolting right through.the lot.

If you have the depth between the support walls, add 6 inch deep joists and
notch the ends at he support walls ? (I seem to remember that notching to
about one third of a joists depth is considered acceptable )
rgds.
Ian


"Steve" wrote in message
k...
Steve wrote:
Ian G wrote:
18mm ply glued and screwed to the top of the joists (instead of unglued
chipboard) , forming a T beam was a solution proposed by a structural
engineer and adopted by me when I did a loft conversion some years ago.
We didn't put a piano up there, but the purchasers of the house did
install a water bed and there have been no adverse reports from
neighbours whom I have remained in contact with. The increase in
strength over the unglued floor was impressive, confirming in practice
what the Moment of Inertia calculations indicated. We were able to jump
up and down on the floor and detect very little spring.

Our joists were 6x2 inches, at 16inch spacing and spanned 12 feet, to
give you an idea of our configuration. The resulting stiffness was, if I
remember correctly better than what would have been achieved with
recommended joist depth for that span. Sorry I can't remember if 8 inch
joists would have been sufficient for that span. I remember that the
addition of the ply to form a T beam gave lots more improvement over
doubling up the beams. Where our span increased to 14 ft . the solution
was to double up on the 6x2 beams

I'll try to find my own calculations which I did to "prove to myself"
the efficacy of the scheme, and perhaps substitute your 4x2 beams.

regards
Ian

Thanks very much Ian, that is very encouraging.

If you do come across the calculations, I would be very interested
indeed.

My email address is not valid so try g8izy_ @ _ blueyonder.co.uk,
obviously minus the underscores.

Thanks again

Steve

Reading your post again, I wonder did you put the ply on top of the joists
to form a T beam or did you sandwich them between joists as I proposed to
form a T beam.

Just that I cannot afford the extra floor height.

Steve