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