Good points – and as well as what pedropete says, if you also used ply for the ceiling instead of plasterboard, and glued and screwed it to the joists, you’d have I beams and a very stiff, thin floor.

you can get timber i beams, might be worth looking into and seeing what size you can get away, with have used them on jobs before but have always had designers/engineers involved.

If it was my house and I was paying for it and as I have joinery skills and would think that adding steelwork is going way OTT (even if it is structurally ideal). I would either: Lift the floor upstairs and glue + screw 18mm plywood on top of the joists making the long length of the boards parallel to the joists making sure whole sheet lengths were central (ie no joins in the middle portion of the joist where all the flexing happens). Or, I would add 150 wide rips of ply, glued + screwed to the sides of the central portion of the joists.

This ^^^.

Plywood to stiffen the joists. I like it.

Got a joiner and structural engineer type going round today, so I’ll put all these suggestions to them. Cheers folks.

There are some good options here. How about 22mm Caberfloor, which is the tongue & grooved flooring for the room above? Could also glue the joints and that’ll help stiffen things up too.

And before you make a final choice of reinforcement, don’t forget that you need to fit dimmable LEDs and accommodate the other services that are in the void, and possibly access them later 🙂