Hello guys, I'd like some advice. I am decorating one of my old bedrooms that was used as a spare room (aka - open door and thrown in without looking room) On removing carpet and lino (yes lino, not underlay!) from 1975, I discovered that almost all the L-shaped nails had risen half inch out from the floorboards and in some cases there was no nails. I lifted one of the floorboards out and found steel beams and really rough uneven concrete, no wooden joists, this is a 4th floor block of flats. Walking on them makes them jump, squeak and bow making it extremely loud. Is there a cost efficient way of fixing this? Being a flat I'd be very reluctant to use leveling compound as I wouldn't want to add weight and probably wouldn't be able to fully seal it anyway. Any ideas?