The old plasterboard ceiling in my upstairs bedroom is quite uneven and the boards have sagged. I would like a nice flat smooth ceiling. I don't want to pull the boards down because the loft insulation is all loose fill and I cannot face the mess. Will screwing new plasterboard to the ceiling achieve this or will it just bend over the sags in the old ceiling? If it is possible what do I do at the edges of the room where there are no beams to screw the plasterboard to? Is it possible to use board adhesive on a ceiling? Would I be better off doing a coat of bonding over the old ceiling and then just replastering?