The ceiling in my kitchen has always had a crack down the middle, where the joists are joined. There is a slight sag there, I guess because the joists dont have enough support. Now that Im about to replace the ceiling, I want to take out the sag. There is just an empty attic above no access. The room runs the full width of the house (24 ft.) with a standard ridge roof above. The ridge runs across the room and is about 12 ft. long. The joists are sistered at the middle but there is not much overlap. It has a piece of ¾ plywood laid on top along the center line (where maybe a beam should have been?). And also there is metal strapping to tie the joists to the rafters (see pictures). I dont know if all the joists are tied this way or just most of them. So whats the best way to fix this? Is it as simple as snugging up the metal straps? Im not so sure about this. I could also replace the metal with wood ties, so its more like a truss setup. If I take this route, would I be adding too much stress to the rafters? We really dont want to add a beam below the joists that is not a style we want to add to the kitchen. There are a couple of collar ties up there, and one option we discussed was to raise the ceiling to that height, giving it a bit of a vaulted look. But we think that might open us up to other problems.