I have a non-contractor built 1960 home in Southwestern KY (Hopkinsville, KY near Clarkville, TN). The people who built this failed at building dog houses, but that's another story. The ceiling joists are spliced on the run (groan) and are cement board with non-annular ring nails and skim plaster. Obviously I get cracks (ooh, THAT'S an understatement!).
I have bungee'd the joists in the dining room to the (lamentably) under-code-even-then (full 2 x 5 fir maybe local milled) roof rafters so that the ceiling doesn't sag 3" in the center anymore. However, the seams of each sheet/piece are cracked out 3 years later. Do you think I can cheat and rout in the old circular plaster repairs just on the seams? I kmow - probably not, but I'm a 51 year old transplanted Massachusetts Yankee now with a spinal fusion. We can't buy a decent contractor at anything less than ruinous costs due to the building of new housing 'mushroom towns' for nearby Ft. Campbell. I'm hoping to get the scale of job down to something I can do myself. Thanks!!