Have lived in our 1957 ranch for 14 years. Sometime back, there was an errant step by someone working in the attic, and a bit of cracking occurred in the plaster ceiling in the master bedroom.
The crack was stable for years…. But recently we discovered it was worsening. There has been major road work less than two blocks from our house (think once every forty years rebuilding of eight lane freeway) which I'm sure hasn't helped. Tiny bubbling in the plaster has also been noticed in the last month.
We went up in to the attic and found a small leak on the air conditioning unit (boiler heat through baseboards with AC unit in attic). Placed a pan under it until the spring when it's warm enough to repair.
We want a fix for the plaster, but aren't sure how to go about it. 
There's the crack. Missing a teeny piece of plaster in the middle. Can't appreciate any lath up under the plaster. It's a grey substrate material directly above the cracked plaster? Can we patch the plaster without making it a huge deal and pulling the entire ceiling down one piece at a time?
Best Answer
You should be able to just pick off the worst bits and apply new joint compound. Do it in 2 thin layers if the plaster is thick. It will look a bit patchy if you don't sand and paint afterwards.