A house hunter asked me, “….I wanted to ask your opinion… about something I saw recently. So if you saw a building with a basement ceiling like this:”
So, I answered:
“Basements used to all have to have plaster ceilings, for fire prevention. Then that regulation was abandoned as different fire safety requirements replaced it. What you are seeing is old, aging plaster. Over time, the plaster dries out and gravity pulls it off the lath beneath it. Or, someone is doing work on the plumbing above that ceiling, and they remove the ceiling. It looks like a bit of both in that basement.”
If you look carefully at the first two pictures, you can see plaster between the lath in a couple of spots. When the ceiling was installed, the ceiling would have all the space between the laths filled with plaster.
In the third photo, someone deliberately cut the plaster ceiling, lath and all, to get to the flooring above. The wood there looks replaced. So, I suspect there was a repair done here.
Upstairs — where it matters — the first sign of the plaster drying out and pulling away from the lath will be cracks in the ceiling. If you ignore it, bulges develop. Eventually, the bulges let go and drop plaster into the room. It takes years for cracks to develop. It takes more years for cracks to start to bulge downward. Once you have noticeable bulges, the ceiling section may fall at any time. (A truck going by, or a vibration from the heating system or dishwasher, or a child jumping around can be enough to signal an avalanche of plaster.)