We just moved into a 1940s house last spring. The upstairs is all original plaster on the walls and ceilings, and the bedroom at the front of the house has a number of cracks in the plaster ceiling, especially around the ceiling fan. We're on a major road and there's some vibration from traffic, plus there was major construction a couple blocks away over the summer, which could be why the plaster is cracking but I'm not sure because we haven't been in the house that long.
The cracks aren't wide, but some are more than a foot long and I'm wondering if I should be worried about them or not. Should I just keep an eye on them to make sure they're not getting bigger, or are cracks that long a serious issue?
I deal with plaster on a daily basis and in the vast majority of cases cracks in plaster are not a significant issue other then aesthetic. Having cracks comes with the territory of owning an older home and because these homes were not build on piles back in the day they are much more susceptible to movement. The soil conditions can also be a major factor. If you are situated within a few blocks of the river that can also affect soil conditions to make the house shift as well. If you are dealing with hairline cracks I would bet that it is not a significant issue. Cracks that appear due to a foundation or structural issue are typically wide set and appear very quickly. If they are indeed hairline cracks, even very long ones I would not worry about it.
Given the age of your home, it is fair to assume that these are hairline aesthetic cracks. They happen to plaster walls/ceilings due to minor movement caused from a number of different variables (temperature fluctuation,etc.)
Best thing to do is to monitor the situation to see if any major gaps/cracks appear.
Definitely keep an eye on them to see if they continue to spread. But, other then looks you should go and touch them to see if any of the plaster is loose and maybe moving. If it has become detached from the lats or whatever surface is beneath then, definitely, you should be concerned. If no they are easy to fix if that is the route you may want to go.
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