Plasterboard base with a plaster skim coat the skim coat is coming loose as if it had gotten wet. The house is approx 40 yrs old in so. cal. this is in a bathroom and the fan works. Also outside wall opposite the bath is wood and the paint is pealing. There is not sign of a leak and it rained substancially the day before I checked the attic. The plaster is pulling away only on half the ceiling and down 2 walls.
Please help if anyone has a suggestion for me to check
Thank you

Way too much moisture in bathroom getting into wall finish. Let me guess the half of ceiling and wall is toward the outside wall. Bathroom vent where does the vent go ? Trapped moisture. To repair wall and use a binz or kilz to seal properly so moisture doesn't pentrate wall finish in bathhroom. Over time the finish coat exposed to moist/wet conditions losses it "stick" properties form the finish coat to the sub coat IE plaster and lath for example. This is common in my town 1754.
Check bath vent exhaust.
Age of paint on walls?
Insulation factor from interior walls to outside source of cold. Differentially moisture being stuck and trapped. Is ceiling "attic" insulated well above bathroom? Another big factor and that in combo with attic not vented properly.
Signed 21 boat

Thanks, I forgot to add the fan is vented outside but I guess my daughter's showers are too long and too hot as there is a fog coming out the door when she is done. She started using this shower approx 6 months ago and the problem started about 3 months ago. The plaster is actually separating from the plasterboard. Yes the prob is toward outside wall. Attic is very well insulated, approx 12" blow in cellulose. Should I get a mmore powerful fan and add a timer or delay?

Yes that would help or get smaller daughter or shorter shower Or have daughter repaint seal haha . I have the same daughter problem. Anything to reduce moisture is the ticket.
Signed 21 boat

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Thank you

You might want to add a dehumidifier to that part of the house to dry the area's extra moisture.