Please can anyone say what this might be?
We live in an old house. The ceiling in question is in the original, oldest part of the house so at least eighteenth century.
Recently we extended the house and when we did, the roof above this ceiling was replaced as it was found to be entirely rotten. This was 2 years ago.
We have noticed a patch on the ceiling which reappears even after it has been painted (white). ie Dh put another coat of paint on the ceiling yesterday evening and by this morning, the brown patch was showing through and the paint was cracking. Previous to this, the ceiling had paper on it which did come away very easily from the corner where this patch is.
So everything points to damp, but dh can't see any signs of damp in the roof space and there is a new roof over it anyway, so hardly likely, I would have thought.
Is there something else this could be? eg have we missed something obvious like priming the ceiling before attempting to paint it?
Thanks in advance for any answers.