Have damp spots on corner of basement ceiling - across a beam and where it meets the external wall (though wall is also covered with plasterboard). House is on two levels, so externally, the wall corresponding to this basement is 1.5m or higher above ground level - basically on this side of the house, this is more like the ground storey rather than basement, and a door opens into garden.
Plumber doesn’t think it is a plumbing issue based on external inspection of pipes and areas around them, and doesn’t think it is wise to cut holes in wall for this then. It has been suggested it is the pointing outside - which does look a bit worn. Have had specialists out who have all said totally different things - from rising damp (2 m above, no signs at ground level!) to simply trying our luck.
however, the patches disappear significantly with dehumidifier use and reappear in their entirety. This is despite hot weather recently.
have attached two photos - one after dehumidifier had been off for 12+ hours and one taken after it has been running for an hour or two. Because of hot weather plus recurrence, I am increasingly worrying repointing won’t solve the issue.
has anyone had a similar experience? Any clues?