Strange damp(?) marks

[FinTutuola]

Does anybody have any ideas what this could be in our bedroom? It started about a year ago with a small patch on the chimney breast and is now working its way around the corner.

It appears on and off, mainly when we have had heavy rain so we assumed it was damp but we have had the chimney capped, repointed and the flashing replaced and it just seems to be getting worse. We have had two separate roofers up who were aware of the problem and neither of them mentioned anything being wrong with any of the tiles so I am really at a loss! We are semi-detached and it isn’t an external wall so I have no idea what we can do now!

We had similar in a bedroom - fixed the roof - and still got it. Turns out it was a major roof leak from our neighbours - if it's a wall that you share with them worth checking it out.

@Minimonkeysmum oh that’s interesting, the house next door is being (very slowly) done up, it was bought not too long ago but before that was owned by a very elderly lady. Will have to text next door and ask if he can have a look. Thank you!

Where is your water tank! It could be that the pipes are leaking. Get a plumber into look around your loft.

@Freefrops we have a combi boiler which is downstairs on the opposite side of the house. Will send DH up into the loft though to see if there is anything untoward there.

Yeah, it could be the decomposing body of the stranger in the attic who was notorious on Mumsnet for a while

Definitely check your loft - if the roof is dry, but wall is wet, it might well be water seeping through from a bad leak next door. We had damp patches, but they had a bowed ceiling - why they didn't do anything before we did, I don't know!

My neighbor had the same issue. It has now been resolved. The issue turned out that she didn't have any ventilation in the chimney breast. She now has vents in the actual chimney breast and the problem has gone. My side of the wall never has been damp but my chimney breasts have always had vents in. The top of the chimney stack should also be vented so air can always flow through.

Do you have an active fireplace? We had something similar where due to an unlined chimney the smoke has affected the plaster over the years and made it hydrophilic (water attracting) so the damp patches were moisture from the air accumulating on the wall. You can try a quick fix of a coat of aluminium paint, but otherwise plaster needs removing and replacing. Try googling hygroscopic salts.