We moved into our house in March this year. Recently I noticed a shadow on our upstairs bedroom ceiling near to the chimney breast. I had thought it was a shadow from the lampshade but I inspected it a couple of days ago and found that it felt cold. I have a damp meter so I started to prod around and found that the meter goes off the scale (33%). I tried to find the outline of the damp area and it seems to surround the chimney breast like a halo/ring about 6-12 inches wide even going over the bedroom wall and into the landing area. I went up into the attic and found that the back of the plasterboard is damp and I can see a defined outline of the dampness. The area that is damp seems to be pretty well defined. Within the area sends the meter off the scale but move it about half an inch outside the region and it goes down to below 10%, which seems to be the standard when I check areas I am confident are dry, nowhere near the ceiling. However, what confuses me is what is not damp. The area of ceiling directly around the chimney of about 3-6 inches is not damp at all. The chimney breast itself is not damp above, nor below the ceiling. The walls that contact damp parts of the ceiling are not damp. I went into the attic to investigate and I cannot work out where the source of the water is. I haven't yet been able to get around to the front of the chimney but I have been able to get to some of the damp areas and inspect them. All wood I can see in the roof looks fine. My damp meter registered the plasterboard as being damp but the wood next to the damp spots was very dry (I did notice that it does seem to have about a mm gap though so not sure if that is why). The insulation material also felt dry. The chimney breast was dry although there were a couple of dry brown water trails on one side but they may have been there for a long time. There are no pipes at all that I can see above the plasterboard. I went up during some morning rain (not storm level but reasonably heavy) and saw nothing that looked wrong. Some history: - some of the ceiling paint in the area that I now know to be damp was cracked and flaking when we moved in 6 months ago. I assume that this means that the damp issue has been going on for a long time. The cracking has not seemed to get progressively worse at all and the mild "shadow" staining I've noticed may have also been there for a while. Our survey highlighted that there were signs of previous water leaks but that none appeared to be current. - the house is a 50s concrete house with the chimney entirely within our house. It is not shared. - we have not noticed drips or anything of that matter - the chimney is in use with a gas back boiler, although not all day and night - I cannot see any signs of mold, just damp plasterboard It appears to me that the chimney is the source of the problem but I can't understand why it's damp all the way around yet the chimney and the area immediately near the chimney is dry. I have read about hygroscopic salts and wonder if a previous leak has left these in a ring around the chimney and they are pulling water out of the atmosphere. I thought these were from coal/wood fire chimneys though whereas we have gas. Also, would they really pull in that much moisture to cover such a large area and all the way through the plasterboard and paint? I have thought about the flashing failing, but I'd have thought this would have caused the plasterboard near the chimney to be damp too rather than just a ring, plus would be on the side that is failing rather than all around the chimney. Can anyone advise on further steps to try and find the source of this problem? I don't want to get a builder in who tells me to do flashing, pointing, chimney replacing, roof replacing, etc. without knowing what the cause is. I'd prefer to try and find the source and then get it fixed properly. I don't have any photos as yet but I've attached a sketched image to hopefully help. Any advice would be greatly appreciated. If you want more details, please let me know.
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