Damp patches on chimney breast – leak or condensation?

4 Jan 2022
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United Kingdom

I’m having a damp/water ingress issue on the old chimney breast wall of my 1890s terraced house. The chimney was removed before we moved in. There is ventilation of the chimney breast upstairs and downstairs.

We started seeing some damp patches on chimney breast wall both upstairs and downstairs a couple of years ago. Had the roof repaired and that back wall re-rendered, and it seemed okay for a while, but the damp patches have returned. We try to open the window in the upstairs room as much as possible but that doesn’t seem to help.

Could this be a condensation issue??

Or is it likely a leak in the roof? The original slates are still on the roof but were relaid a couple of years back so I’m unsure why the issue has returned.

Any suggestions would be very much appreciated.



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Do you have 11" cavity walls? If so, do you have CWI?
Would you please post a pic showing the outside of the wall from gutter down to lean-to flashing?

The chimney stack was removed, & the slates made good.
The chimney breast & flues (in the loft?) and on the floors below have been left intact, and are open top and bottom for through ventilation?
Were the flues swept at any time?
Why not go into the loft, take a strong light, and investigate the under roof for water penetration? Check the open top of the remaining chimney breast.

The peeling paintwork and behind the skirting board appears to be signs of damp?
Why no window board at that window?

I'd suspect:
1. due to soot residue in the flues then hygroscopic chemical penetration of the flue brickwork could be causing damp patches?
2. damp penetrating through the outside wall?
3. Is the upper window frame well sealed - does it have a projecting sill?
I would say it’s water ingress from somewhere, and not condensation. Some good advice and starting points above.
A customer of mine had damp patches on his chimney breast in the top room. He had the exterior wall repointed. He put caps on the pots. Two year later I redecorated the room over easter. Once winter rolled in the patches came back. The roof was checked- no issues.

He spoke to a damp specialist. They told him that they only deal with rising damp but they did kindly explain what was going on. The soot in the chimney breasts contain hygroscopic salts which absorb moisture during the colder months. Their advice was to remove the plaster, seal the brickwork and skim with sand and cement.
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