Flues/ chimneys/ fireplaces etc.

A flue is the hollow channel built into the thickness of an external wall, usually a gable wall, which ventilates a fireplace and carries any smoke out to discharge into the atmosphere.  This happens by the differential in pressure caused by wind blowing across the chimney at high level which draws out smoke and air from the room it serves.

As well as smoke, flues are also very important in carrying stale air from a room, stale air caused by breathing, sweating, cooking....all the usual reasons.  Where these are blocked or sealed various other problems occur including reduced air quality and increased condensation.

Partly blocked chimney flue

This shows a disused flue being cleaned out.  Damp debris congregated and blocked the flue completely.  The reduced ventilation of the flue caused dampness to form at the blockage and transfer that dampness to the walls internally. 

Please note - an injected damp proof course or waterproofing plaster will not solve this!  This is a problem 'internally' within the flue and likely at high level too.

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It's not the fault of the fireplace.  It's not the fault of the builder or the old stone, nor is it just because it's an old building - lack of maintenance caused by a lack of understanding of old buildings by owners and contractors leads to failures.

Plasterboard in attic blocking fireplace

Traditional buildings are tricky.  That's why you need to understand how they work - and the common defects which can be misdiagnosed by contractors wanting work.

This situation is common but not always the reason for damp on a chimney wall.

This Victorian semi in Falkirk is in generally good condition despite being around 110 years old.

It had been experiencing water penetration associated with the chimney with one roofer suggesting to REMOVE IT COMPLETELY!

What utter nonsense.  The chimney structure is perfect - some cosmetic repairs and it will last another 110 years (if maintained).

Repairs with cement have caused saturation and the moss growth as well as lack of proper maintenance.

It is important to ask the right person for advice as often you just get a best guess which is not helpful and can be downright bad.