We are often called out to investigate damp chimney breast problems. Each case can be different so a site visit is usually necessary to identify the range of external and internal issues in order to carry out a methodical approach. For instance, is the chimney alongside a gable wall? In which case, failure of pointing / rendering / rainwater goods may play a part. Is the property single storey? In which case, water may penetrate from the chimney capping area down to the ground floor
However, since you mention ground level, unless the chimney flue is directly vertically situated above the fireplace then it is very, very unlikely that water will penetrate so far down from the chimney UNLESS the flue (liner?) that you had installed is not terminated correctly and is in fact channelling rainwater down into this area?
Those considerations apart, the problem appears to develop from ground level and could be a combination of rising damp and salt contamination (hygroscopic salts)
You mention that the chimney breast has been DPC’d (but not the adjoining alcoves?)
I would question the method of applying the DPC and the initial diagnosis
There are a number of assumptions that I make here due to not seeing the problem first-hand which may make things long-winded but here are my ‘thoughts’:
Since the dampness ‘begins’ at a height above 1m on the chimney breast rather than below suggests to me that the damp treatment has been successful for the first metre but has failed above this.
Why would this be?
Here are two considerations:
1. The contractor may have limited their application to the ‘textbook’ rule of one metre height. Dampness and salts can rise higher and the treatment needs to respect this. Otherwise, even after controlling the damp from rising, (say, by injection or by a damp proof membrane) there can still be a salt band above this 1m mark which will draw in moisture from the atmosphere
2. The plastering material used above the 1 metre mark may be different from that below: there are certain plastering specifications that need to be followed when dealing with damp
The pattern of the dampness in the alcoves does suggest that the salts have migrated from the chimney. (Salts from the burning of fossil fuels include sulphates, which are more aggressive, as well as chlorides and nitrates)
There is also the added possibility of the DPC (if any?) being compromised in this area by being bridged with the plasterwork behind the skirting
There are other scenarios / possibilities but it would be labouring the point to continue – a site visit from an independent damp surveyor is advisable, more effective and less costly.
