Damp / stain on a chimney wall - 1st floor period house (Haringey, London)

[Olivier D]

Hello!

First of all, as a new member thanks for welcoming me to your forum!

(also sorry in advance for my English, but I'm originally from France, so I will do my best with technical terms).

 

Here is my problem:
I own a flat with my wife on the 1st floor of a period house in London (Wood Green, N22).

I have had an issue with damp / wet stains on a chimney wall for a few years now. 

2 roofers came to inspect the roof/attic and chimney. They fixed a few things (brick repointing, roof flashing), but the problem is still there... (The last roofer who came also sealed the chimney with ciment).

 

It's very strange as there is no water dripping or mould, just a damp stain on the wall which is wet and cold.

It's drier in summer and then gets bigger in the winter. I would think that if it was a water leak problem, it would have become bigger and bigger after a few years with more water coming and more damage to the wall.

 

The other strange part is that the evolution of the stain is not really linked to the rain / no rain period.

For example, after a whole week of rain the patch is not bigger (and sometimes starts drying a bit), but then a few days later and with the weather being ok, the stain will increase... (especially on cloudy / foggy days).

 

So maybe it could be linked to condensation, or low air pressure?

 

If anyone has an idea on how to fix this, that would be amazing.

 

Thanks in advance!
OLIVIER

 

(Here are some photos to understand better):

[JohnMo]

Do you have a chimney feeding it or are the fireplaces all closed off?  If so is the chimney vented from inside or outside at a low level?