In article ,
Andy Dingley writes:
Damp patch inboard of an outside chimney, with no obvious cause or
broken thing to fix.

Late-Victorian three storey, with an external brick chimney for the
old kitchen range.
http://quercus.livejournal.com/240103.html#cutid1
http://quercus.livejournal.com/32427...43408#t1343408
The chimney is capped at the top (paving slab), but unventilated at
the bottom. It has a lead gutter into the corner of the roof. Access
is a pain, as I don't have ladders to do it and there's also a
protruding conservatory beneath.
http://quercus.livejournal.com/324272.html

Had a laddery chap up there a while ago to tidy the other chimney
stack, and he reported no obvious cause.

Inside has been damp for some years, to the point of peeling paper and
the plaster starting to fail. As there's nothing visible outside, and
I plan to insulate inside anyway, I've now started hacking the plaster
off inside. I hope to find an obvious damp spot, otherwise to tunnel
out onto the roof from the insides (at least with an endoscope).
Pulling the plaster showed at least three vintages of plaster on
there, so I think it's an old problem with some partial repairs done
in the past.

In addition to the other comments, the flue needs ventilating top
and bottom, or it will gradually fill with condensation, which will
come through to the inside as a damp chimney stack. The vents can
both be to the outside, so you don't lose heat from the house up
the chimney.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]