My girlfriend and i bought a two storey Victorian property a year ago
built in 1890's.
The original slate roof and exposed chimney stacks (2 of them) have
been replaced due to leaks in january last year by a reputable company
from the area. When we moved in we noticed a damp patch on the ceiling
in the corner of the chimney breast in one of the bedroom upstairs. We
hired a damp meter and recorded a very high level of damp on the
plasterboard ceiling, on the large part of the chimney breast too and
also on the bricks showing in the loft space as well as the wooden
beams surrounding the bricks too.

We contacted the surveyor who did the full structural survey report
before we bought the house in which he never mentionned any damp
issues in that bedroom. He came along and advised us to remove the
wallpaper which was holding, from his point of view, the residual damp
due to a probable leak of the original roof and let the house breath
for a little while. We did that but after a few weeks we didn't notice
any improvement. We also applied a waterproof solution on the external
wall thinking that the water coming through the bricks.

We then contacted the company who replaced the roof thinking that the
lead flashing was leaking or there was a loose tile. A representative
came along but he was really confident there was no prolem at all with
the roof but he advised us to remove the plaster which was holding the
damp and replaster using a waterproofing aditive in the plaster. We
did what was recommended and it cured the damp problem on the wall
however the ceiling, the bricks and wooden beams in the loft space are
still very damp.

After almost 2 years since the roof and chimney stack have been
replaced we struggle to understand why the ceiling is still very damp.

We heard of damp caused by salt leaching from the bricks into the
plaster and leaves a hygroscopic damp patch is that what the real
cause is? is this also valid for plasterboards too? if that's the
cause what can i do about it?

People advised us to use a stain block on the ceiling to stop the damp
patch.
It might work however it's not going to solve the real issue.

If you have any advice to suggest that would be really useful because
we are simply running out of ideas and also getting stressed about it
too.

Thank you

Sebastien

Could be;
poor flashing where roof meets chimney
roof at chimney junction may have collapsed slightly with persistent
damp - channeling storm water which can blow through tile gaps
open chimney gets wet inside and out and becomes soaking wet
permanently, especially if it isn't used or only used for gas
appliances.
If not used it wants capping with ventilating clay mushroom pot - not
enough to just put a slate on top with a few holes, AND also
essential is a good vent from the outside as low down as you can get
- to generate a good flow through of air. Perhaps removeable so that
you can remove debris and old soot.
Or if not used could be removed entirely and roof covering continued
over
If used then cap with good chimney pot and repoint, reflash with
proper traditional stepped lead flashing and soakers.
If you can get up there inside the roof with a torch during a heavy
downpour such as we have been having lately you might see the source
of damp unless its just soaking in through the masonry.
Whatever the cause the moisture comes from the outside, there is
nothing very effective you can do from the inside to stop it except
perhaps good pointing to the chimney breast up to the roof.

cheers

Jacob