Orb wrote in message ...
I live in the top flat in a converted Victoria house dating from about 1850
which has a lot of damp problems. There is a lot of water damage to the
inside of the gable wall in the loft. The wall is basically brick and
aggregate which is literally crumbling away (in the worst place, about 10cm
depth of brick is missing).

I want to repair the wall. My first thought was to apply some sort of damp
seal to the wall, then some mortar. However, if damp is coming through the
wall, this could make matters worse by trapping moisture. My current
thinking is maybe I should repair using a lime mortar without any damp
seal.

From what I can make out, the last time the exterior wall was decorated was
in 2000. Where necessary, render was repaired using cement render and
painted with an oil-based paint. I've used quite a bit of bitumen paint
around the chimney stacks and top of the gable wall. There are no obvious
points where water is penetrating (no windows, no guttering etc.).

This sounds like a problem for the whole building rather than just your
flat. How many flats are there? Do you have a residents' association, or
proper arrangements for maintenance?
In an ideal world the whole wall should probably be hacked off, a membrane
installed, and re-rendered but of course the flats which aren't affected
will be reluctant to pay their share. Same with the roof. Nobody on the
ground floor thinks it's anything to do with them.