Damp inside loft wall

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.).
Thoughts?
thanks, Andrew
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1850
10cm
seal.
was
A few thoughts (disclaimer of expertise, though). I am assuming from what you have posted that the gable end includes a chimney stack .
(1) Oil based paint and bitumen on the exterior wall may not be a good idea - if the damp cannot escape outwards it can only go inwards. External paint should be breathable to allow moisture to escape, including moisture generated inside the property which will have a tendency to rise to the top with warm air.
(2) If there is a lot of damp, suspect the roof and the chimney pots. Broken or weathered tiles, especially the place where the tiles meet the top of the wall, and the mortar on top of the chimney stack around the chimney pots. Also the flashings where the roof meets the chimney stack. Possibly best to get a roofing contractor to check it all out - gable ends can be quite dangerous! If the chimneys are no longer in use, they should be covered with a cap (e.g. a ridge tile) which allows them to breathe but prevents water dropping down.
(3) If the chimneys are still in use (e.g. for a gas boiler or open fire) then are they lined properly or are they leaking? Gas boilers, gas fires and wood burning stoves all have low flue gas temperatures which leads to condensation in the flue. All these should have flue liners.
Main point - find the source of the damp and cure it. Trying to trap it in the wall and hide it generally just causes more problems.
HTH Dave R
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Orb wrote in message ...

seal.
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.
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