Ok, so a bit of a long one, but bare with me...
Key Facts:
My house is a victorian mid-terrace on a slope. In my front room (street) on the wall shared with my upper neighbour, I have a damp wall/ plaster starting from about 1-1.5m up the wall, but it slowly curves to floor level and disappears just after half-way across the wall (towards the street).
My house is a "mid-terrace" house built in approx. 1886* - but my house is the first of the second phase of building, and my upper neighbour was the original end house. So my house was 'joined onto' the end of the original terrace after.
My upper neigbour's floor is roughly 1m higher than mine, and is solid slab floor with no void space underneath. I have timber joists and a void with an unobstructured air-brick.
I have found that there is water coming through (very small trickle when raining a lot/ heavily) underneath the floorboards and pooling against the wall in the area most affected (I have since redirected the water away from the wall to pool further into the middle of the room *see picture*).
My house was bought and 'renovated' prior to my purchase, and I found some questionable practices/ work when looking more closely (such as plasterboarding over a built-in wardrobe rather than neatening it up...). The original paint on the wall was a latex-style paint.
A prior 'investigation' resulted in a couple of sections being drilled/ chiselled out to inspect the damp levels within the wall. They were filled with wood and topped with filler (temporary fix). After putting up lining paper to see if the damp was still penetrating, these sections resulted in being the only dry parts within the original damp area.
Upon inspecting the plasterwork (see photo) the skim had been done all the way down to the floorboard, whereas the plaster underneath (I'm assuming that the renovators just skimmed over), stopped about 5-6 inch above.
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My Theory: The water pooling underneath has evaporated and climbed the wall through the surface skim as opposed to the brick or plaster (although both are damp tbf), causing the damp patch, but also explaining why the damp patch curves down across the wall.
Other's Theories: The lack of sub floor venitalltion in my neighbours room will be creating a very cold/ damp effect on that part of my wall. Also, that we can't be sure there isn't water getting in higher up than my floor boards.
Also, maybe the skim was not left to properly dry before being painted with non-breathable latex paint?
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Ultimately - What to do next? Just take the damp skim off? or damp skim and plaster? Drop the plaster off the entire wall?
Thanks for taking the time to read through.
