Neighbor is building a house next to the garage, neighbor's wall is wet

Horrido,

we are currently in a small dispute with a neighbor concerning our old house. In this residential area, the houses stand directly next to each other without any space, arranged like this: House | Garage | House | Garage | House | Garage, and so on.

Our house with garage has been there for nearly 35 years. About 10 years ago, the neighbor attached his house to our garage and created the connection from our garage to his house wall himself. For this, copper sheeting and plenty of silicone were used. The wall is on the weather side, and for some time now, he has been complaining that his interior wall becomes damp during prolonged or heavy rain. However, the adjoining garage wall of ours is (measured) bone dry. He, or a person he hired with supposed expertise, now claims that our garage roof drainage does not function and that rainwater flows exclusively toward his wall over the edge of the garage roof. The drainage, however, works perfectly, and neither the roof nor our garage show any signs of moisture penetration.

I have now climbed onto the garage roof to take a look myself. He placed a copper sheet over the edge of our garage roof at that time and plastered over it in beige. A picture of this is below. My suspicion is that water simply runs in between the plaster and the sheet and therefore he is responsible. As I said, our garage wall is absolutely dry.

What do you think?



The "collar" of our garage roof looks like this, for example:


This is how it looks along the entire length:


The "closure" to the neighbor on the other side was done like this, in my opinion better:

Very difficult question without local knowledge. If it is measured dry, nothing could have penetrated. Could it possibly be condensation (dew point)? I doubt it, you will find a civil engineer on site who can advise you. It does not have to be expensive. May I ask which federal state?

Depending on how this develops, we will definitely bring in someone knowledgeable. The house is in Hessen. I am also somewhat surprised that it is dry on our side and not on his. However, I cannot say whether there is some sealing medium in between and if the moisture only runs on one side of this medium. I need to look into the dew point... according to my current understanding, with a dew point problem the moisture should form on the exposed part of his wall, right? But it is wet where the house is next to the garage. By the way, our garage wall is a normal concrete block wall.

So, I can't help with Hessen at the moment. And yes, it would have to be the coldest wall. I think the phrase "someone with expertise" is great. Do that! It's really hard to assess here at the PC.

The roof without gravel is yours? Is the bitumen membrane under the sheet metal pulled up along the wall?

Roof with gravel is our garage (the house with the damp wall borders on it), roof without gravel is the other neighbor's garage, which in turn adjoins our house wall (no problems).

Do you mean the copper sheet on our garage with gravel? Nothing is raised there, the copper sheet simply protrudes over the edge of our garage roof.