I live in a three family condo where the sewer is located in the basement of the first floor apartment and if the sewer pipe becomes clogged the basement becomes flooded with water. Are the people from upper floors liable if it gets clogged? Even if it might be their fault or not their fault? And if they are liable, if it happens in middle of the night do they have to pay the emergency fee?
This question splits into two: The formal halachah, and the halachah based on the minhag.
The formal halachah will depend on who caused the problem, and on whether there was an element of “direct damage” or not. In some cases the upstairs neighbors will be liable, but this will not always be the case, and it is possible that one will be liable, and the other not.
However, in general the custom is that under such circumstances, the damage caused by a “communal pipe” is shared by all parties. This is true unless one of the neighbors is clearly at fault, whereupon he would foot the bill.
If there is a specific case, please specify.