Leakage on below flat from common drain pipe

I stay on the 11th floor of a 23 floor multi storey building (which is a housing society). There is a small balcony/flower bed outside the hall, and a common drain pipe (vertical) running across all floors (which has been built by the builder). nnI have made no alterations to this part of my house, and there is no water leakage on my floor, but the apartment below is facing leakage on the ceiling around the area of the common drain pipe. So in this case, who has to bear the cost of carrying out the waterproofing work?