Damage to a property due to rain

Dear Sir/Mam, My neighbour often complains to us whenever it rains. Basically, we have a elevated wall which holds the overhead tank atop the roof (inside our property) and whenever it rains, the rain water slides through that wall and reaches the roof of the neighbour's toilet (external), down below. Before the elevation was constructed to hold the tank, we had a parapet wall of the same dimension. My neighbour has constructed the toilet adjoining the wall on our property and wants us to provide him a solution/ repair his property for the damages caused by rain. Also, the rain water drops from the parapet wall on his building. Our argument is that he had to think before building the toilet adjoining our wall and we cannot actually do any remediation to stop the damages inflicted by rain. He pesters us each and every day and it is becoming hassle to come out of the house. Please advise about the legal course of action and let us know if we have to correct something. Thanks and Regards, Shashidhar GS