Who should bear the expense for seepage issues in a flat

Sir Good evening! I stay in Mumbai (Kandivali West) in a co-op society on the 2nd floor since last 8 years. The house owner staying below my flat has been complaining to me about some bathroom seepage issue, which is dampening his ceiling walls, since last 5 years. On his complaint i has called a plumber and asked to look into my bathroom for any possible leakage. He did some waterproofing work and i had to bear all the expenses. Now again the house owner below me is complaining about water leakage from my flat. I would like to bring it to your notice that I have not made any repaid work to my flat since I bought it. Also I ensure that there is minimum water usage in my flat (because I really care for conservation of portable water). Now even after conducting waterproofing work on my neighbour's compliant, how many times should I bear the cost of the repair. Shouldnt the society or the builder bear such expense, because I am not at any fault. I a using my flat like any other flat member in my society. The seepage problem is because of the faulty construction of the builder.