Whose responsibility is to for waterproofing of terrace in society

I live in a cooperative society where terrace needs waterproofing as the water is leaking to a high extent and hence waterproofing activity is needed to be done. But our chairman claims that the whole expense is to be incur by the flat holders of top floor. If needed society will share half contribution to expense but not full. Here I believe that the expense has to be incur whole by society and so I need help.. Please let me know which laws and codes are applicable under my case to fight them... Also a fact is that the water leakage have led much damage to my personal belonging..please help.