A terrace or rooftop is a common area of in a residential society which is for the enjoyment and benefit of all its members. Thus when the roof is a common area, the repairs or the developments to done on this are to be done by the society through its association usually called as the Resident Welfare Association( RWA). From the facts of your case, you stay below the terrace and there has been leakage problems both at your bathroom and your terrace.
Well the society is responsible for only waterproofing the terrace or the roof area. You are responsible to waterproof your area. It is your duty to waterproof your flat or your bathroom. The society will not do waterproofing in your personal area. You must hire a plumber and a waterproofing person and must do the needful. You must not delay the process as this will cause both you as well as your neighbour problems.
You must request your society to get the repairs done on the common area that is terrace area. If they refuse to do it, you can file a suit against the society. Send a legal notice to the association before filing the case. First get the problem identified and convey it to the society association. Request them to solve the issue as soon as possible.
Reference: Co-operative Societies Act, 1912