If you're experiencing water leakage from an upper flat in your co-operative housing society, the owner of the upper flat is generally responsible for repairs, as the source is within their property. While initially seeking a polite resolution is advised, if the owner refuses to act, you can involve the housing society or builder. Persistent issues may require escalating to consumer forums or civil courts, potentially involving legal notices. The society also has a role in ensuring structural integrity and may be involved in resolutions.