My second floor shower-tub drain is leaking heavily. How is the top of the tub drain connected to the black PVC drain pipe underneath. I can access the drain pipe as it is exposed in the roof of the garage underneath. I need to take it apart and replace or reseal it.

Are you asking about a leaking tub drain that's located in the tub or the tub waste and overflow tailpiece that connects to the trap? Regards, Tom