I have a second floor bath tub that has developed a leak in the drain. Upon removing a portion of my ceiling in the room below, I can see that the leak is coming from the black rubber gasket at the waste drain. I don't see any way I can twist the elbow and horizontal waste pipe (that connects to the overflow drain) to remove it (a floor joist is too close). Am I correct in assuming that the tub drain threads in from the top and that there is a way to screw it out, thereby allowing me access to the rubber gasket? Thanks for any help you can give.