Heard what sounded like water slowly dripping from stairwell outside the 2nd-floor bathroom when the bathtub is draining. Following sound location down to the 1st floor, discovered a black drainage pipe with soaked floor and stud but no water on the outside of the pipe. When I opened up the wall on the other side I found that the pipe was wet. Later when the shower was used in the 2nd upstairs bathroom, I checked the pipe and found it was wet on one side. I am thinking there is likely a leak at the joint where the line changes from vertical to horizontal since there is leaking whenever draining water from both bathrooms. Am I on the right track? If I am correct and find the joint leaking is reapplying the joint glue all I need to do or will I need a plumber?