I have a flat roof on the back of my house.Every time it gets cold(below 20degrees) I have been getting a leak in two parts of the house.I finally cut a hole in one of the areas and found the drain pipe runs horizontally through the ceiling.In the one area the pipe is coming down from the roof to an elbow that seems to be leaking from the bottom and top of the joint.I have read about marine epoxy and epoxy putty but I would rather have a permanent fix.This is 4" pipe that appears to be glued.What is the best way to cut it out when there is not much space to work in?Any suggestions would be greatly appreciated.

Mike in the U.P. of Michigan.