Are pre-made rain caps made for plumbing waste vents?
My house was built 1938 with galvanized DWV pipes which have a series of elbows before reaching the roof. All such elbows retain a certain amount of rain or dew water. I quite incidentally discovered one elbow is paper thin at the bottom:
The run is more or less sloped. See also: Covering / Cap over DWV vent pipe This is 1.5 inch pipe.