Bathroom venting is simple and necessary to remove excess moisture from the bathroom, a fan, a pipe and a vent on the roof or exterior wall.
This professionally remodeled home in a Dayton historic area appeared to have it all until we questioned the direction of the piping in the attic. None of the pipes were going out through the roof.
It turned out that the vents in both upstairs bathrooms were connected together so that when the fan in the master bathroom was operated the moisture blew into the other bathroom and visa versa.
We got a good laugh from this one.
NOTE: Bathroom vents cannot be connected together to vent out through a common exterior vent.