Older homes I've seen here, built 1950 and earlier, don't have
a main vent that runs out the roof. They have one on the
sewer line, near ground level, where it exits the house. Then
they have vents for bathrooms and such that do extend out
the roof.

Interesting, I haven't seen that (in my very limited experience).

My in-laws house is about 100 years old. They had a single main roof vent
(only 2 inches for the whole system), but none of the individual fixtures
had any vents at all.

Anthony Watson
www.watsondiy.com
www.mountainsoftware.com