A typical scenario is:
The bottom of the flue pipe is located
on the fireplace. There is one joint in
the flue exposed inside the house,
without any securing screws. The flue
pipe is indirectly supported by the
escutcheon plate in the ceiling. The flue
pipe has one joint inside the roof cavity
that is secured with 2 screws, and sealed
with silicone. The flue is then supported
indirectly at the roof level by the roof
flashing