To get a good draw you need to raise the flue outlet away from the roof - a surface (like a roof) creates wind eddies and slows the wind down thus reducing draw, so by raising the flue outlet you bring the top of the flue up into unimpeded air flow.
To make sure that there is no hazard the flue outlet needs to be sufficiently far away from the roof, adjacent buildings, and roof windows.

https://www.stovesonline.co.uk/flue-outlet-height.html?adwor...

A coring ball test is a heavy ball, with a diameter about 25mm
less than that of the flue, which is lowered on a rope from
flue outlet to the bottom of the flue.
...Smoke should be seen to issue freely from
the flue outlet or terminal.

ww.rutland.gov.uk › GatewayLink

If the efflux velocity is weak, there is a risk that conditions of down-draught or inversion may occur.

https://books.google.de/books?id=CmhMYo8cLn4C&pg=PA157&lpg=P...

The positioning of a flue outlet externally should be governed generally by the same considerations as for other flues....

https://books.google.de/books?id=mhYCsyHvdTIC&pg=PA199&lpg=P...

I don't think 'sezione' is necessary in English.