You need to tell us where this is being built ??

Climate changes are rapidly changing traditional estimates for water and snow loads as well as insulation for increasing temperatures. - In addition the extremes of seasonal climate adds to stress cracks from expansion and contraction.

you can check these....

https://landingpage.bsigroup.com/LandingPage/Series?UPI=BS%20EN%2012056

and this

https://www.cibse.org/media/ohubb2qd/geberit-bs12056-drainage-design-without-compromise.pdf

this video

https://www.youtube.com/watch?v=pMIZnnrLTPU

and this design manual

https://eprints.hrwallingford.com/495/1/Manual-roof-drainage-system-HRWallingford-SR620.pdf

and article on EU standards

https://www.en-standard.eu/bs-en-12056-1-2000-gravity-drainage-systems-inside-buildings-general-and-performance-requirements/