I pride myself on not being an early adopter but in this case all I can say is

Yep, Dylan and Martha are right. Roofmaterial is part of the underlying original picture, no modelling done on that. Thanks again for comments

Thank you. That was it.

Thanks Rich, that's a good idea! I'm gonna give it a go. If anyone else has good suggetions, please let me know

Thanks Cyberdactil, I guess in that case it is indeed a ballasted membrane. Gonna study this info from you, as my clientbase is expanding to include more international clients it's a good thing to know the english words