This is easy. Thanks. Didn't know about the surface stuff.
I will investigate further to build a complete roof.
Maybe a tutorial about building different kind of roofs with slates would be nice. With end capped slates / shingles and different shapes.
I used to use roof designer but it reached the end of life over a year ago and lacked some features.
I think Railclone is the best and versatile solution for such roofings we have a lot here in Germany and Europe. So Tutorial on this topic would be really nice.