I try the same... but no way.

If only roofs could find snap points on other roof parts, including one in the middle that would allow them to overlap (with collisions turned off), it would solve a lot of problems. It would be nice to not have to always use a wall, and it would be nice to be able to have more than one roof piece extend from a wall. I had to use a ramp from the settler set to complete the roof on my most recent build. Getting that inside corner is a pain. Actually if you could overlap the roof at an offset diagonal to the center as well, that would remove the problem of the end corner of a roof overhang, much more neatly than the way that Ozocraft handles the same dilemma.