I have tried to build with Sloped Roof Corners. I have a problem. When attaching below the item in question, you can only attach properly to the right hand of it, but not the left hand of it (looking down at it from above).
This seems to be a issue in the game that needs minor correction, because the left hand side is perpendicular to what it is meant to be.
This is with full support underneath, so it is not a question of support but orientation of the snap to in the UI.
If you need screenshots I have it available.