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In short, only logical roof solutions will be allowed.
You can add nodes to a roof layout by hovering over the along the side of the roof and selecting Add Node or Divide Segment. Divide Segment will put a new node exactly in the middle.
For the option to be available the cursor must be over the layout of the roof, not over the overhang.
You can drag a window around a node (or nodes) to select them.
When nodes are selected The Delete Node(s) option will appear in the local menu.
You cannot delete peak (Green) nodes on a roof.