I tested for other polylines and it says insufficient curves. Kindly have a look.
boxes-query.gh (17.9 KB)
Hi, the reason is that the curves you inputted as rectangles are not so tidy. I created new rectangles from your curves by generating the bounding box of each one.
Boxes Axes_v02.gh (20.3 KB)
Interesting. Thanks!!
Amazing work ! I am wondering how it is possible to find the centreline between two interpcrves like the example here 
It could be more simple here for “closed centerline”. You can use a classical Medial Axis. It will be better work for “smooth” curves.
For polylines, my script could be transformed to work for roof with patio (holes) .
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If you want it really fast, turn that surface to a mesh and use z axis mesh rays instead of Brep closest point. 