This was cut and pasted from Xsyon's full description of new building system on 1/30/2015:

"Building Support Requirements
-Platforms: support Floors, Walls, Roofs, Trusses, Posts, Ramps, Platforms."

Unless it was changed since then it should work but, from what I'm seeing you may be correct...?
I may have to post this as a bug or as help ticket to get an official answer.