Perhaps read up on the aecAnchor tool options. They are hidden for some reason from the Ribbon.
I would think you could draw lines (not plines) to your roof slab edge and then add your structural object and add it as an aecAnchor at spacings using an array option.
I haven't used them much but would like to. Just haven't taken the time to explore them and understand the nuance of how they work but they look useful. I use a railing object and just align it with my roof object (not slabs 🙂 ) I still have to add the hip separately (manually) and I campaign for the roof object to be updated and that is one of the things I would like to see.
The creator of the Roof Object went on to recreate the tool in another program and continued to improve it to the point of being able to autoframe so rafter tails would be easy. Unfortunately Autodesk have never seen a priority for a comprehensive roof tool in an archtitectural project. Too niche maybe. Go figure!!
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