Henry wrote:
Can I convert a mesh to a roof, then trim columns to the 'roof'? I am sure I read something like that but of course now I can't find it...
Yes, but this may not be what you want to do, depending on your version (always post version and platform) and the specific result you're after.
With 8.1 and above, you can just use Solid Element Operations and subtract-with-upwards to trim the columns to the mesh.
With 10 and 11, the 'mesh to roof' add-on was moved to the online Goodies download site. Go to the Help menu to get there if that's what you really need/want.
Cheers,
Karl
