It's not the best model as the lines and not truly vertical and planar so you have small offsets and errors causing the problem as it cannot cut correctly (as below)
you need to review why these lines are not truly Vertical and Horizontal as this will only cause more errors later especially if you are looking to esport to analytical programs.

Making sure the Roof Covers the Column to allow it to be cut

This Grid is Vertical in comparison to the Roof Edge

Again to show it does work in your model with the correct alignment I created the same example where the roof covers the Column so it can be cut to the underside and it works as expected.

As regards the CAD drawings, If you linked it (which is the correct method), go to Manage tab --> Manage Links --> CAD Formats and Remove the link.
If you inserted the CAD file directly then this gets more complicated, go to Manage tab --> Purge Unused, click Select None, then scroll down to where you see the name of the file. If it's not being used in the project, you'll be able to see it here. Just check the box next to it and click OK.
If you don't see it there, then there's still an instance of it somewhere in your project, and you'll have to hunt it down and easy way is a 3D View with everything turned on window Select everything then use the Filter Tool to pinpoint the item

Jay Colcombe
Autodesk Certified Instructor
Revit Architecture & Structure Certified Professional
AutoCAD Certified Professional
B.Sc. Hons Civil & Structural Engineering
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