The roof of this building consists of a simple 3D floor and 4 sloped 3D floors. To
achieve the sloped roof notice how the control sectors are aligned with the play area.
For the slopes to occur add a sector with the approptiate height and set the common
linedef to 181:Plane_Align.
The result is this
Example pwad: 3D-01a.wad
The problem with the above setup is immediately noticable in large maps. The control sector for the alignment has to be constructed in a void fairly closeby, which is not possible in most case, so must be constructed far away at the edge of the map, which makes the alignment a bit tricky.
To the rescue comes boris's 3Dplugin, which accomplishes the alignment of the slope in the sloped sector itself, with a control sector somewhere on the side of the map.
Example pwad: 3D-01b.wad
It may take a few moments to get the hang of how the plugin works, but the effort is well worth it.
Kappes Buur, Sept. 09, 2017