After trial and error we noticed that setting wall roughness is ignored on suppressed volumes and surfaces. The wall behaves as if completely smooth.
The effect is that you always need a meshed volume next to your wall, which complicates the setup of the CAD and increases the number of elements.
According to autodesk this is intended behavior but is not put in the manual.
Please make roughness robustly working whenever it is specified on a surface, similar to the competitors. Or allow for suppressed volumes, which reduces the number of elements in the model.
