if the top of the wall is below the cutplane of the roof plan, it will not show... as on any other storey...
the question is, why would you want to show the walls that end below the level that you are representing?
if you need to show them, just set the distance to show one storey below in the view setting. if they actually exist on this level, extend the walls to where they should be on the roof level, and then they will show.
if you just need it for reference, use Trace and Reference...
ArchiCAD since 4.55 ... 1995