I noticed that the slope tool configuration in the toolbars supports not only 'straight' slopes, but all kinds of slopes. So I made a menu with every possible slope. That works for all slopes which have at least one corner at zero height. Those which have no zero-height-corners can be used and create correct slope walls, but no correct "roof".
I guess this is because slopes without at least one zero-height corner do not really exist, and it was probably never planned to have anything but straight slopes in the toolbars anyway, since they are not that useful. Still, I wanted to report it in case it's easy to fix, or to get an 'official' statement that it's not intended.
This is intended (in the sense that the terraform code cannot handle slopes with no zero-height corners besides all-up slopes).