Multiple Wall Plate Heights on Footprint Roof

[xtchall]

I am trying to add height to one of the gable roof's sidewalls by selecting the wall sketch lines and changing the Plate Offset From Base (all walls currently have common base). Revit complains it can't make footprint, with only option "Delete Roof" to finish the sketch - not really an option, but maybe should take the hint :)

I attached a Roof Plan.

Any ideas much appreciated - wasted way too much time banging head on this!