I changed just the shed roof room height, under room properties/structural change the ceiling height, yes make an exterior wall, you can uncheck "ceiling over this room" to get the sloped roof, there is a ceiling structure just below that can be used adjust ceiling thickness.
if you open a second window with a cross-section of the room you can see the changes
FYI the program does have a mind of it's own, auto-making the roof is not always possible, but roofs can be manually drawn, they have tutorials on company web site, but it is a little tedious
