Roof — shaped by hand

Press Rectangle (or type rect) and place it where the roof belongs.

Use Edit to shape the roof however you want.

Change its building role to Roof in the right panel. Then let the rooms below stretch up to it with Fill to Neighbor.
You can reshape the roof anytime — the rooms follow automatically.

Don't see it in the panel? You're probably still holding a face or an edge from the last step. Press Esc once — that drops the part but keeps the roof selected.