Posted this on the wrong site, so re-posting the question and answer for posterity...
I'm trying to get my plan to build a simple gable roof, but for some reason it just won't do it even though I followed the tutorial exactly. The house plan is a simple Aitch, with a flat roof garage (observation deck), a portico at the rear of the house, and a flat roof master suite opposite the garage side of the house. I made the East-West walls of the attic full gable, and unchecked Full Gable on the the North-South walls. When I try to build the roof, though, nothing happens. Any suggestions?
The solution,.. Rename the "Attic" to anything else.
Sure enough. I renamed the room from "Attic" to "Utility" and voila... Instant roof. (Needless to say, it's not finished yet.)