Thanks for assisting.
What I'm trying to do is pretty uncomplicated. I'm not trying to create a flat ceiling. Nor am I trying to create a ceiling plane that uses the underside of the roof rafters as the "ceiling". I'm simply trying to input scissor trusses, in which case the underside of the bottom chords forms the ceiling. And so I followed what appeared to be a very straight forward tutorial to do so until it led to a dead end (because this uncheck of "flat ceiling over this room" feature is apparently unavailable in my not-so-old software). Perhaps that tutorial, which is supposedly applicable to fairly recent versions of Home Designer Pro like mine, should either be taken down or revised so that it gives proper instructions for creating a scissor truss ceiling. Just a suggestion...
JimC
www.chelanhouseworks.com
Home Designer Pro 2017