For the arched ceiling you can copy and paste your arch brushes, rotate them, and just put them together at the corner of your roof (overlapping them). Then just lop off the overlap with the clip tool. That will line up the ceiling but it will be a hard 90 degree turn.
If you want to have the arch of the ceiling itself curve around the 90 (since much of FF is curvy) you'll want to use the torus tool.
|