Hey Paul. I have a 1998 Beaver Patriot Brandywine and it just so happened I had the pleasure of replacing a section of the ceiling in the kitchen area that had rotted from a long time leak in the roof. The ceiling is a 2" foam board sandwich with very thin plywood (less than 1/8th inch) on either side glued together (so plywood, foam, plywood). Then on top of that was 3" thick foam board between the ceiling and the roof. Extra 1"x1" wood support was around the fixtures. Oh, I'm 300 lbs and haven't fallen through yet!