I've used smooth FRP as ceiling panels in my last 3 builds. On my first I used that whiteboard type laminated hardboard, which was cheaper, but it wore through in a couple spots from packed up items rubbing against it so I wanted something more durable and with color all the way through the material. The FRP is kind of heavy and is 0.09" thick, but one of my local building suppliers stocks it in 4x10 sheets in smooth or pebbled. I can order it in other colors and sizes, but always only 4' wide. Free freight if I wait for their next shipment and include it with their order. Other manufacturers may offer it in 5' but I simply cut the ten footers in half and joined them together with a union type trim piece (readily available too). You can even get FRP in large rolls or pay to have them precut the sheets. I did not use a backer of any type and my spars are a foot apart. Space the spars closer for more support but once it's bent to a curve it's pretty rigid. My only concern were my joined edges near my kids bunks. I was a bit worried pushing or kicking too hard against it might pop the sheet out of the trim. I insulated my roof cavities with foamboard so that should help support it from behind. I also glued the trim pieces on but I didn't use the proper glue. I don't think much, if any, adhesives actually chemically bond this stuff. Overall my ceiling is pretty solid now, but better planning would have been to place spars at the joints. The adhesives I used were for plastic panels like shower and tub surrounds or stuff specifically mentioning FRP. I tried several types and they all seemed decent when going to the wooden spars. Two non porous surfaces is different than going to wood for some reason. Glob massive amounts on because the curves never seem to match up quite as planned. These glues take time to cure and harden up pretty good. Holding the sheets in place was a pain. Afterwards I put tons of generic adhesive/sealant on the back along every edge, spar, and joint. I used end cap trim to give the wall edges a nice clean look but I guess you could caulk it too. I do the ceiling first and then the roof. I also wish I had remembered to thoroughly rough up the back side for better adhesion. Another suggestion would be to ask the manufacturer what adhesive they recommend. I did not use any physical fasteners though I was tempted to, but they wouldn't look good. The roof vent will help hold it once the inside trim flange is screwed on and the screws for my ceiling lights help a bit too.
My build thread:
http://www.tnttt.com/viewtopic.php?f=50&t=65949There are other options for ceiling material but most are thin and easily bend. Probably not much in 5' wide though. Regardless of the material the ceiling is one of if not the most difficult part of the build for me. Maybe it's my designs or large ceiling area, or my technique, but whatever it is I loath that part of the build.
Lucas