by noseoil » Sat Aug 16, 2014 8:08 am
Just in general, 1/2" sheathing grade ply or OSB (actually 7/16" now) will span 24" on center with 2X joists. Most residential roof systems are laid-out with 2' on center being the norm for roof trusses or stick framing. This allows for a 20# psf live load, 14# psf dead load. In plain words this means the roof sheathing will withstand a total load of 34 pounds per square foot of roof surface area with 2' centers. Your basic construction worker type with a hammer, tool belt, nail pouches, etc. can walk around up there without falling through just fine. A mattress will distribute the load across the area more evenly, so really 3/8" ply will work if it's supported properly (16" on center). I'm using 1/8" baltic birch under 4/4 poplar framing, with pink foam & 1/4" baltic birch for the decking. I'll cover the 1/4" with some 1/2" osb scrap while working on the inside, so I don't hole it, but the mattress will cover it once the trailer is finished.
I'm building with 3/4" exterior grade fir plywood "skeletonized" panels for the walls, 3/4" foam insulation to fill the voids in the 3/4", 1/8" baltic birch inside & outside for skins. This will then get skinned with aluminum or fiberglass & paint, haven't decided, since I'm not there yet. The 3/4" sides will be cut down to 1 1/2" for the skeleton's ribs. This will give plenty of strength for through-fastening of the walls to the cabinets with screws & glue, light fixture attachments, doors, 3/4" roof slats, etc. It needs to be strong, but it needs to be light as well.