I've been thinking.... and researching on the internet... and learning lots.
And I've come to the conclusion that simply laying down sheets of 3/4 plywood may not be the best way to go. It's simple, yes. But, there will be virtually NO insulation from the cold on a chilly night. Ever slept on the bare ground without a foam pad? I have, and unless you have a really good sleeping bag or bed roll , the ground just _sucks_ the warmth out of you.
Taking another, less arrogant look at the floor design of this trailer, I realize now that the original designers had created an insulating barrier of air and corrugated cardboard sandwiched between those sheets of 1/4" plywood. A good idea. But I still feel that it's not structurally strong enough.
So, I've rethought the floor design and have come up with this one.
The idea is to glue and screw sheets of 3/4" inch ply over a framework built from 1" x 1" lengths of yellow pine. Paint the whole thing with exterior paint. Then coat the bottom side with roofing tar or brush-on truck bed lining. Then, glue and a screw in 1" or 3/4" pieces of insulated foam boards. Finished off with a layer of Tyvec paper stapled to the bottom of the floor.
Than, the assembled floor is lain onto the steel trailer frame and bolted down via lag bolts and washers from underneath. (I still like the idea of being able to unbolt the frame from the trailer body in the future should the need arise.)
The original floor was about 1 1/4" thick. And the walls were screwed to the sides of that wall. I still plan to bolt the wall frames to the top of the floor edges rather than the side. This means that (theoretically) with the addition of the 1" wood yellow pine frame, the 3/4" plywood, and perhaps an additional 1/4" for the final layer of glue and linoleum flooring.... I will lose close to 2" of headroom inside the trailer body.
Really, with actual thickness in the real world of contracting supplies... it will more likely be around 1 3/4".
I'm prepared to accept the loss in headroom in exchange for a stronger, insulated floor.
Will the Tyvec paper release the water it collects after a day of driving through rainy weather?
Will kicked up road gravel and small animals shred the Tyvec paper into uselessness?
I've read several warnings from other vintage trailer owners that covering the bottom with a solid surface like metal is an invitation to trap moisture. That's why I think it will be best to just use a layer of Tyvec paper so as to breathe out moisture.
I'm itching to get to the frame and start the welding. But I've got to finish the restoration of a clients' Honda ct70 next week first. Both will be satisfying to work on.... I just want them both done nownownowNOW!