Torsion box for camping platform and alternate maerials

[Travis Baits]

Yes, camping platform. I have a 5'x8' trailer that I've put an aluminum frame on top. The frame is 80/20 extrusion 24" tall bottom and top rails of the 1.5" x 3" extrusion. Strong and light. For my yearly camping trip, I use 4x4x8' 3each to support 2 4x8' ply sheets. That is where the tent goes. ( it almost always rains REAL heavy for the festival. I sleep better when dry.)

Here is a pic: 355381

I want to put HDPE plastic on the sides...aka, marine board. Same basic stuff as cutting boards are made of. not a problem there, other than my wallet gets smaller.

The top...I need a foldable top that I can then fold out to 9'x9' or even 10'x10' for a sleeping platform. I am thinking of a set of torsion boxes. The issue is waterproofing them. Or has anyone tried alternative matierials for a torsion box?

My concerns with plywood torsion box:


Marine plywood is expensive.
replacing the warped sheets will be problematic if the coating leaks.
unit will be stored 300 miles from home most of the year, so maintenance (recoating of exterior of TB) is an issue..


Support legs will be easy, as Unistrut make galvanized holey-tube that telescopes.

Looking in serveralforums to see what ideas might be had. Please let me know!

[Eric Schmid]

For comparison, I built a box for a utility trailer five years ago that has marine ply skins on the inside. This trailer holds yard debris & soil year round. It is almost continuously full or partially full of decomposing yard debris. It lives outside in the elements year round. It was coated with a boat builders concoction of linseed, varnish, pine tar, and a couple other elements when it was built. The marine ply shows little sign of wear except for scratches from shovels or rock. One difference is these are vertical surfaces, although the continuous exposure to decomposing material may create a comparable environment to horizontal surfaces?

The torsion box principle will apply to any material, so you might take a look at Azek sheets. These are going to require more internal support, but they won't absorb any moisture or rot. If you wrap the whole box the joints can be sealed with PVC glue. Azek is light weight. It's more expensive than marine ply, but the only maintenance you'll ever have to do is cleaning.

Azek is soft and isn't suitable for structural, so legs, hinges and the like would need to anchor into wood, aluminum or other structural material to be secure. This could be blocking placed inside the torsion box.

Which ever way you go, I would avoid fasteners protruding through the top skin. You can glue and clamp. Pocket hole screws would be a handy way to assemble.

[Mac McQuinn]

Travis,
I've used a good quality MDO 3/4" ply on torsion box construction and found it held up well and was a nice product to work with. I believe MDO is used for highway signage and considered to be a decent product for longevity. I sandwiched 2X material w/ the MDO and used TB3 and a good quality, coated deck screw for fastening.
Good luck,
Mac