does underside of floor need framed?

[Poorlytiedfly]

Is it necessary to frame the underside of your floor? I am just beginning my build and have a custom built frame and was planning to start with a laminate floor consisting of 1/2" ply + 1"x2" frame/insulation + 1/4" ply top skin (all screwed and glued). If I build this way do I also need a lumber frame on the bottom of the 1/2" ply floor resting on top of the trailer frame? Seems a bit pointless...

[daveesl77]

I way, way overbuilt mine, so your numbers are probably fine. With Conch Fritter it has a 9" overhang on the sides and 12" on the rear. My floor structure is 1/2" ply, 1" framing I made, fully filled with eps foam board, then 1/2" on top. Everything is glued and screwed together. The bottom of the floor is coated with PMF, the top with the "mix". I can jump up and down on an overhang corner and no movement at all. Yes, it added strength, but also a bunch of wasted weight. If I do another, the max I'll go is 3/8ply, 3/4 insulation, 3/8 ply.

My floor deck is throughbolted to the frame, using nylon washers and silicone between the deck and frame. All bolt holes are filled with silicone.

If you aren't doing an overhang, then your design will be rock solid.

dave

[Woodbutcher]

I kept using less for my floors on each build. The current trailer only has a 1/2" floor. My frame has cross members every 18" so those support most of the weight. I used a good exterior grade plywood with exterior glue and coated both sides well with waterproof finishes. Works fine and keeps the weight down.

[Poorlytiedfly]

more info... I have absolutely no overhang. I had my trailer frame custom welded to be an exact footprint of my build.



and the floor will be bolted to the frame with 16 1/2" grade 8 bolts into 1/4" tabs welded onto the frame



I am copying the technique that JSS06 used on his "dog house" build.

[linuxmanxxx]

Lay the 1 x 2 flat place the 3/4 foam either pink home Depot or blue from Lowes in the spaces and glue it and it will be more than strong enough. Use the waterproof on the cheap 8mm luann and save money and weight. I used it on all my builds and works fine. Make sure all joints are sealed correctly and it'll last many many years.

[KCStudly]

TPCE has 5 mm ply top and bottom with 1-1/2 insulation and cedar frame. No subframe under the floor. Some people do that to space their cabins up over their wheels, while others allow the wheel wells to bump into the cabin and cut the mattress a bit.

If you can't justify swapping out the 1/2 inch ply (perhaps using it for another project, and provided that the core frame members land on the steel members below, I would put it on top with the thinner stuff on the bottom.

[angib]

No, you don't need to frame under the lower skin, assuming your 1x2 framing is connected to both skins.

It would be sensible to make sure there is framing inside the sandwich directly above any trailer frame members that will be in contact with the lower skin.

[noseoil]

My floor has 1/8" baltic birch for the bottom skin, 3/4" stick framing at the edges & cross-members, 3/4" foam & 1/8" BB on top. I did chicken out & put a layer of 1/4" ply (58" X 30") on top of the 1/8" at the doors, worried about over-stress at that point. Standard framing for a house deck on a roof is 24" on center for 1/2" plywood (a roof load). 1/4" ply will take 12" on center & support a roof load with no problems. My 1/8" skinned deck has nowhere near that amount of framing & works well enough.





Your frame looks like it has plenty of support for the deck & lighter is better in the build. Too many people think they're building a house on wheels.

[Nobes]

Kind of a silly question I think but...

I am building a sandwich floor. Bottom OSB ply, 1x2 frame with 3/4 insulation, top OSB ply. I assume I should glue the foam to the top & bottom OSB, and that I do not need to sand or waterproof the inside of the plywood before I glue the foam. However I should sand the top and bottom and waterproof both the top and bottom.

Is that correct?

Since I am using OSB as many others have done, should I use The Mix or move up to the black stuff for the road side of the sandwich?