Why are the subfloor 2X4s necessary

[IamJerryP]

When I check out pictures of people framing up their TDs, I notice everyone seems to be putting stud lumber over the trailer frame for the subfloor plywood to be laid over.
I am curious if there is an actual reason for it????
I figure i am missing something, because I can't think of any reason why the plywood cannot be affixed to the trailer frame, and give a pretty good weight savings.
When I build utility trailers, I just screw wood floor directly to the frame.

Just curious if I am missing anything.
I have no desire for under floor storage, it would lower the roof of the trailer at least 2 inches, it would save weight, and a few dollars.

[KCStudly]

This question comes up frequently. Short answer, there is nothing wrong with putting plywood directly over a level frame (shim the xmbrs if required). Some people use framing to extend out wider and/or space up over the wheel wells. Others like to sandwich insulation, although some just attach insulation from below a single deck. Some people like the idea of under floor storage, and the frame gives a few inches for certain things (while others find it impractical to access the under mattress area once they have built).

For me, the sandwich floor idea was attractive because of the insulation (both thermal and acoustic benefits) and the ability to use a much thinner skin; so for about the same weight as 3/4 inch ply, maybe less, I have 1-1/2 inches of insulation and a floor that will not warp.

Do what you like, just make sure that you waterproof well, especially the edges.

[citylights]

Sub floor 2x4 are not necessary and is considered overkill. 1x2 over the trailer frame is common. That gives a solid wood connection from the frame to 1x2 and plywood floor. It also gives a better thermal barrier than connecting the plywood directly to the frame.

One thing you need to think about is your side wall connection to the floor. With the plywood bolted directly to the frame, will you glue and screw the sidewalls to the end of the floor plywood? With the 1x2 under the floor, the sidewalls can glue and screw to that as well as glue to the plywood floor. Screwing into the end of plywood is not a good practice.

[len19070]

As KC mentioned, sometimes trailers are built wider than the trailer frame and a way to do that is to build a wood frame over the trailer frame.

I don't use full 2X4's either. I generally rip a 2X4 into 3 pieces about 1 1/8" wide.



But other than that short of shimming up the frame to level out for metal thicknesses and Bolt heads theres nothing wrong with going right to the frame.

And again as mentioned this creates a situation with connecting the side walls. A lot of builds that do this leave the frame exposed which is fine. exposing a frame to some is fine, to others its not. (and covering a frame with the side walls creates an all new situation)

That said, there "Might" and I said Might be a condensation situation between the mattress and the floor. I didn't want to find out there was so when I would put my trailer in storage I would put a milk crate under the mattress and elevate the mattress off of the floor.

Happy Trails

Len

[bobhenry]

Mine was just an osb sub floor with a 2x4 halo around the edge to screw the walls to.






Of course you do wind up with these to contend with.

[Kharn]

I used ripped 2x4s (so 2" tall and 1.5" wide) because I was working with a 4x8 trailer for a 5x9.5 camper, lifting the bed 2" let me have only ~2" wheel wells inside the cabin which are hardly noticable under the mattress (plus giving me an easy way to attach the walls to the floor via recessed desk screws), vs having 4" wheel wells and the floor directly on the frame. It also gave me 2" more ground clearance below the battery and storage compartment.

[rowerwet]

My plywood sits on the frame, the walls sit on top of the plywood floor and hold the 6" overhanging floor plywood up, the front wall and galley wall support the sidewalls. Wood below the floor is wasted weight IMO, it comes from thinking about house building rather than boat or airplane construction.
Of course I have different ideas about construction anyway, I used no metal fasteners in my boats or tear, instead every seam is fully glued, making it waterproof and stronger than the wood.