Planning the Floor

The work station, here pictured minus the usual cat.

Now that I have my trailer, the real work begins. The first task will be to build the floor, but before that... I need to figure out how to build the floor.

Until yesterday, I thought "Yeah, yeah, I pretty much know how the floor goes together" and didn't worry much about it. But then I sat down to actually draw out the plans and... oof.

The floor will be made of up four major components: a plywood undercarriage, a 2x6 lumber frame, rigid foam insulation, and a plywood subfloor. Basically, a sandwich of lumber and insulation between two layers of plywood. The lumber is glued and screwed together to create a frame; the undercarriage plywood is glued and screwed to the frame; that assemblage is laid undercarriage-side-down inside the trailer bed and bolted to the trailer; the insulation is cut to fit inside the frame; and finally the subfloor is glued and screwed on top so it covers the wooden frame, insulation, and trailer bed extensions. I plan to use sill plate gasket (i.e. foamy material that comes in a roll, normally used to block air leakage between a house's concrete foundation and the wall built on top) where the metal trailer makes contact with the wood/plywood. I hope this will act as a thermal break (i.e. insulation) between the metal and the wood, so that less of the cold from outside gets conducted inside. I will seal up any other gaps, in particular gaps in the rigid foam insulation, with spray foam insulation.

That all sounds reasonably easy, right? The challenge is that--yet again--the design has to meet a few different requirements that don't perfectly match up with each other. Namely,

1. The edges of the plywood subfloor--and, ideally, also the plywood undercarriage--should line up with the 2x6 joists and beams so they are supported and the floors don't squeak and bend under footfall. 
2. The joists should line up with the ribs of the trailer, so that they are supported as fully as possible.
3. The joists should be spaced no more than 24" apart, measured from the center of the 2x6 (that is, the joist spacing must be at most 24" on center).

Then, consider these facts:

1.  The ribs of the trailer are spaced unevenly, anywhere from 18" to 31" on center--no two ribs the same distance apart.
2. None of the trailer ribs line up with where the edges of the subfloor plywood will be.
3. 1:24 scale drawings are hard when you've misplaced your quad-ruled graphing paper and your ruler unhelpfully divides inches into 1/16s instead of 1/12s or 1/24s.

But, after a whole lot of eraser dust, measuring and re-measuring the trailer, and unit conversions... I have it! The (initial...) floor framing plan!

Look upon it and weep, ye mortals! (Maybe click to view full-size and then weep...)

The purple dashed lines in the drawing above represent the nine trailer ribs; the purple outlines delineate the trailer frame and extensions; the shaded pieces around the inner edge are the outside of the floor box frame, made of 2x6s lying on edge; and the striped-looking pieces are the joists and beams, which are also 2x6s on edge. The joists and beams are attached to each other with joist hangers. I did not draw the wheel wells; in real life, I will need to cut out rectangles from the plywood subfloor pieces to fit around them.

Subfloor plywood (rough) positions

I decided to start with the joists and beam that will support the edges of the plywood subfloor. I'm going to consider their placement non-negotiable; they must fall under the edges of the plywood. The 4'x8' subfloor plywood will be arranged like so...:

(Update: I may have to stagger the seams of the subfloor plywood, as many sources recommend you do. I'll look into it more and see if it's really necessary, since to do so will involve cutting plywood, which I'd prefer to avoid...)

...With the interior lines denoting both the edges of the plywood and these principal joists and beams that support them. The pieces running across the width of the trailer bed are joists (joists A and B run under the short edges of plywood pieces 1-4 and along the left long edge of piece 5) and the piece running down the length of the trailer bed is a beam (divided into 3 sections, C, D, and E).

With the plywood subfloor now supported adequately at the edges, now I have to fill in with joists that are spaced 24" or less on center and line up with the trailer ribs, as much as possible. I gave each rib a lower-case letter designation, a through i, which also applies to the joist if there is a joist directly on top of the rib. Most of the ribs are spaced 24" or less on center, but I need to add two extra joists between ribs to make sure there aren't any too-wide gaps. Those extras are labeled c1 and d1 in the image detail below (more visibly, they don't have a purple dashed line on/near them):

I'm about ready to weep, myself.

Note: ribs d and h do not have a joist directly over them, since the subfloor support joists A and B fell too close--but not directly on top of--them.

Also note: I plan to put additional pieces of 2x6 around where the wastewater lines exit the house, under the shower and under the sink. However, I need to look up the specifications of the tub I want before I know where exactly I'll be putting that drain, and... yeah... I'll get to it.

Also also note: each lower-case-letter joist is divided into two halves, one on each side of the central beam. I decided to break up the joists rather than the beam because that way, both the joists and the beam are supported by the trailer's steel ribs; if the joists went through and the beam were broken up, then it would hardly be a beam anymore--it would just be blocking, spacers between the joists--and it would be supported only through attachment to the joists, not by lying across the ribs.

...Let me tell you, it makes a lot more sense when I explain it now than it did when I was fumbling my way through the drawings and calculations. The calculations were the worst--mainly the ones to do the 1:24 scale, but also the basic but relentless arithmetic of figuring out where exactly the center of each joist is in relation to the other key points of the diagram. And I consider myself pretty good with numbers!

Now I'll just wait for some new piece of framing knowledge to show up and force me to rehash all of it...

(For these floor framing plans, I am basically following the guidelines set forth by Dee Williams in her book Go House Go: How to Build a Tiny House on Wheels. I also found Kate from Naj Haus to be a helpful source of ideas, particularly in this post. I'll think about her technique of lining the bottom of her floor box with Tyvek housewrap...)