The Floor Begins

 

Installed 0.025" 5052H32 Aluminum Outer Flooring Layer

For review, when I removed the old flooring I found: an outer layer of roofing felt paper; 1" wood cross members on top of the steel frame; and 1/2" plywood floor. Sandwiched in between the roofing felt and the plywood was (nasty) Kimsul cellulose insulation. I'd need to replace the whole thing.

I read a bunch and saw what lots of other people and manufacturers did and do today. In the end I've come up with something that's a bit unconventional, i.e. I haven't seen anyone else do something precisely the same.

Plywood is still the standard. Airstream just transitioned to a composite product for the 2021 model year. 

For insulation I've seen everything from nothing to insulation with blown in insulation, batting type product, or rigid foam products. Some people just lay down plywood. I'd say this is actually the approach of 80% of trailer renovations. It's cheap and easy and the lack of insulation is no big deal if you only plan to camp in the summer.

The exterior layer is another story. Airstream uses an aluminum "belly pan". Essentially it's another shell but on the underside. Some use roofing felt. And many use a corrugated plastic product that's sort of like plastic cardboard.

I strongly considered the simple plywood approach. The roofing felt can tear exposing the insulation and flooring, and maybe worse, creating a habitat for mice and insects (I found lots of black widow egg sacks in the Boles when I removed the floor - eww). No insulation means no cavity for creatures to lurk. But I was thinking I'd do better than raw plywood, I'd coat the underside with something like an applied waterproof roofing product like "Liquid Rubber". 

Then I saw a picture of a more modern Boles. It had an aluminum skin as the exterior layer. It made sense - it would be more difficult to tear and provide a nice outer layer so you could insulate.

I read up on aluminum products I might use and decided that 5052H32 alloy would be best. It's very tough and corrosion resistant. When I went to search for suppliers online I found it's what Airstream renovators use, is readily available, and not unreasonable in price. So I decided I'd go with 5052H32 aluminum for the outer layer.

There are some downsides and I'll cover them in a later post where I detail the total flooring system I plan to install.

So I was almost at the point where I could start the flooring. "Almost" only because I thought it would be a good idea to fabricate the wood cross members that lie between the exterior and the plywood floor. So I did this. The are load bearing basically only in compression (sandwiched between the frame and the plywood) so nothing special is required. The 70 year old cross members were in good shape and they were some pretty awful looking lumber, basically firewood quality.

I ended up using some kiln-dried 2x4 material ripped to the appropriate thickness. I half-lapped the joints but that wasn't really necessary, it was the woodworker in me overdoing it.

For good measure I added a couple unsupported (by the frame) cross members where there is a longer run with no frame support. These aren't really necessary, but when I screw the plywood to them they'll act like battens providing some stress relief for the plywood. It was cheap (I had the wood) and easy so why not.

After a dry-fit of the wood cross members I removed them and got to installing the 5052 aluminum. The old adage of "measure twice cut once" was substituted with "measure 10 times cut once". I had just enough 5052 to get the job done and it was all in one long 20' x 4' roll. A wrong cut would have been an expensive (in time and money) mistake. I'm glad I took the extra care because it prevented at least one major mistake.

The first cut marked and ready

First piece in place and ready to rivet

The aluminum doesn't need to be real securely fastened to the trailer frame. The wood cross members will be bolted down from the top through the frame some light duty connecters are all that's necessary. I chose to use 1/8" stainless closed-end pop rivets through the frame and aluminum ones where the seams of the aluminum meet. The stainless rivets will reduce the amount of dissimilar metal corrosion between the frame and the connector, but the tradeoff is the potential for corrosion between the connector and the aluminum sheet. I'm counting on the POR15 frame coating to minimize the 5052 to frame galvanic corrosion. I'd consider using UHMW polyethylene tape between the frame and the aluminum but decided this was expensive over engineering.

Stainless steel closed end pop rivets that no one will ever see

I ended up cutting three sheets, each slightly more complicated cuts than the previous. I didn't plan it that way but it worked well to build my confidence as I went. I really like the Eastwood sheet metal shears I bought, they are easy to control even to get a nice clean cut even for a novice like me.

Second piece cut and laid down ready for riveting

I had a choice in how to overlap the pieces. There is a 3" overlap between them and I chose to lap them so that water flowing "down" from front to back (say when traveling down the highway in a rainstorm) would shed best. Along the seams I used Sikiflex 221 as I've done on the exterior patches I've riveted on.

The last piece in place

It was getting dark when I knocked off (the sun is going down at ~4:30 since it's late November). I had a couple rivets yet to complete in the morning, but I was tired and needed to clean up the garage before my better half arrived home from the clinic. I wrapped up the final few rivets and sealed a few mistakes this morning and I'm calling it a wrap.

Next I install the wood cross members.