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  • Writer's pictureJim Martin

Bolting On

Updated: Oct 20, 2020

Because a tiny house uses a trailer as a foundation, and because one hopes to be able to haul that trailer (and therefore the house) at least a few times during its lifetime, a tiny house needs to be securely attached to the trailer.

For this part of the project, I chose eight-inch long, 3/8 inch galvanized carriage bolts.

The bolts have to span a total of about 7 inches to reach vertically through the 2x6 floor joist, the ½ inch plywood underneath, that thin layer of aluminum sheathing, and finally the roughly 2x2 inch steel framing member of the trailer.

This required first drilling a 1 inch diameter hole about 2 inches into the floor joist to countersink the head of the bolt. Then, using an extra-long 3/8 inch bit, drilling a hole through the reminder of the 2x6 and continuing down through the plywood and steel framing member of the trailer. (You can see the long bit going all the way through in the picture.)

I spent about a half day climbing up on top of the trailer, drilling a hole, then climbing underneath to check my work, threading the bolts through, then attaching and tightening the nuts and washers.

As I drilled each series of holes above each framing member, I made sure the floor joist was in its proper place, the standard 16 inches on center from the joist next to it. That way, they run perfectly parallel which will make it easy to attach the plywood subfloor when we get to that stage.

Other tiny house builders have suggested leaving enough bolt protruding beyond the tightened nut underneath the trailer.

I found this suggestion helpful as some of the bolts will inevitably work themselves loose ether as the trailer is moved from place to place, or as the floor joists potentially shrink a tiny bit as the house ages. The idea is that by leaving a little tail of bolt sticking out you can grab it with some vice grips and spin the nut tight again with a wrench.

What I found in practice, however, was that using an impact driver with at deep socket is probably all I’ll need to do.

Not only did it save me from having to tighten all those bolts by hand, but the driver spun the nut with enough vigor that the carriage bolt head didn’t spin in place as the nut tightened underneath.

This probably means that if the nuts ever do come loose, I’ll be able to re-tighten them by the same impact-driver method and avoid the slower, two-handed vice grip and wrench process.

Finally, I knew the work that the extra-long drill bit was doing was going to be demanding. And knowing that I needed to drill about 30 holes (there will be more coming later), I bought two bits at my local hardware store assuming the first would either dull or break before the job was done. Lo and behold somewhere around hole 22, the first drill bit simply snapped mid-hole. Feeling clever, I switched to the second bit to finish the job. But the second bit snapped before I finished even two holes!

Here’s my shout out for local hardware stores: I brought both bits back to my favorite local hardware purveyor: Earl’s True Value of Fredericksburg, VA. I spoke to the manager explaining that I felt the first bit’s performance was fine. The 22 holes it managed seemed about right to me. But the second bit just seemed defective. The manager looked briefly at the bit and said, “Sure, I can see how cleanly it snapped off right here. Sorry about that!” and replaced the bit happily. He saved me eleven bucks and increased my sense of loyalty at the same time. (Carriage bolts, nuts, and washers also happily purchased from Earl's.)

So, here’s where we stand: I still need to finish off the last few holes on the joists themselves, then I’ll need to attach the external 2x4 joists out on the edge of the flange (the “outboard” section of the trailer in line with the wheels). But those will be a piece of cake by comparison. The bolts are shorter, and they can be reached without having to crawl underneath!

After all that we get to insulate and put the subfloor on. Then the walls go up. Stay tuned!

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