To protect the underside of your house, we recommend installing a metal barrier under your subfloor. There are a couple different materials that can be used as this barrier, but what we have found to be extremely durable, easy to work with, and inexpensive, is metal roofing. The roofing material is turned upside down so that the raised portions are facing down and don’t interfere with the subfloor installation. It is then temporarily secured in place with metal screws. Later, when the subfloor is installed, it will be sandwiched between the metal frame of the trailer and the wood subfloor, thus securing it permanently.
For the list of reasons why we prefer not to attach the flashing to the underside of the trailer, check out the section on what makes our trailers different.
On our tiny house trailers we’ve added a steel flange along the side edges of the trailer and replaced the wood decking with two steel ‘runner’ beams. These combined changes offer a total of up to four* connection points to securely attach each of your floor joists to the steel of your trailer. On a 20 trailer this can be as many as 48 connection points!
At each attachment point, we start by drilling a 1” wide hole just deep enough to countersink the washer and head of the galvanized hex bolts we’ll be using later. Next, using a 3/8” drill bit suitable for both wood and steel, we drill a hole down through the floor joist and through the metal piece below (either the flange or runner beam). Finally, we secure the joist and trailer together using a 3/8” hex bolt and nut, with a washer at both the head and bolt ends. The length of the bolts that go through the runner beams will need to be at least 6-1/2” long, while the bolts that go through the flange will need to be at least 5” long.
People are often concerned about the perceived difficulty of drilling through the metal of a trailer, but as I demonstrate in the “Ask Dan” episode below, it’s not difficult at all.
*Not every potential connection point can be used on every joist. For instance, on some joists the angle of the fender may not allow enough room for the tools needed, or rear lights mounted close to the flange may make a connection point unavailable.