The underside of your tiny house needs to be protected from water and road debris. To accomplish this there are a couple of choices, and your selection of insulation will help drive this decision.
If you plan to use an insulation that is not spray foam, you will likely need to install metal underside flashing (sometimes referred to as an ‘underbelly’ or ‘belly pan’) on your trailer. Because most insulations are loose and can fall, something rigid must be installed to hold them in place.
The easiest material to use for this purpose is inexpensive metal roofing, called ‘5V crimp’. The roofing material is turned upside down so that the raised portions are facing down and don’t interfere with the subfloor. 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.
If you select metal underside flashing, it will likely be the first thing that is installed on your trailer.
For the list of reasons why we prefer not to attach the flashing to the underside of the trailer when the trailer is manufactured, check out the Underside Flashing section in our trailer guide.
If you are using spray foam insulation (what we use), you have the option to use corrugated plastic under your trailer. This is only possible when using spray foam since the foam expands and adheres when it is sprayed in, thus it does not fall and does not need to be supported.
Corrugated plastic is lighter, less expensive, easier to work with, and allows for more insulation to be installed than when using metal flashing. This is the same product used on the underside of almost every RV on the road.
If you use corrugated plastic, it can be installed any time after the installation is installed. This makes it easier to work on the plumbing, and potentially any gas or electric lines under the trailer.
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.