Once the aluminum flashing and insulation were finished, it was time to cut and install the plywood subfloor. We walked into Home Depot fully intending to pick up the necessary materials but instead of plywood, we mistakenly bought a cheaper version of plywood called OSB (oriented strand board, or pressboard). OSB is a common material used for subflooring and wall sheathing and is popular due to its lower price tag. It's not as strong as plywood and much less water resistant. Now, the subfloor shouldn't really ever get wet, but if it ever does, a plywood subfloor would react much better than an OSB subfloor. When in contact with water OSB swells, causing problems. We didn't realize that we'd used OSB instead of plywood until we'd already cut the 4x8 pieces to size, so we decided to stick with the OSB and make a few changes to compensate for it. Originally, our plan was to anchor the sill plates on top of the plywood subfloor then build the walls on the sills. Because the OSB is sensitive to water, we decided to do a bit of extra work to shield the OSB subfloor from unwanted moisture. On top of the trailer, we cut the OSB sheets 3.5" short of the perimeter of the trailer frame to allow space for the 2x4 sills to be anchored directly to the steel trailer frame. We also installed strips of L flashing between the sills and the OSB subfloor to prevent any rogue rainwater from getting to the OSB. If water somehow got into the walls and down near the sill plates, it would be stopped from entering the subfloor and instead, would be guided out of the bottom of the wall by the L flashing. With this flashing installed, we then installed the pressure treated sills and prepared them to be anchored to the trailer. Anchoring is one of the most important parts of the tiny house building process because it is how the structure and all of its weight is connected to the trailer. By firmly anchoring the sill plates to the trailer frame, everything built on top of the sills is held strongly to the steel trailer frame. Instead of welding threaded bolts to the side of the trailer frame, we opted to bolt down the sills by drilling holes through the sills and trailer frame, dropping in 4" long 5/8" bolts, and fastening these bolts with heavy-duty washers and nuts. We temporarily removed small areas of the insulation in order to reach down into the trailer frame to tighten the nuts. We placed the bolts no more than 4' away from each other along the perimeter of the trailer and a bolt within 12" of each corner. We also chose to place our hold-downs at this time in order to maximize the strength of the walls. To do that, we marked on the sills where each stud would go followed by a mark for each hold down.
Once the anchor bolting was done for each side, Mike drilled holes for each hold down with 5" long, 5/8" bolts. We wanted to ensure the integrity of the trailer itself so we chose to bolt the hold downs on the inside edge of the sill plate, avoiding the c-tubing, and attaching them to the trailer by using a 2" by 2" metal plate, sandwiching the c-tubing and another piece of metal to keep it flat and secure. Once all twelve hold downs and sixteen anchoring bolts were tightened, we set the subfloor back in its place and we were ready to screw in the subfloor and build the walls!
A couple of the most common questions we receive about our tiny house project is "How do you know how to do all of this?" and "How much construction experience do you have?" I understand why people ask this. It seems like only people with lots of experience in the field can do this (that's what I thought too). We were asking ourselves these questions before we started this not-so-tiny project. We were intimidated by the amount of learning we had ahead of us. There is so much that goes into building a house and we knew about none of it. Going into this project, we had absolutely no experience in construction but we do have family and friends who've consistently been helping us plan for and work on this project. Specifically, Jen's dad and my uncles have been helping us a ton. They've invested their time and effort into our house. Furthermore, many people have offered us tools to borrow and whatever knowledge they have about construction. We just want to make it clear that without the help of everyone who have invested in us, we would not be able to build this house the way we are.