Hello DIYers! As promised here is a companion blog post to go along with my youtube videos regarding my tiny house subfloor design.
The design stage of the tiny house is probably the hardest, as there are so many cool builds out there on HGTV, YOUTUBE, and personal blogs its hard to narrow down what you want. Its only after months of research and design that you can get a picture of the direction you want to go and this will also keep changing as you find new ideas to inspire you. My advice is to keep designing until you have a plan that doesn’t experience any major changes for at least a month. That’s when you know you are ready to start. Don’t get caught up in the idea that you need to have all of the plumbing, electrical, and interior design figured out before you start because you will never be satisfied and you will become stuck in the planning stage. Have the general layout of where your bathroom, kitchen, and bedroom will go, and you are set for the subfloor. The rest of the designs will work themselves out as you build. Once you are ready, the most important step is to act! Buy your trailer, and the momentum will carry you to starting your build!
For my tiny house frame I am using 2 x 4 (really more like 1.5” x 3.5”) non-treated boards for the subfloor, walls, and ceiling. I decided to go with these instead of 2 x 6 because I am trying to save on weight for the house, and also because my structure will only be about 12’ long and 8’ tall (without trailer HT), so 2 x 4 should be enough support. I am using non-treated wood for the subfloor because the entire cavity is sealed off from the outside world, so I have no worries about water damage to the boards. I used 3” construction screws for attaching the framing together, and I also added aluminum gusset angles at all main corners for added strength. The wheel wells are a bit unique, as I used 2 x 2 lumber to construct a frame with will be sealed and flashed. I will go over more of the details of how this will work on my wall build.
For the subfloor and exterior of the house, I am using Tyvec house wrap as my vapor barrier. The bottom of my trailer is a flatbed aluminum, so while I don’t have to worry about damage to the underside of the subfloor, I still wanted the vapor barrier around the entirety of the subfloor. The vapor barrier is held in place by staples and sealed on edges with Tyvec Tape. Make sure to overlap pieces at least 6” when installing. Also, I built my subfloor out over the wheel wells, so while the majority was protected from road debris by the flatbed aluminum trailer floor, about 8” of the edges were exposed. Therefore, I used aluminum sheathing to protect those areas, and also provide a metal edge between the sheathing and trailer to seal the subfloor cavity with Flashmate sealant.
Securing the Framing to the Trailer:
So first off, I was a bit wary about putting holes into my brand new aluminum trailer, as I didn’t want to lose structural integrity. However, after a bit of research, I discovered that once a bolt is through the hole the area can actually be stronger than surrounding parts of the trailer. I decided to go with 21 bolts throughout various areas of the subfloor to give uniform stability. I started out using 3/8” x 8” galvanized hex bolts going through the frame and trailer, however, Home Depot ran out! So I switched to carriage bolts of the same size and found out I actually liked them a bit better. I also countersunk the heads of the bolts into the subfloor so they would be flush. Side note, make sure to used galvanized bolts if they will ever see the light of day! Otherwise, they will just rust out on you and you will be replacing them in a couple years. I also strongly recommend buying a 12”+ drill bit for this part so you can go through both the subfloor and trailer in one go. Otherwise, you will end up having to widen holes when they don’t match up.
I decided to run grey water piping through my subfloor instead of the walls because I did not want to deal with raising the shower stall. I used 1.5” PVC piping for the project instead of ABS piping because PVC has more flexibility and should be able to take more road stress. Through the length of the subfloor, the piping goes down 1/8” through each floor bracing from front to back, with 5 braces = 5/8” slope for drainage. I believe this should be more than enough for such a small structure, and it’s not a huge deal if there is a bit a water sitting in the pipes. All pipes were connected using Tees and Elbows and were sealed together using purple primer and PVC cement. After 24hrs curing, they were incredibly stable and held up to running water through them. But, for peace of mind, I also ran Gorilla duct tape around each of the connections. If I ever do have a leak, I’m going to have to take a saw to the floor, which will be a pain in the ass, but I’m pretty sure I have it sealed well enough that won’t be a problem. Also, make sure you write down exact dimensions of pipes and studs in case you need to excavate in the future! Lastly, I’m using a product called a HEPVO valve in lieu of a traditional P-Trap. It is hooked inline with the main drain pipe and will make sure no sewer gases creep into the house whenever I am hooked up to main city lines.
The big con for using a 2 x 4 frame for your subfloor is the lack of options for high R-value insulation. In my area, the highest insulation value I could find was R-15 for my size. I spent many hours researching different types of insulation and weighing the pros and cons of fiberglass, denim, spray foam, and Rockwool. Ultimately it came down between Rockwool and Fiberglass for me, as I was looking for something with a higher R-Value that was non-flammable. In the end, I chose Pure Safety R-15 Fiberglass insulation. I chose this because, in addition to its insulative properties, it also has up to 50% noise reduction, is mold resistant and is non-flammable! It assuaged all my insecurities in one package. I also used Reflectix double-sided radiant barrier to help reduce heat transfer through the trailer into the subfloor and sealed all nooks and crannies using Great Stuff foam gap filler. In total I estimate my subfloor to be about R-18 insulated.
For the top flooring, I went with 3/8” OSB flooring. There wasn’t any particular reason I went with OSB instead of plywood aside from cost. There is a little give in a few areas where there are open cavities underneath, which could have been avoided by using 1/2” OSB, but I am trying to save on weight, and the laminate floor that will go on top should add the last bit of needed rigidity. I used a 3.5” borehole in the OSB for where the pipes come out. Also, I highly recommend drawing all of your stud lines and piping placements out on your flooring, as it will be a quick reference for any screw placements and help you avoid a disastrous PVC puncture!
Some Tools you absolutely need at this stage (highly recommend cordless!):
- Skill Saw
- Mitre Saw (Trust me, you don’t want to have to use a skill saw for everything)
- Impact Wrench (Will save your back when screwing the framing together)
- Work Gloves
- Staple Gun
- Caulk Gun
- Drill Bits
- Safety glasses
- Bore Hole attachment
I had a hard time deciding which brand of cordless tools to invest in, as battery packs only work with the same brand. I went with Ryobi tools, as opposed to Makita and Dewalt. The main reason was cost, as these tools can cost up to 50% less than their counterparts. Some reviews have griped about the quality, but I have found that they work perfectly for this project! They also come with 3-year warranties if they break, and the batteries are unanimous for their tools.
Also, here is a list of all purchased items to date for the subfloor. While the sum might look huge, don’t forget that the cost of the tools and trailer are added in there. The actual cost of materials for the subfloor comes out to be about $580, with some leftovers for the wall build! Also, note some of my costs are a bit lower due to discounts (Black Friday tool shopping!!) but the general expenditure should be representative of what you will spend for your build.
If you have any further questions, advice or encouragement, please message me below!