2x4s at every 24" on center were used to build the walls. When designing your tiny house, it's a good idea to plan out your walls and use where the studs land as the edges of your windows and doors. This will make framing easier. The walls can be build on the sub-floor since it should be a level surface. Building on the ground is not recommended. After a wall section is built, it can be stood up and nailed into the sub-floor. Additional temporary kick braces should be used to reinforce the walls until they are all built and connected together. Headers need to be built above all window and door openings that cut through a stud and that are more than 24" in width. In my case, the headers are two pieces of 2 by material with a piece of 1/2" sheathing sandwiched between to make up the 3 1/2" wide wall cavity from the 2x4. A 2x4 header can be used for openings up to 48", a 2x6 header can be use for opening up to 72" according ANSI A119.5, the construction code for Recreational Park Trailers. I didn't know this until I was almost complete and used 2x6 headers almost everywhere. Overbuilt is better than under-built.
I constructed the walls in two methods that I haven't seen done while researching. The loft walls were constructed as full heights continuous wall. This removes a thermal break that happens when the loft wall is constructed as a second wall on top of the first floor walls. I had to use 10' 2x4 lumber to accomplish this. A little bit of ingenuity was used to construct the higher walls since they were built on a 7' wide subfloor. I used horses to support the walls that hung out over the floor to support it. The other decision I made was to use a ledger board to support my loft. 2x6 ledger with Simpson Strong Tie 4x4 deck loft support hangers were used. Timberlok 4" screws were used to fasten the ledger boards deep into every 2x4 wall studs. Simpson #10 - 2 1/2" screws were used to fasten the hangers to the loft and Simpson #9 - 1 1/2" screws were used to fasten the 4x4x into the Simpson hangers. I consulted Daniel Bell for this construction method.
16 gauge metal strapping was used to tie the walls together. I strapped just above the 1st floor window elevation around the entire house. I would do this differently if I went back. I would have strapped at approximately 45 degree angles across the walls and corners if I did it again. It seems like that would have been more structurally sound. Also I used a double top plate to tie all of the walls together. This may not have been necessary but I liked how it connected the corners of all the walls together.
1/2" plywood with liquid nails fastened with 2" screws at every 6" around the edges and 9" in the middle to the studs were used. It was at this point that I realized the my walls from the bottom of the sub-floor to the double top plate exceeded 8 feet, which is the length of a standard piece of plywood. The sheathing acts as strapping to connect the sub-floor to the walls. My lower walls were 8'-1 1/2" high, which meant that I had to rip a piece of plywood 1 1/2" wide to make up the difference. Double check your design for this. The other mistake I make in laying out my studs is that I started my first stud 3/4" away from where it needed start. The 4' wide sheathing landing apast the middle of the stud, meaning that the next piece of plywood would be able to be fastened at the edge; thus causing me to rip 3/4" off the plywood. Not the biggest deal, but both of this oversight probably created an extra day of build time. I sheathed my house without initially cutting out the openings for windows. When the sheathing was complete, I drilled holes in the corners of the framed out windows, drew lines on the outside of the holes with a straight edge, and drove the circular saw blade to cut out the windows on the outside.
Special thanks to my Wednesday night bros Chris and Hall for bearing the weight of the walls.