The tiny house was designed as an upside down boat. The ridge beam is the keel, the rafters are the frames, and the floors go in the ceiling!
Floors are the horizontal boards that connect the frames to the keel. They strengthen the hull and prevent wracking (twisting) and flexing. Tiny houses need this as they will be exposed to hurricane force winds while driving down the highway! I could not find the name of this piece of wood in house building, but I have heard of people complaining about the house twisting in high winds. These wooden floors should strengthen up the roof and the entire tiny house.
Installing the floors was rather straight forward. A length of 2x4 was cut and screwed to the rafters. If the roof were to try and lift, the floors would move upward and be stopped by the ridge beam. If the house were to twist, the triangulation of the floors, rafters, and ridge beam would resist the motion.
The floors are eventually going to be covered up when we insulate the inside and then cover the frames with interior siding, but for now, they can shine in the sun with all their structural glory!