Thoughts on making a thin loft platform
How much headroom do you have in a loft? Simple equation. Total inside height (from the peak to the finished floor) minus headroom under the loft minus thickness of the loft. How to get more headroom in a loft? Let's see, raising the roof or lowering the floor are probably out of the question, and lowering the loft itself just takes away headroom from whatever room is underneath it (kitchen and/or bathroom, generally). The only option left is to make the loft thinner.
Now, let me be up front, I haven't been in any tiny houses other than the one I'm building. My opinion that most lofts are too thick comes from looking at pictures of tiny houses, and seeing a person in a loft straining his neck to one side because he doesn't have enough headroom. I understand, lofts are for sleeping, and you work with the space you have. But: most lofts could be built two inches thinner. An extra two inches of headroom can make a big difference.
Most of the lofts that I see are built with 2x4s as the joists underneath the loft. Spanning from wall to wall, spaced on 16 or 24" centers, depending on how the house was built. Then they have a platform of 3/4" planks sitting on top of that. This makes the whole platform a bit over 4" thick. There are two simple ways to make this platform thinner. First approach: don't use 2x4s, use 2x2s, or ripped pieces from a harder wood. The key is to use more of them and place them closer together. Instead of attaching a 2x4 to each stud, attach a ledger plate to the studs, and then sit the thinner joists on it closer together. Look at any joist span table, and you will see that different sized lumber can safely span different distances at various spacing. So, to span a 12' floor, 2x8s may be required at 24" OC, but 2x6s can span the same distance at 12" OC. See here for reference.
If I were making a loft this way, I would do a little test first. Screw a ledger plate (section of 2x4) across two studs, and do the same on the opposite wall. Attach some 2x4s across, and sit a piece of plywood on them in the middle. Put some weight on the platform and see how much weight it can hold without bending too much. Now do the same thing but with 4 or five evenly spaced 2x2s (or ripped 2x4s). Put the same weight on it. Does it bend too much? Would you be comfortable sleeping on a loft made this way? Try adding another 2x2 if it doesn't feel stiff enough. Also, remember that when the platform surface is securely attached to the 2x2s, this will stiffen the whole structure up significantly. When the 2x2s are firmly sandwiched (glued and screwed) between two sheets of plywood, you basically have a torsion box. Hell, you can even buy torsion box panels from Lowes now.
The second approach (which is the one I chose) is to replace the 2x4s with steel. Steel C-channel members are readily available at Home Depot/Lowes/Menards and other places (I think I got mine at Lowes, under the brand name "superstrut"). They are used primarily for attaching conduit and HVAC equipment to ceilings. They are relatively lightweight, and they are strong. I chose this option because it looked easier than messing with a whole bunch of 2x2s. In retrospect, both approaches are probably about the same amount of work. Either way will let you build a thinner but still sturdy loft.