Wood – Is this loft bed flooring strong enough

It might be good to consider a sliding dovetail.

Edit 11/16/13 To assemble your bed frame, the M&T joints with captured bolts with nuts to hold the corners together with the legs. Just as a mention, a good hardwood needs to be used for the assembly. regular 2X4, 2X6s and other similar materials used in the construction of homes will be too soft, and when the joints are put under stress SPF, Hem fir, etc is going to compress and loosen the joints. To use this material you need glue and screws to hold all this together. That is not an option for you I think?

To assemble this, white oak, ash, Black walnut and a dense variety of mahogany would be a good choice, if it is not cost prohibitive. There are other hardwoods too, just give a look. Red oak and Douglas Fir are hardwoods, but very prone to readily split under the stresses you are going to expose the joints to.

Another good thing about some hardwoods, is that you will not need the same dimension of material to have the same strength as 2X framing lumber.

The sliding dovetails are ideal for the cross members into the side rails. To use the same joint for the attachment of the legs would produce a weak joint in my opinion, I have seen it fail in pedestal table legs.

To dissemble this in time, and to ease the assembly, which sliding dovetails can be a bear to slide together, I suggest tapering the dovetail so as it goes in, the joint gets tighter, it doesn't need much, a 1/16th of an inch over 4 inches on each side would dramatically increase the ease of assembly/disassembly. A router jig with a bushing guide on the router would make the female cuts in the rails, and a router table with a tall fence and a shim on the appropriate corner of the support for the slats would cut the male dovetail. A push bock would be handy to control the top heavy stock for this cut.

I can't find a floor joist span table that goes smaller than a 2x6. However, the American Wood Council calculator gives an allowable span for #2 Southern Douglas Fir 2x4s (~35mm x ~90mm) at 16" (405mm) centers at 5',8" (~1750mm) with a wet condition 10lb dead load and 40lb live load.

If the dimension measurement is accurate, the joists wouldn't get past my local inspector (around here it's 2x6 minimum for deck joists), but even at 12" (300mm) centers you could probably park a small car on it with 900mm spans.