True, Tim. We can't prove any of this. Probably ever. Al we havve learned through the past few centuries of studying the history of these things is that just about everything we have thought about origins is wrong. That's why I'm not putting these ideas down in any published material -we can't really do any more than speculate over a lot of it. But exploring and learning new things, changing our viewpoints and perspectives is always rewarding.

The planks in that picture, here for the benefit of others, simply lodge atop the joists (these beams are called 'Unterzug' in contrast to the normal word for a joist which would be 'Deckenbalken'. They are in function closer to a summer beam, but at the same time like a joist. Hard to explain, since they are unlike anything we are familiar with here in this country)



The planks are set into grooves cut into the walls -here this is a log structure- which are the primary bearing surfaces. Other than the groove -1 1/2 or 2 inch deep- the unterzug is the only support for the floor planks. Much of their strength is derived from the fact that they are wedged tightly in place.

In the timber frame structures, we sometimes use this same flooring method. It is also common for the floor joists just to sit atop the plates and the 'deckenbalken' (as they are called in this case, different region=different terminology) rather than be set into grooves. Newer structures used more beam support and thinner floor planks, and often even just set planks into rabbets cut onto the top edge of the support beams. The floor boards remain stationary by virtue of the fact that they are splined together. Fasteners are uncommon. Nails are rare.