[sologoub]

The renderings they are showing for newer plans (at 1:27 of the video) seem to indicate a soil later running a long the wall of the building, in addition to the roof. As the result water would naturally travel down that wall and into the surrounding soil, preventing the buildup on the roof and negating need for a traditional drain.

That said, what would be required to make sure no barrier formed, say from the roots system or something else is really hard to say.

You'd also need some sort of lining that would prevent the moisture from being obsorbed by the building materials themselves.