Likely solution to eliminating the plastic membranes on a wofati

Apparently, birch bark is waterproof and rot resistant. Treating and sealing it with birch tar might possibly improve these abilities. Perhaps larch or black locust planks can help segregate the birch bark from soil contact. Insights? Has anyone worked with birch bark? http://naturalhomes.org/permahome/birch-bark.htm

I've thought about this a little bit.  I think the problem would be that if the birch bark is buried, water can travel horizontally a lot easier.  So if you shingle pieces of birch bark like you would on a roof, I suspect the water will still wick through the gaps in the bark pieces and get through.  Especially in the flatter areas near the top.

You'd also have to be pretty selective to get bark without cracks or holes in it.  Tar would resolve that issue a fair bit.

Lastly, as the building settles and the dirt shifts during the covering process, I think the pieces would keep getting out of alignment.

I should add that it is waterproof and very rot resistant.  I had a piece of birch bark in my compost pile for several years and it didn't degrade while everything around it rotted several times over.

Awesome to know! So sealing it is a good idea. Maybe a layer of clay on top of the tarred bark would be good. Need to find out the traditional techniques for green roofs using birch bark.

I think the key difference between birch on a roof and birch in a wofati is that it's buried in the wofati.  I believe water underground acts differently from water running down a roof.

Wood shingles are fairly waterproof and for them to work on a roof I think they need to be a fairly steep pitch.  Flatten them out and water gets through.  Cover them with 6" of wet soil and the water can take its time getting through.  I think it would be the same with birch bark.

I do hope it is a viable option and my beliefs/assumptions are incorrect.  

The Scandinavians are one step ahead of you, there. They use birch bark as the underlayment for a living roof.

To be clear, sealing each joint with birch tar or pitch would be my method. That would hopefully eliminate any capillary action of the kind you’re worried about.