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.

wow, the wood being buried underground for so long and it doesn't rot, seriously?! seems amazing to me. i like the suggestions made here, and as i just finished the Building a Better World book, on pg. 138 Paul brings up the need for a solution this thread is attempting to derive.

what about natural rubber? too expensive? i don't mean to discourage the more natural, less processed ideas, but at least natural rubber's biodegradable

I suspect most organic materials will be tricky in a buried state, and as Mike says the water flow uderground may be more likely to wick through any joints. So something like thatch or slates will likely not work. I wonder whether natural pond liners might be a possibilty? Clay has been used for wall cappings on blackhouses in the Hebrides, maybe a clay roof layer? It actually might be less successful in a dry climate like paul's of course - a pond is usually full of water so the clay stays hydrated. I'm not sure how you'd go about making a gley layer on a roof?
I've been pretty impressed by my kombucha scoby - I'd love to explore the physical properties of those a bit more.

Mines are a similar structure and I am sure in the olden days wood was used.

Charring the wood might protect the wood from the elements.

Nancy Reading wrote:I've been pretty impressed by my kombucha scoby - I'd love to explore the physical properties of those a bit more.

I love this idea -- particularly if it would work. I've always wanted to grow a SCOBY in a little wading pool or something to get one 6-8 feet across and see what can be done with them, but there are always more important projects. Getting a single sheet as big as a roof sounds challenging, but maybe they can be grown together or something.

Birch bark roofs, covered or not, I have heard that the bark sheets are layered six to twenty times first, so that is a lot of protection! Additionally, the sheer weight of the soil on top might effectively seal the sheets together. For more of a vapor barrier, caulking with pitch or other natural sealants seems like it could be beneficial. But then again, maybe we want some exchange of gas between soil and home. It’s a long topic so will leave it at that and not veer too far off topic…

As a side note, Plains tribes made their earth lodges with earth on top of big bluestem grass thatch, which is a lot less of a moisture barrier! On the other hand, most pre-modern societies always had a fire going, which would likely dissipate the moisture and smoke would preserve and make the thatch resistant to excess moisture. Someone told me  that in parts of Ecuador, smoke is intentionally used to darken and “impermeabilizar” (impermeabilize/“waterproof”) a new thatched house before it’s used, otherwise it would rot in the tropical, humid climate.

I have also heard that in Iceland, some houses do not even have birch bark in between the roof and the soil. And they’re well insulated enough that the main heating source is the human body! (Iceland being a wood scarce region.) Or at least no one said they used it in the video…

Christopher Weeks wrote:I've always wanted to grow a SCOBY in a little wading pool or something to get one 6-8 feet across and see what can be done with them, but there are always more important projects. Getting a single sheet as big as a roof sounds challenging, but maybe they can be grown together or something.

I'm not sure if there is a limit to what you can grow (swimming pool?) but something like this would prove the principle:
source

For what it's worth, on an open air museum I know of, there's a building roofed with birch bark and turf with the top portion of the roof nearly horizontal, and it's not leaking at all. It's possible, but I'm inclined to think that a simpler solution would be to pick a roof construction that has at least some slope at every point. I've never heard of caulking between the bark sheets being done traditionally, it's probably too labour intensive to be practical for an entire house, but it was clearly used for birch bark canoes...

M Lijn wrote:
I have also heard that in Iceland, some houses do not even have birch bark in between the roof and the soil. And they’re well insulated enough that the main heating source is the human body! (Iceland being a wood scarce region.) Or at least no one said they used it in the video…

I've been wondering about that, and I think I might have a theory. In the video (if you're referring to the one I think) he mentions that the turf has to be taken from wet places. The Icelandic rocks are obviously volcanic. When volcanic rocks weather, bentonite is one possible end product, and I imagine any clay-like material formed would tend to end up in wetlands. Bentonite, once it gets humid, acts as a moisture barrier. Probably not a perfect one, but he does mention in the video that they used a thick cover of turf, which would mean that the majority of the water is shed before it goes through.