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.