Look closely at the top of the photo and note the rafters that curve to meet and buttress the wall. These aren’t bent into this shape; they’re made from the “knees” of trees — the place where the trunk naturally curves down into the soil to meet the roots. It’s the strongest part of the tree, perfect for making brackets, arches, and other load-channeling, curved architectural supports. The crotch and the curved joints between branches and trunk were also used for curved members.