When you travel through parts of northern Scandinavia, especially in the countryside, you’ll probably notice quite a few old log houses with grass growing on the roof. No, these aren’t run-down, overgrown buildings. The grass is actually part of the original structure. The practice of putting sod on the roof of a building, especially a log cabin, dates back to the prehistoric era. To help keep in the heat and stop drafts, people would add a layer of sod to the outside of their homes. The birch bark that is found underneath the layer of sod ensured that the roof remained waterproof. This simple yet effective design was used well into the late 19th century when more efficient roofing and insulation systems were invented.