Sod Roofs, Norway

From these reservations, sod roofs have begun to reappear as an alternative to modern materials. The more recent idea of the green roof is developed independently from the traditional sod roof, but could benefit from the experience gathered during hundreds of years in Scandinavia.
Birch bark is eminently suitable as a roof covering because it is strong, water-resistant and soil-resistant enough to last for generations, although 30 years was considered the normal lifespan of a sod roof in most places. Birch is common everywhere in Northern Europe, and its bark is easily stripped from the trunk in spring or early summer, while the sap is running. One incision with a knife lengthwise down the trunk between two branches will produce a correspondingly wide sheet, its length equal to the circumference. The outer, flaky (white) bark is pried loose from the inner (green or brown) layer, the phloem. Removing the phloem will cause the tree to die, but removing the outer bark will not harm the tree, although the scar will remain for many years. A new, coarser bark will replace the stripped bark.
The sheets of bark must be stored flat under pressure to prevent curling. Left alone, a sheet will curl up into a tight roll, in the opposite direction of the natural curve of the trunk.