Norwegian Farmhouses


Folk museum at Oslo



Near Oye



Sod Roof

The three traditional roofing materials in this area were slate, wood shake, and sod. However, precisely cut slate tiles were expensive, and in the absence of nails, it was not easy to keep wood shake permanently in place. Iron was available but nails were scarce and not often used as fasteners. Iron was more often used for hinges and supports around fireplaces. Made of soil and turf which were readily available and requiring less manual labor to install, sod roofs became the obvious choice and the most common roof in this area to protect against the combined threat of wind, sun and water. Nonetheless, sod roofs required special materials and knowledge to install, and would be required to be upgraded and reinstalled over time.

The base of any roof was supported by logs placed about 24" on center, with wood planks about 2" thick placed above. To support the short overhang at the low end of the gable roof, outrigger beams were cut into the top of the wall logs and the direction of the roof planks were perpendicular to the roof slope. Some houses had a right angle key at the end of the outrigger to support the fascia board. This end of the roof fascia board supports the weight of about 4" soil and turf with water in it and keeps the sod from sliding off the roof.

The sod roof is a particular and unique feature in the traditional Norwegian farmhouse, providing several benefits beyond availability of materials and ease of construction:
  1. Thermal insulation against bitter winter cold;
  2. Thermal insulation against humid summer heat and shade from the summer sun;
  3. Moisture retention and weight to hold the water-repellant birch bark underneath;
  4. Water shedding properties.

The traditional sod roof was constructed in three layers, beginning with a bottom layer of water repellant birch bark. Prior to the use of oil based, waterproof membranes, birch bark was readily available and water repellant, as long as it was kept moist. The oil in the bark itself protected against moisture, kept cracking to a minimum, and kept the bark's water shedding properties alive. Birch trees that grow in this area tend to be quite narrow and so the strips of bark were layered to reduce gaps and the possibility of leakage.

Grass growing on the roof is important because their roots will act as binder for the soil. Without it, the soil will wash off with heavy rain and sliding snow. During winter, the grass will be dormant but the roots will continue to hold the soil together. Under the freezing temperature, the water in the soil will freeze. Snow could cumulate up to 8 meters in the area and will actually act as insulation keeping the heat within the building.





© Kotaro Nakamura, 2009