Sod Roof

A sod roof, or turf roof (Norwegian torvtak, Icelandic torfþak), is a roof covered with living turf laid over a waterproof layer. In the traditional Scandinavian form, the waterproofing is birch bark, and the sod's main job is to weigh the bark down and protect it. Until the late 19th century this was the most common roof on rural log houses in Norway and much of Scandinavia, and it is closely associated with the log-building traditions of Norway, Sweden and Finland. Turf roofs were also traditional in Iceland.

Traditional construction

• Birch bark is laid inside-up on gently sloping roof boards, from the eaves upward, overlapping like shingles and lapping over the ridge. It is usually not nailed; friction and the weight of the sod hold it in place.
• Six layers of bark were considered adequate; high-quality roofs had up to 16.
• The first layers project about 8 cm (3 in) beyond the eaves to form a drip edge.
• A turf log along the eaves, held by wooden hooks or pegs, stops the sod sliding off.
• Sod is cut in pieces about 30 cm (12 in) square and 7.5 cm (3 in) thick. The first layer is laid grass-down to protect the bark, and a second grass-up.
• About 15 cm (6 in) of sod is enough to survive a dry summer; more than 20 cm (8 in) adds unnecessary weight.

Weight

A traditional sod roof weighs about 250 kg/m² (about 51 lb/ft²), rising to roughly 400-500 kg/m² (82-102 lb/ft²) with winter snow. The load was useful in log buildings because it compressed the logs and reduced draughts.

Lifespan

About 30 years was considered a normal life for the bark layer in most places.

History and revival

Sod roofs were probably used since prehistoric times and were almost universal on rural houses during the Viking Age and the Middle Ages. During the 19th century tile and later corrugated iron replaced them except in remote areas. National Romanticism, open-air museums and demand for mountain cabins revived the type, and turf roofs are still widely built on new cabins in Norway. Modern versions replace birch bark with bitumen roofing membranes and a dimpled drainage sheet, an approach that overlaps with modern green-roof construction.