Torvtak (Scandinavian Turf Roof)

Torvtak is the Norwegian name for a turf or sod roof: a low-pitched timber roof covered first with a waterproof layer and then with living turf. Turf roofs were the ordinary roof on rural houses across Norway and much of Scandinavia for centuries, and similar roofs were used in the Faroe Islands and Iceland. They are still built today on mountain cabins, holiday homes and heritage buildings.

Traditional build-up

In the traditional method, the turf itself is not the waterproofing. That job is done by birch bark:

• Sheets of birch bark are laid on gently sloping roof boards, inner side up, overlapping like shingles from the eaves to the ridge. Six layers were considered enough, and up to 16 have been recorded on high-quality roofs.
• The lowest layers project about 8 cm (3 in) beyond the eaves and curve down to form a drip.
• Turf pieces about 30 cm (12 in) square and 7.5 cm (3 in) thick are laid in two layers, the first grass-side down and the second grass-side up.
• A timber log or board at the eaves, held by wooden hooks, stops the turf from sliding.

A total turf thickness of about 15 cm (6 in) is enough to survive a dry summer. A finished roof weighs roughly 250 kg/m² (about 51 lb/ft²), and with winter snow the load can reach 400-500 kg/m² (82-102 lb/ft²). That weight helped compress log walls and made them more draught-proof.

History

Birch bark and turf have probably covered Scandinavian roofs since prehistory. In the Viking period and Middle Ages most houses had turf roofs, while churches and other steep buildings used boards, wooden shingles or lead. Turf remained almost universal in rural areas until the early 18th century. In the 19th century clay tiles, and later corrugated iron, replaced it in most places except remote inland valleys.

National romanticism and the open-air museum movement helped save the craft, and demand for mountain cabins kept it alive.

Modern turf roofs

Contemporary turf roofs usually replace birch bark with a bituminous or synthetic waterproof membrane and a dimpled drainage layer, with turf or sedum above. They are a close relative of the modern extensive green roof. Typical service life for a traditional roof was about 30 years in most places, with periodic repairs.