Birch-Bark Roofing

Birch bark was the original waterproofing membrane of the northern forest. Across Norway, Sweden, Finland, Russia and neighbouring regions, builders stripped sheets of bark from birch trees and laid them in overlapping layers on roof boards. The bark is naturally rich in water-repellent compounds and very slow to rot, so it could keep a roof dry for generations, provided it was shielded from sun, wind and mechanical damage by turf, poles or shingles laid on top.

Use under turf

In the Scandinavian sod roof (torvtak), birch bark is the real waterproof layer and the turf mainly holds it down and protects it. Bark was laid on gently sloping roof boards; six layers were considered adequate, and up to 16 have been recorded on high-quality roofs. The lowest layers project about 8 cm (3 in) beyond the eaves to form a drip. Birch bark could last for generations, though about 30 years was a normal life for a sod roof before it was relaid. See torvtak and Scandinavian sod roofs.

The Finnish pole roof

Until the early 19th century the traditional Finnish roof was the malkakatto: a base of wooden slats covered with several layers of birch bark, held down by long timber poles weighted with boulders, and left unpainted. Wooden shingles replaced it during the 19th century.

Under shingles and domes

In Russia, birch bark has been used as a waterproofing material since ancient times. On the Church of the Transfiguration at Kizhi in Karelia, birch bark covers the domes beneath the carved aspen shingles. See aspen ploughshare shingles.

Iceland

Iceland had little usable timber, but dwarf birch was sometimes packed beneath the turf, as at Þverá in Laxárdalur, one of the sites on the country's UNESCO Tentative List.

Decline and replacement

During the 19th and 20th centuries, clay tile, corrugated iron and other industrial coverings displaced bark-and-turf roofs in most places. When turf roofs are built today, birch bark is normally replaced by bitumen roofing felt nailed to the sarking, with a dimpled drainage membrane above it. Bark is still used in heritage conservation, where open-air museums and preservation bodies maintain traditional roofs.

Why it worked

• Birch bark is flexible, light and water-resistant, and can be peeled in large sheets.
• Layering and overlapping, like shingles, sheds water down the slope.
• Turf or poles on top protect the bark from ultraviolet light and wind and hold it down.