Sod and Birch Bark Roofs

A living roof

For centuries rural Scandinavian log houses wore sod roofs - turf over birch bark - until tile and corrugated iron replaced them in the late 1800s ¹. National romanticism and mountain cabins kept the style alive, and they’re still common on Norwegian cabins today ¹.

The layers

From the bottom up ¹:

1. Roof boards over rafters or purlins.
2. Birch bark, usually about six layers (sometimes up to 16), lapped like shingles from the eaves up - the actual waterproofing.
3. First sod layer, grass side down: the wilting grass protects the bark from acidic humus and acts as a drain ¹.
4. Second sod layer, grass side up: the living surface.

At the eaves, a turf log (torvvol) - an undressed fir log 5-6 inches thick - holds the sod from sliding, fastened with wooden hooks or pegs ¹. Barge boards at the gables hold the edges.

Weight and slope

• Weight: about 250 kg/m², rising to 400-500 kg/m² under snow ¹. That’s several times a shingle roof; rafters, purlins, walls, and foundations must be sized for it.
• Slope: gentle - steep enough to shed water through the bark but shallow enough that the sod stays put; roughly 20-30 degrees is typical.
• Insulation: the sod is a reasonably good insulator in a cold climate ¹, and the grass keeps the roof cool in summer.

Modern versions

Today’s turf roofs replace birch bark with bitumen felt and dimpled plastic drainage membranes ¹, or EPDM rubber, with a root barrier, drainage layer, and growing medium. Sedum or meadow plantings are lighter than full sod. Birch bark is hard to harvest in quantity and requires felled trees (see Birch Bark Work in the Green Woodworking course).

What to take from this

A traditional Nordic sod roof is roof boards, six or more lapped layers of birch bark, sod grass-down, and sod grass-up, held at the eaves by a turf log on wooden hooks. It weighs about 250 kg/m² and up to 400-500 with snow, so the frame must be built for it. Modern versions use membranes and drainage mats instead of bark.