Læsø Seaweed Roof

The seaweed roofs (tangtag) of Læsø, an island in the Kattegat between Denmark and Sweden, are among the most unusual roofs in Europe. Instead of straw or reed, islanders covered their houses with great mounds of dried eelgrass (Zostera marina), a marine flowering plant that washes up on the shore. The roofs are massive: up to 1.5 m (4.9 ft) thick and weighing 35-40 tonnes (34-39 long tons).

Origins

Læsø's salt works consumed huge amounts of fuel, and by the 17th century the island had been largely deforested. Thatching reed was difficult to ship to the island and straw was too valuable as animal fodder, so islanders turned to eelgrass. The oldest surviving seaweed roofs date from the 1650s, and the last building in the tradition was built in the 1870s. Similar roofs once existed in Halland on the Swedish coast opposite Læsø, but none survive there.

Construction

Building a seaweed roof was a collective task. Women harvested the eelgrass and did the roofing, while men, often away at sea as fishermen, gathered driftwood for the frame. Teams of 40-50 people, or up to 100, worked together. The eelgrass was laid in layers directly on the roof beams, often over sheaves of twisted seaweed, and built up into a very thick covering.

The material has several advantages. It insulates well, is air-permeable, resists rot and pests, and resists fire. A well-made seaweed roof can last up to about 300 years, according to the Danish foundation Realdania.

Decline

Seaweed houses have almost disappeared over the last 75 years, partly because disease reduced local eelgrass beds and partly because the labour-intensive seaweed economy declined. Accounts of the numbers differ: one count gives about 230 seaweed-roofed buildings in the 1930s and 34 in 2014, while Realdania gives about 300 once and about 20 surviving with intact seaweed roofs.

Conservation

Realdania By & Byg bought and restored Kaline's House in 2010, a small protected seaweed-roofed longhouse from 1865 on the meadows south of Østerby. A new seaweed-roofed house was built in 2012-2013 as part of a conservation project, and old roofs continue to be repaired with the traditional technique. Most seaweed houses are privately owned; a marked "seaweed route" lets visitors see two of them. The houses form part of the salt-production cultural landscape of Læsø, on Denmark's UNESCO Tentative List since 2023.

Significance

The Læsø roofs show how a roofing tradition can adapt to scarcity. Realdania describes the material as non-toxic and carbon-neutral, which has drawn interest from architects seeking low-carbon materials.