A thatch that can last up to 400 years...


This is a roof made using a seaweed called eelgrass. It's one one of the traditional homes on the island of Læsø in Denmark. The timber used in this house is from a ship that ran aground on the island in the 1860s.

In the Middle Ages the island of Læsø became famous for its salt industry. Hundreds of salt kilns were built on the island requiring constant fuel to refine the salt.


But the island's finite natural resources, feeding hundreds of salt kilns, eventually led to its deforestation. This left the islanders with no timber for construction so they built their homes from driftwood and eelgrass. Because the seaweed and timber had been impregnated with saltwater the seaweed homes were not susceptible to decay, standing virtually unaltered since the 1600s.

In the 1930s a disease attacked the Island's eelgrass making it difficult to maintain the roofs. This marked the decline of this type of roofing on the island. In the late 18th century there were 250 homes and farms thatched with seaweed, now there are only 19. In 2009 a heritage project to save Læsø's seaweed homes was established. Part of the project was to teach farmers in the south of Denmark to harvest and prepare eelgrass. A so called 'Seaweed Bank' was created to provide the materials to re-thatch the remaining homes.

Because of the high concentration of salt in eelgrass it doesn't burn and has a very long lifespan. A seaweed roof typically lasts 200 years, some have lasted as long as 400 years. At about 5,000 DKK/m2 ($90/sq.ft) the seaweed costs 4 times more that Denmark's straw thatching which lasts some 30-40 years.


Click 1 to 4 to see the thatching steps


Normally thatching a roof would be done in a day with the help of the entire community, just like the gassho-zukuri homes in Japan. Up to 100 women twisted long drop-shaped bundles, called 'vask' (wash), of seaweed with a long thin neck [see picture No.1 right]. The vask were wound around the lower three rafters [No.3]. This formed a thick bank of woven seaweed.

Over the rest of the rafters, up to the ridge, a layer of branches are laid to act as a barrier for loose seaweed to rest on [video]. The loose seaweed is held in place by the bank of vask on the lower rafters. As the loose seaweed is laid it is trodden down to bind and compress it. A ridge of turf is laid [No.4] which eventually binds together with the seaweed. Finally the hanging eelgrass is trimmed [No.5] to open out the windows.

Initially the roof is not waterproof but because it is very thick water doesn't penetrate the building. In about a year the roof goes silvery grey, solidifies and becomes waterproof with plants establishing themselves in the thatch. As the roof ages the mass becomes solid.






Part of the project to save the homes, and the knowledge almost lost to build them, has been to use eelgrass in a contemporary design.

The idea of this new cottage is to help build a deeper knowledge about seaweed as a building material, a natural, non-toxic, non-itching, non-smelling, locally-sourced, CO2-neutral building material, with comparable insulation properties to mineral wool.

In this design (right) the seaweed is used as an insulating wall panel with additional sausage shapes on the exterior, not to provide a waterproof membrane but as insulation.

Video No.1 is a presentation (in English) about the history of the seaweed thatched homes including film of the thatching process.

In video No.2 the architect of the contemporary seaweed home discuss the design and materials (in Danish with English subtitles).