The island village of Läsö off the Danish mainland has preserved houses built hundreds of years ago, when owners would thatch the roof with tons of seaweed.
Via Natural Homes:
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.