"The practice of building roofs with seaweed—actually a marine grass called eelgrass—dates back to the time when the island of Læsø had a flourishing salt industry. The island was practically soaking in salt. The ground water had over 15 percent salt, and during the hot dry summers they crystallized out of the ground naturally in large salt meadows. Hundreds of salt kilns were built on the island to refine the salt. They needed fuel which was provided by the island’s limited woodlands, until a day came when the islanders had cut down the last tree. With no wood left to fire the kilns, Læsø’s salt industry collapsed. With no trees left to break the wind, Læsø’s villages were buried in sandstorm. The air became full of sea salt and they inhibited the growth of any kind of plants, even grass.
Læsø, however, had plenty of eelgrass and driftwood. So the people began using driftwood to build their houses and the eelgrass was used for the roof.
Eelgrass grows on the island’s seas and were once so abundant that it frequently washed up on the shores. They have long, bright green, ribbon-like leaves about a centimeter wide, and up to 2 meters long. Farmers would collect them from the beach and once dried, they were bundled and twisted into thick ropes that were then woven through a home’s rafters to form a roof. Traditionally, weaving bundles of seaweed together was done by the island’s women folk. As many as one hundred women and young girls would take part in the activities.
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Due to the high concentration of salt in the air, the eelgrass does not burn and has a very long life. According to Meeting the Other in Norse Myth and Legend by John McKinnall, a seaweed roof typically lasts 200 years; some have lasted as long as 400 years. At about 5,000 DKK per square meter ($90/sq. ft) the seaweed costs four times more than that of Denmark’s straw thatching, which lasts some 30-40 years."
https://www.amusingplanet.com/2018/02/the-seaweed-houses-of-ls-island.html
Læsø, however, had plenty of eelgrass and driftwood. So the people began using driftwood to build their houses and the eelgrass was used for the roof.
Eelgrass grows on the island’s seas and were once so abundant that it frequently washed up on the shores. They have long, bright green, ribbon-like leaves about a centimeter wide, and up to 2 meters long. Farmers would collect them from the beach and once dried, they were bundled and twisted into thick ropes that were then woven through a home’s rafters to form a roof. Traditionally, weaving bundles of seaweed together was done by the island’s women folk. As many as one hundred women and young girls would take part in the activities.
...
Due to the high concentration of salt in the air, the eelgrass does not burn and has a very long life. According to Meeting the Other in Norse Myth and Legend by John McKinnall, a seaweed roof typically lasts 200 years; some have lasted as long as 400 years. At about 5,000 DKK per square meter ($90/sq. ft) the seaweed costs four times more than that of Denmark’s straw thatching, which lasts some 30-40 years."
https://www.amusingplanet.com/2018/02/the-seaweed-houses-of-ls-island.html