On Læsø, a large Danish island in a bay of the North Sea, there are centuries-old houses that are unlike any others. Their roofs are made entirely of a thick cushion of seaweed, harvested from the water and layered to make long-lasting and uniquely charming structures, which look a little like giant, manmade mushrooms or like ‘80s rockers on a great hair day.
Once, Læsø had about 250 houses in this style; over the years, they disappeared, and their numbers dwindled down to a couple dozen or so. Today, some are preserved as parts of museums and tourist destinations, but they aren’t just relics from the past. Some of them still serve as homes, and one of these tangtag houses, originally built in the 1700s and called Andrines Hus—Andrine’s House—is up for sale, for around $400,000.
The backstory of the seaweed roofs isn’t quite as charming as the results. Læsø has an unusually high concentration of salt on its land and in its groundwater, and back in the medieval period, a salt industry hub developed here. Harvesting salt meant concentrating that brackish water down over huge fires, fueled by the island’s wood supply. Only, the fires needed to burn so hot and so long that eventually the industry had denuded the island of all its trees, making it all but unlivable and hurting the salt industry. Around the same time, in the 17th century, the Little Ice Age changed the salinity of the island’s groundwater, and soon the industry collapsed altogether.
In the years that followed, the people who still lived on the island made do with the resources they had. The bay around them was abundant with eelgrass, Zostera marina, and they harvested it as a building material. Teams of builders (usually women) would form the seagrass into giant bundles with long necks, used for the lowest layer of roofing, wound around a building’s rafters. On that base, they would add a layer of branches, then pile on loose seaweed, climbing up to the roof to trample it down. The roof could be around five feet thick, a deep enough layer to keep water from pouring into the house. On the very top, the builders would add strips of turf to help the roof form.
Over time, the seaweed roof would harden into its final form, turning a silver gray and becoming water-proof. The very last step was to cut away the overhang around the windows, letting light into the house. Resistant to fire and pests, the roofs could last for hundreds of years. Almost every building on the island was topped in this way: Only special buildings, including two churches, had tile roofs.
But in the 20th century, this ingenious solution to a human-made resource crisis started to fail. In the 1930s, a disease started killing off the seaweed around the island, limiting the supply available to repair the roofs and make new ones. As the economic conditions on the island improved, too, people moved into more standard houses. As charming as the tanghus might look, inside it often felt dark and damp.
By the end of the 20th century, only a handful of the seaweed houses remained. The island, too, had transformed again, reforested with trees. This new forest may have also contributed to the decline of the seaweed roofs. It’s possible that the salty winds blowing across Læsø helped preserve the roofs; once the trees grew in, blocking the winds, wildflowers and grasses had an easier time growing in the roofs and contributing to their rot.
Over the past decade or so, though, there’s been a push from the government, nonprofits, and locals to restore and conserve the houses. In 2007, the national government listed them as a wonder of North Jutland, the region on the northern tip of Denmark, and the next year, some of the restorers started a “seaweed bank” to source their roofing materials from nearby islands. Slowly, the roofs of older buildings started to be repaired or redone. The architectural and planning company Realdania also created a modern seaweed house that used the eelgrass as hidden insulation and as a building material on the exterior.
The roof on the Andrine’s House has been recently restored in the traditional style and could last for hundreds of years more. Inside, the scullery shows the roof’s underside, but most of the rooms have white, low ceilings. The house has been on the market for a few months already—it will take a rare buyer to take on the responsibility of a seaweed roof. But imagine coming home to this small wonder, a place unlike any other.