The roofs are made from 40 tons of seaweed.

For almost the entire summer on the Danish island of Læsø, Henning Johansen is busy working. As an islander, Johansen works as a roofer. But on Læsø, the work of a roofer is unlike anywhere else.

Located 19km northeast of the Jutland coast, this island, steeped in Norse mythology, is home to the giant Ægir (Aegir, meaning Sea). It is dotted with shipwrecks and is mentioned in Danish maritime history. However, the most unique feature of Læsø Island is its houses, with roofs made of thick, heavy bundles of silver seaweed.

Seaweed has been used for roofing on the island of Læsø since the 17th century and is famous worldwide for its unique use, although seaweed roofing later appeared in the Orkney Islands of Scotland. Læsø, which had a thriving salt industry, suffered from deforestation – trees were burned to power salt kilns – so the islanders needed to find an alternative roofing material. The island itself was scarce in resources, consisting mostly of mudflats and sandy shores that were difficult to exploit, so the islanders began to look to the sea.

Floating wood salvaged from shipwrecks can be used for building houses, and the abundant eelgrass (scientific name Zostera marina) harvested from the shore can be used for roofing. Until the early 20th century, most of the island's inhabitants had eelgrass roofs. But after a fungal disease wiped out most of the eelgrass in the 1920s, knowledge of roofing techniques gradually faded. Today, only 36 eelgrass-roofed houses remain on the island of 1,800 inhabitants.

Since 2012, Johansen has revived this skill by replacing roofs, becoming the first person in his generation to do so. "It's one of the greatest parts of Læsø's history, so it's very important to the island," he told me over the phone while roofing a house. "Before, it was women's work," he added, explaining that because most of the men on the island are seafarers, the women stayed home to look after the farms and the house. "They were alone on the island and had to take care of themselves. They figured out how to make these roofs, and they don't exist anywhere else in the world. So, of course, we are very proud of this heritage."

Approximately 40 to 50 women typically work together to build a roof. They harvest seaweed after autumn storms, spread it out in fields to dry for about six months—a step that removes microalgae and prevents the seaweed from rotting. When they begin construction, they take eelweed, which looks almost like wool, twist it into large bundles (called vasks), and tie them to the rafters to form the underlayment. Then, they add more layers of seaweed on top of the initial layer and seal it with peat to bind the layers together: an average roof is about 1 meter thick and weighs about 35 to 40 tons. These women then bring chairs onto the newly built roof, sit there, and gaze out at the treeless horizon, watching for shipwrecks (perhaps there's a chance to salvage something), and walk around, even dancing on the seaweed to help it stay tightly packed.

Johansen said they could roof a house in a day. With his team of five roofers, it usually takes him about 21 days. "Because I don't have 50 women."

But Johansen sought to revive this seaweed roofing technique not just for conservation purposes. "Eelweed is a very interesting material," he explained, "because it doesn't burn, and there's so much salt deposited in its stems."

Although eelweed is often considered a type of "seaweed," it is actually a type of seagrass, with leaves that can grow up to 2 meters long. Eelweed can be found worldwide, although the majority are concentrated in the Northern Hemisphere. Eelweed not only possesses natural properties such as fire resistance, rot resistance, and insect resistance, but it also absorbs CO2.₂And because it doesn't require high temperatures for processing before use, it's carbon-neutral when harvested and used on-site. Seaweed becomes completely waterproof after about a year and has insulation properties comparable to mineral wool, a thick fibrous material made from molten glass fibers, rock, or industrial waste. A roof can last for hundreds of years – one of the remaining seaweed roofs on the island is over 300 years old – compared to the typical 50-year lifespan of a brick or concrete roof.

This combination of sustainability and heritage caught the eye of American architect Kathryn Larsen, who lives in Copenhagen and is currently researching how the traditional seaweed roofing method on the island of Læsø can be adapted for use in contemporary sustainable building materials around the world.

While studying at the Copenhagen School of Design and Technology (KEA), Larsen's thesis focused on the seaweed roofs of Læsø Island, a subject with little English literature. "It became a huge mystery to me because most of the information was written in Danish," she recalled. "I was really fascinated by it for several years: I learned more Danish and then was able to research it further and keep returning to the topic. It became a real obsession."

After learning that her school sourced its eelweed from local seaweed farmers, Larsen began experimenting "for fun." She developed prefabricated panels for use on building facades and roofs as a sustainable insulation layer, reinforcing the primary insulation material, and she installed these panels on the school's roof to see how the eelweed would react to other elements over time.

However, she soon realized she needed to install them somewhere more public. "A lot of people were really skeptical," she shared about using seaweed. In late 2019, she started The Seaweed Pavilion project, a structure incorporating seaweed panels into a wooden structure in the KEA schoolyard. "I wanted people to sit under them and experience it firsthand. People are always surprised when they think it would smell or be sticky, so with more positive interaction with them, I hope to change that perception."

"What I really want to convey is that this is a very good natural installation method," she shared, explaining that with this material, you don't need a damp-proof layer, which is not breathable. "You can use a lot of this material and the air will still flow through the building, not become stuffy inside, so the whole structure can breathe and the air quality is much better."

Larsen wants to change the construction industry's perception of older techniques, which are not only "better for the environment because of the use of renewable natural resources, but can also build healthier buildings for us. Basically, we discovered them, then we lost all this knowledge, and now we're trapped in an even bigger hole where indoor air quality is getting worse and worse."

Because eelweed grows all over the world, she sees great potential in turning it into a sustainable building material globally, "provided it is harvested and used locally."

The eelweed used at the Seaweed Terrace is harvested and processed by Kurt Schierup. He uses harvesting and drying techniques employed by the women of Læsø Island, albeit with machinery rather than manual labor. And although he only founded his eelweed harvesting company, Møn Tang, in 2016, Schierup has been doing this since his childhood in the 1950s on the island of Tærø in Denmark, where eelweed is exported internationally for use in upholstery, insulation, and mats. This small detail in history clearly shows how popular eelweed once was.

"When I was a child, 50 years ago," Johansen recounts, "everyone slept under eelgrass roofs in Denmark. But then plastic came along, and it was very cheap. So, in 50 years, all the knowledge about eelgrass disappeared."

On the island of Læsø, Johansen still has 10 more roofs to replace. It's a long-term project, but one he says he doesn't want to finish "because it's easier to tell you the history when you come and see me repairing the old houses." Visitors to Læsø are invited to visit him at work on weekdays during the summer, where they can see firsthand "how it works, the materials, and how interesting it is."

He said eelweed is part of the world heritage. "You can come to Læsø and I can show you your own history from your country" because it was once widely used all over the world – from the Wadden Sea of ​​the Netherlands where it was used to build seawalls to the New England region where people insulated their homes with Cabot's Quilt insulation panels made from dried eelweed.

"It has a rich history, one that's quickly forgotten, and now I'm starting to tell that story. It's easy to tell a story when you're standing next to an extremely old roof, looking up at it, and people say, 'What a funny material.'"

Huong Thao