The world’s last master eelgrass thatcher

01 / 05 / 2026

In 2023, the remote, sparsely populated Danish island of Læsø was shortlisted for UNESCO World Heritage Site status. While little known beyond Denmark, the island is the site of a remarkable revival: the return of its eelgrass-thatched roofs, which lie at the heart of UNESCO’s interest. This long-overlooked, sustainable building material is the reason Søuld exists – but the story stretches back thousands of years.

The use of eelgrass in construction dates back to the 1600s, when Læsø’s settlers built thick roof thatching for their distinctive ‘seaweed houses’ – a technique found nowhere else in the world. Exceptionally durable, these roofs could insulate buildings and withstand the elements for more than 350 years. By the early 20th century, eelgrass thatching shaped the island’s built landscape, with around 250 seaweed houses. The tradition declined in the 1930s as construction practices evolved, and it remained overlooked until recently, when local craftspeople, farmers, and designers began to rediscover its potential.

The world’s last master eelgrass thatcher and Søuld co-founder, Henning Johansen, has been central to the revival of this endangered craft. In the 2010s, when only a handful of seaweed houses remained on his native Læsø, Johansen and his partners at Søuld raised awareness and led efforts to document traditional techniques and restore the roofs. Over the following decade, more than thirty seaweed homes were reinstated on Læsø, reviving cultural interest and an entire eelgrass supply chain, and paving the way for contemporary applications of the material.

In this interview, Johansen shares what he has learned about eelgrass over the last 15 years.

Q: In the 17th Century, why was eelgrass initially gathered, processed, and used in construction on Læsø? Who traditionally used the material, and how did they work with it?

It’s really a story about natural resources and how they drive the development of a small island. From around 1200 to 1600, Læsø was a salt production island. Before that, the island was covered with forest. Around 1600, there were only a few trees left on Læsø, so it was forbidden to cut them down. The salt huts were fired with grass. The overuse of wood and grass had led to serious sand drift that made parts of the island uninhabitable. This led to a new period on the island, where the men went fishing, and the women stayed at home to take care of the small farms.

On this impoverished island, there was not much to build with. The fields were very barren, and the small amount of short, poor straw they produced was used for the animals. There was no wood, only clay, lumber from shipwrecks, and eelgrass. With these three elements, the women and a few old men developed a new building culture. The houses were simple and very narrow with a timber frame and an eelgrass roof. To build an eelgrass roof house, you would need to collect materials for years to have enough. The houses often started as very small ones that, over the years, were enlarged as new eelgrass floated ashore.

It is likely that the eelgrass roofs were invented by women. You can see how they found inspiration in spinning wool. The eelgrass is also spun into ropes that are tied around the wood construction to keep the eelgrass in place.

Before roof construction could begin, the eelgrass had to be collected along the shore and dried in the sand dunes or at the farms, typically by a widow trying to make some extra income. Tons of eelgrass were collected for one roof and carried on small horse wagons. Today, we use 50–80 tons of eelgrass for one roof. When modern trucks deliver the many tons in compressed round bales for our current restoration projects, it is hard to imagine how an elderly lady would have been able to carry out the same work by hand and horse wagon.

Up to 100 women and children gathered to construct an eelgrass roof in one day. Teams consisting of an elderly lady and a couple of children made the vaskere: bundles of eelgrass with a long tail, and a rope made out of eelgrass. Working on the roof, a team of an elderly man and some women attached the eelgrass vaskere to the roof and walked on it to compress it. Finally, the host threw a party for everyone who had helped that day.

In the 1920s, the eelgrass roof tradition began to die out. This was due to modern building materials becoming affordable while the eelgrass population declined. Society also changed: the idea of gathering 100 individuals to build a roof became outdated.

“ I discovered through my work that no one knew how to save the eelgrass roofs. Intuitively, it felt wrong to take down a roof that had lasted for 200–300 years and replace it with a material that might last 40–50 years.”

Q: You are the first in a generation to work with eelgrass in the traditional way. Why did you begin restoring eelgrass-thatched roofs?

I am a straw thatcher who makes straw roofs on Læsø. I have been taking down quite a few worn-out eelgrass roofs to put straw on them instead. I discovered through my work that no one knew how to save the eelgrass roofs. Intuitively, it felt wrong to take down a roof that had lasted for 200–300 years and replace it with a material that might last 40–50 years. That is why I decided to teach myself how to make the eelgrass roofs again. I learned it from looking at the roofs that I was taking down, from old descriptions of the methods, and by simply doing it. 

Q: Please tell us about the 2009 Læsø Tangbank project, its mission, members and impact.

I was not the only person who found it sad to see the old traditional roofs slowly disappear. We were a group of people working on how to save the roofs. Some of the most dedicated were my friend, Poul Christensen, who also started the Læsø Salt Workshop and Museum, and an architect who had written a PhD about the roofs, Marcelle Meyer. Together, we created the Tangbank project. Its purpose was to create an organisation that could secure funding to start working with eelgrass again, and to have a stockpile of eelgrass for the restoration projects.

As there is not a lot of eelgrass in the waters around Læsø anymore, we searched for eelgrass elsewhere in the Kingdom. This idea led us to Møn and Bogø, where small-scale eelgrass collection had begun as an employment project. This was led by a farmer whose father had collected eelgrass for export up until the 1950s. A few farmers who had land in places where eelgrass was historically collected got involved, and that is how a notable production of eelgrass was reinstated.

The first pilot project, a small house for sheep (Kjeld Posts Fårehus), was carried out in 2010. Since then, funds have helped the owners of eelgrass roofs to finance restoration work. The first projects were supported by Realdania and Kulturstyrelsen, and later, a large number of houses have been sponsored by A. P. Møller Fonden.

Q: What eelgrass restoration, harvesting and production challenges do you face today? What are the potential solutions? 

The current method of collecting eelgrass and drying it on grass fields is very close to the methods used back at Læsø and in the 1800s and early 1900s all over Denmark: pick up the fresh eelgrass, spread it out and wait for rain to clean off the salt, wait for good, dry weather, and then pack or press the eelgrass for storage until you need it. This method has worked well, but the challenge is that it is very unpredictable. And also with climate change, we are seeing longer periods of either wet or dry and sunny weather, which means we need to leave the eelgrass out on the fields for a longer time than in “the old days”. It may sound like a cost-effective way of handling the eelgrass, to let nature do the work, but the truth is that it is very labour-intensive to spread and turn the eelgrass over several times.

Today Søuld is working on new, gentle methods to wash and dry the eelgrass that are easier to predict and will give a better quality of eelgrass. It is all in the test phase, so let us see where this will take us.

Q: How does your work help to draw attention to eelgrass and its potential? 

I have insisted that the work sites always remain open for visitors who are interested in learning the craft by participating for a week or over a summer. Everyone who is willing to come to our island is welcome to learn what I have to teach: craftspeople, architects, journalists, school kids, and tourists.

I am also part of the group working to have the history of salt and eelgrass at Læsø listed as UNESCO World Heritage.

Q: What future do you envision for the use of eelgrass in construction? 

Eelgrass roofs only make sense in Læsø due to the way they look, their history, the labour-intensive method, the huge amounts of eelgrass required, and the economy. Despite this, eelgrass has a huge potential, and the roofs at Læsø can help prove this: what other natural fibre will last for hundreds and hundreds of years if piled up on a roof on a wet and windy island? Eelgrass doesn’t really burn due to minerals in the fibres from the sea. Eelgrass can bridge the gap between high requirements for technical performance and the need for carbon-storing, nature-based solutions in contemporary construction.