Inspired by medieval seaweed thatching methods, Danish designer Kathryn Larsen developed a way to bring it back to modern construction by making prefabricated thatched panels for roofs and façades.

In the Middle Ages, the Danish island of Læsø became famous for its salt industry, and hundreds of salt kilns were built. But because of the island’s limited natural resources, it was soon completely deforested, which left the inhabitants without construction materials for their homes. Therefore, they started to build their homes from driftwood and thatched the roofs with a type of seaweed called eelgrass.

Because the seaweed, and the timber, were impregnated with seawater, the materials are much less susceptible to decay. Compared to a normal thatched roof, which has a lifespan of about 30-40 years, an eelgrass roof can last for as much as 200 to 400 years. In addition, eelgrass is naturally fireproof, rot-resistant, carbon negative, and becomes fully waterproof after about a year. Its insulation properties are comparable to mineral wool. Additionally, the material invites plant growth, giving the effect of a green roof or façade.

The Læsø homes, currently being restored by master thatcher Henning Johansen, served as inspiration for Larsen’s project. Since using eelgrass in construction has so many environmental benefits, she aimed to bring the material back to modern architecture by designing prefabricated panels. Larsen used existing research by Studio Seagrass and Tobias Gumstrup Lund Øhrstrøm’s master thesis, in which seaweed was mixed with (natural) binders to create panels.

Larsen aimed to retain the original qualities of eelgrass, so she only used minimal amounts of binders. Using 8 natural non-toxic binders, plant or animal based, as well as water, she created several test panels. Te natural binders helped improve the seaweed’s waterproofness, but also made the material more stiff and sometimes brittle.

Currently, Larsen is testing large scale panels of water-applied seaweed on the roof of Copenhagen School of Business (KEA). After about eight months outside, the panels are almost entirely intact, and moss is beginning to grow on the eelgrass.

The installation was sponsored by KEA Campus Service, and all material testing was done with the guidance of KEA’s Material Design Lab. In 2019, Larsen received funding from Boligfondens Spirekasse, to build new prototypes and continue her research. She hopes to test the panels’ u-values, to see what insulation properties they can bring to a construction.

Photos: Kathryn Larsen / Anders Lorentzen