Danish designer Kathryn Larsen has developed a method of producing prefabricated thatched panels for roofs made of seaweed—based on medieval traditions from the island of Laeso. Saltwater strengthens the seaweed, making it less susceptible to decay: while a traditional thatched roof can last around 40 years, a roof made of eelgrass can last up to 400 years. The material is also fireproof, resistant to rot, waterproof (after about one year) and carbon negative. Larsen’s design brings this eco-friendly material back into use in modern architecture by creating premade panels, which are already being tested in-use on the roof of Copenhagen School of Business. So far, the project has been a success: after close to a year outside, the panels remain intact. This project also uses waste as a resource: see as waste, approximately 22,000 tonnes of eelgrass are thrown out each year. Larsen continues to receive funding for further developments of this technology.