The Danish island of Læsø uses eelgrass (a form of seaweed) as thatching material. Free, extremely simple to lay, lasts many centuries.pic.twitter.com/lXG3uaKODg
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Thank you. But this is not thatching. Thatching is far more complicated and uses other materials, eelgrass is very limited in availability.
I'm reading up on it now, this is really neat actually, how the eelgrass solidifies into a mass. Today I learned!
It is super cool. In some photos of old tangetak it looks like they have poured concrete over the roof. It should work everywhere it grows.
Eelgrass is a marine plant with roots and leaves (so critically not a seaweed - see @ProjectSeagrass) but that is a very cool photo! 
Former phycologist so aware of the distinction, I had presumed it was a kelp, thanks for clearing that up!
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