Since the Viking age (at least) it was typical to have a layer of dirt on houses and buildings as the dirt insulated from the cold Winters and warm Summers.  Grass was grown on top so the dirt wouldn’t fall off and I guess to also beautify the building.  The Sami even mounded their houses into a little hill of earth and grass, like the one below from Nordland.  Even though Sami were nomads, they would have seasonal houses they returned to every year which made this permanent structure worth while.  However, not all Sami were nomads.  The coastal Sami had permeant structures for both humans and animals.  And today it is only the Sami that have a livelihood herding that live a semi-nomadic life.

Grass roofs sure look cute in the Norwegian landscape.  Nowadays it is usual to see grass growing on boat sheds, however, I think this little one has taken it one step further and is growing trees on its roof.