It's a perfectly serious answer, Nosha. In Iceland, northern Scotland and the Western Isles you'll see cottages with grass turf roofs. Some owners actually encourage sheep to graze on the roof, even to the extent of lifting them up if the sheep can't manage to get up there on their own!
Water-loving plants would have a hard time of it if the roof dried out, and a few days of sunshine could kill them. Grass turf is more readily available, and is more able to survive, especially the tough hill varieties.