When straw or other dried grasses are used to cover a roof, it's called thatch. People have used thatch to keep the rain out for at least 300 years.
Although dry grasses might not seem like the sturdiest roof, thatch is actually a rain-proof, insulating substance. These days it's most common to find thatched roofs in developing countries, but some people use it for a deliberately rustic look. If someone describes your hair as thatch, they mean it looks as dry, messy, and thick as the thatch used as a roofing material. The root of thatch means "to cover."