Thatching is the craft of building a roof with dry vegetation such as straw, water reed, sedge, rushes, heather, or palm branches, layering the vegetation so as to shed water away from the inner roof. Since the bulk of the vegetation stays dry and is densely packed—trapping air—thatching also functions as insulation. It is a very old roofing method and has been used in both tropical and temperate climates. Thatch is still employed by builders in developing countries, usually with low-cost local vegetation. By contrast, in some developed countries it is the choice of some affluent people who desire a rustic look for their home, would like a more ecologically friendly roof, or who have purchased an originally thatched abode.