roof, overhead covering of a building with its framework support. Various methods of construction, such as are suited to different climates, have diversified exterior and interior architectural effects. A roof may be flat, as in hot, dry areas where the shedding of rain and snow does not present a problem, e.g., in ancient Mesopotamia, Egypt, and in the SW United States. Modern structural materials and methods have made flat-roof construction practical in nearly any climate, with the development of concrete slabs, efficient drains, and waterproofing materials. On the other hand, steeply sloping roofs are still commonly found in N New England, in the Scandinavian countries, and in other regions where it is necessary to shed snow. Variations of the pitched roof are in gable, gambrel, mansard, or hip (having four sides sloping from a short ridge or center) form. The pitched roof may be of the lean-to type, as in a simple shed, or it may achieve the dignity and aspiration of a dome or spire and embody such features as the dormer window, cupola, or minaret. Pointed-roof construction includes the tie-beam, trussed-rafter, collar-beam, and hammer-beam types. English churches and halls afford many examples of these various methods, some of which have highly decorative open-timber interiors. The simplest roof covering is thatch (of straw, palm leaves, or other fibers) used by the peasants of many lands. Other finishing materials include wood (usually shingles), tile, slate, tin, lead, zinc, copper, felt, and tar. A roof's ridge is the point where the rafters meet; its principals, the purlins, resting on center or side posts, support the rafters; a valley or trough is formed by the junction of two slopes (e.g., where an ell joins the main structure). The eaves, or overhang, carry gutters or themselves drain water beyond the walls, and in the chalet and bungalow they are very wide. The concave curve of East Asian roofs is said to follow the graceful lines of a sagging tent. The classical Greek roof was of marble slabs upon timber framing and sloped gently. Early Roman roofs also were timber framed (as in the basilicas), but vault and dome construction (as in the Pantheon) were prominent in later buildings. The pointed arch and vaulting gave the slope to the Gothic roofs of Europe, while roofs in Renaissance Italy, except those with domes, were concealed, but France and Germany of this period emphasized the gable. Stepped gables are characteristic of Dutch and German roofs. Cone-topped turrets are common on the steep roofs of French châteaus. Roof ornamentation consisted of finials, crockets, crestings, gable crosses, bosses, and fantastic gargoyles (that also served as waterspouts). Roof decoration was particularly elaborate in early Asian and Gothic architecture. In contemporary architecture, roofs can span great distances with little material and few supports because of advances in the methods of using concrete and steel. Green roofs, which have used mainly since the late 1980s, lessen the environmental impact of traditional roofs, especially in urban areas. The roof surface of a building is covered with soil or another growing medium that is planted with grasses, flowers, or other plants. Green roofs reduce storm water runoff, reduce roof heating (mitigating urban "heat islands" and lowering cooling costs) and insulate the building (lowering heating and cooling costs).
See T. Hamlin, Forms and Functions of Twentieth-Century Architecture (1952).