ROOF, the covering of a building by which the interior secures protection. In ordinary ac ceptation the term includes the covering and the supporting framework, though in carpentry the term is applied only to the framework. Roofs are broadly distinguished as flat or sloping. The former were used almost exclusively among ancient peoples and are at present employed largely in Eastern countries where the roof is devoted to some extent to domestic purposes. The sloping roof, being to a great extent an ornamental consideration has given way to the flat roof in modern business buildings, where economy of construction prevails. Flat roofs are usually covered with tar, metal, asphalt or gravel. framework of the roof has been, until the modern application of iron to this use, almost exclusively of timber; and the end de sired has been to arrange the timbers in such relations as to secure the greatest strength and stiffness consistent with the minimum of weight ; also to avoid lateral strain or thrust upon the walls. The form chosen has been that of two or more inclined planes for the slopes of the roof over the enclosed space. Two in clined planes are formed by a series of sloping rafters having the lower ends tied together to prevent spreading, the upper ends forming the angle of the roof and connected by the ridge piece. The method of tying the rafters is by a horizontal beam laid across the span, receiving at each end the foot of one of the rafters, the two being securely mortised. This is called a tie-beam. When the roof is of wide span it becomes necessary to secure the tie-beam against settling in the middle, and this is done by joining the centre of the tie-beam and the ridge angle of the rafters by a king-post (A in Fig. I).
Sometimes two vertical beams are sus pended from the rafters to the tie-beam, and these are called queen-posts (BB in Fig. 2).
Braces or struts (c) may be carried from the foot of the suspended posts to the rafters to afford additional stiffness to the frame. Further support is given to the rafters in roofs of long span by one or a series of transverse longitudinal beams called purlins. Early Chris tian basilicas, of which San Miniato in Florence is typical, had roofs of the character just de scribed. When the period of Gothic architec ture was reached the king-post and tie-beam were carved and molded, in cases where wooden roofs were used instead of stone vaulting. Open timber roofs were used in England more than on the Continent for churches and halls. The finest example is that of Westminster Hall (Fig. 3) completed in 1399' and having a span of 68 feet.
In this example the central part of the tie beam is cut away, and the great rigidity of the framework prevents any inordinate thrust upon the walls.
The inclination or pitch of the roof seems to be regulated by taste, after the general law is observed that countries of moist climate re quire a sloping roof to carry off the rain and snow. The slope of a Greek pediment, marking that of the roof, varied from 15 to de grees; Roman roofs, the roofs of Romanesque buildings, and also those of the Renaissance era, generally carried an inclination increased one-quarter to that of Greek buildings. The Gothic pitch sometimes reached to 50 or 60 degrees; but since the period of the prevalence of one particular style, great latitude has been observed,— Germany and the Netherlands favor ing a steep roof, and France retaining a mod erately steep inclination even in her Renais sance structures; but England has tended toward the lowering of the pitch.
The commonest form is the gable roof where each end terminates in a gable. Modifications of this are the curh-roof in which the slope is broken, further distinguished as the gambrel roof where only two opposite sides are sloping; the French roof where the slope beneath the curb approaches almost to verticality and that above is nearly flat or with a slightly visible slope; the Mansard roof, named after its de signer, Mansard, where the slope beneath the curb is pierced by dormer windows. The hipped roof is one having a double pitch and no gables, and the ridge shorter than the parallel wall plates. The pyramidal roof is one where the ridge is so short as to approach a point. Various modifications of these general forms are known by a variety of names.
The recent use of iron for roof-frames has become very general, especially where wide spans are required as in the case of railway train sheds. The general principles of construc tion employed in wooden frames are utilized in iron ridge roofs; but for oval or conical roofs, the iron trusses require a consideration of the principles governing those of bridge trusses.
See BRIDGES.
The covering of sloping roofs is of various material, such as thin slabs of stone, thatch, tiles, slate, metal, shingles and cement. Metal shingles and asbestos shingles are increasing in use. Of the 120 factories in the United States making roofing materials, one-fourth make metal shingles and ceilings. There are employed in these factories over 4,000 people and the gross annual product is valued at about $28,000,000. There is also a production of over $5,000,000 worth in factories mainly devoted to other industries.