ROOF. The coverings of houses vary in every climate and every age. In warm countries, such as India, fiat roofs, covered with cement, are almost invariably used. The frequent allusions in the Bible to the house-top show that the roofs of Palestine were flat iu ancient times as they are now. Those .of Egypt and Assyria (q.v.) were also flat, and were composed of wooden beams, covered with thick layers of earth, forming an impenetrable protection from the fierce heat of the sun. In countries where the climate is milder, and rain more abundant, roofs sloping from a central ridge are the usual form. The Greeks and Romans constructed their roofs in this way. Those of Greece were, in important works, covered with marble slabs, carefully grooved together. so as most effectually to protect the interior from rain. In the common buildings of Greece and Rome, roofing-tiles are used.
In the rainy climate north of the Alps, roofs of a much steeper pitch are employed, so as the more readily to throw off rain and snow. The angle at the ridge is not uncommonly a right angle; and roofs slated in the usual way should never be less than of the span (or width between supports) in height. When large slates are used, of the span in height will suffice.
When roofs are well constructed, they serve to bind the walls together, and thus to strengthen the building. In order to do this effectually, they must not be made of too great weight, otherwise they crush the walls. The actual covering of the roof and its supports are therefore made as light as possible, and the strength concentrated in principals or trusses. The following are the commonest forms of these trusses: Fig. 1 represents what is called a king-post roof (A being the king-post), and fig. 2 a queen post roof (B, B being the queen-posts). The latter is used for wider spans than the
former, and has the advantage of leaving the center of the roof clear of timbers, so that attic rooms may be introduced. The other members of the truss are named as folows C,C,C,C, braces or struts; D,D, tie-beams; E,E,E,E, principal rafters; F,F, ridge pieces; G,G, etc., purlins; these and the ridge-piece are laid across from truss to truss, and carry the common rafters J,J. H is a collar. K,K, the pole-plates, and L,L, the wall-plates, are laid along at the wall-head, to bind the wall and feet of rafters together.
The above system of construction has been used from a very early time to the present day. The early Christian, and probably the Roman basilicas, had exactly such roofs. In early Gothic times roofs of this kind were made ornamental by carving the king-post and molding the tie-beam. During the decorated style. an arch, or a series of cants was introduced. As the style.progressed, curved braces were placed under the tie-beam to support it; these were carved, and rested on elegant corbels, the spandrels between the braces and the wall being tilled with tracery. In the perpendicular style, the central part of the tie-bean is eta away, and the beautiful hammer-beam (q.v.) roots of the period Lecome usual. The roof of Westminster hall is one of the finest examples of this kind of roof. These open timber-roofs arc used both in churches and halls, but chiefly in the latter, as the church roofs were frequently vaulted. See VAULT. In modern times, when great spans have to be roofed over, combinations similar to those used in lattice bridges (q.v.) are required. Recently iron has been introduced, and by means of it, spaces of almost any width can be roofed over.