ROOF. In order to cover in a building in which the space to be spanned is greater than can be covered by single blocks of stone ex tending from one point of support to another, it is necessary either to have recourse to the principle of the arch, as in vaults and domes of stone or brick, or to form a framework of lim ier to support the covering. [Aacn ; DOME.] Roofs formed of one level plane, which are ex tensively used in eastern countries, are not adapted for our climate ; and a single inclined plane carries up the roof to too great a height. The best figure for a simple roof is that formed of two inclined planes, rising from the two opposite walls that approach nearest to each other, and meeting over the centre of the edifice, so as to form a ridge : this is called a conzmon or gable-ended roof. Frequently four inclined planes are used, disposed as a hipped , roof, which takes its name from the hips, or , inclined ridges formed by the meeting of the sides and ends. Sometimes the inclined faces ; are not continued upwards till they meet, but the roof is completed by a horizontal plane. Such a roof is called a truncated, terrace, or cut roof, and may havetwo, three, or four inclined faces. A similar saving of height is frequently obtained by means of a roof in which each sloping face consists of two planes of different degrees of inclination. This form, which is denominated a curb roof (or, from its inventor, a Illansarde roof), is very common in London, because it affords more space for the forma tion of bed-rooms in the roof than the simpler forms. These various kinds of roof require many modifications to suit irregularities of shape, or combinations of rectangular forms. Thus, in the junction of different roofs or portions of roofing at right angles with each other, there are inclined lines or valleys, which, at the junction of two planes, form hollows the reverse of hips. When two faces of a roof join, so as to form an angle similar to a valley, but in an horizontal instead of an inclined position, the term gutter is applied instead o/ valley. A further distinction is that betwee] roofs with dripping eaves, and those in valid., the water is collected in gutters. In the former case the roof projects several inches, or even feet, beyond the walls, and the water running from the roof either drops at once on the ground, or is collected in troughs fixed under the margin of the eaves, and conducted by them to descending pipes. In gutter roofs the timbers do not extend to the outside of the walls, which are carried up as parapets, of a reduced thickness, to such a height as . to
conceal the roof either wholly or partially.
l. The gutters, which are troughs of wood, co ered with lead or other metal, are laid at the bottom of the slopes, just within the parapets, and have a gentle inclination (usually about an inch in ten feet), to cause water to run freely towards the pipes.
The timbers of a roof have various names to them, according to their position and pur pose. Thus, those which join the wall are the wall-plates ; that at the meeting of two faces, parallel to the wall-plates, is the ridge piece; and the inclined bars extending from the wall-plates to the ridge-piece are rafters, those which form the salient angles in hipped roofs being distinguished as hip-rafters. In a hipped roof, the rafters near the ends are called jack--rafiers. Additional support is supplied by horizontal rectangular bars called purlins, and these are supported by a series of bars called principals. The lower ends of the principals are mortised into the ends of the tie-beam, which stretches across the building, and rest upon the wall-plates, The triangular tram formed by the two principals and a tie-beam with any bars it may comprise for additiona strength, is called a truss, and such framer being placed at regular intervals, the timber work between any two of them is called a Getz of roofing. The lower extremities of the com mon rafters, being elevated by this arrange ment above the wall-plates, are supported by or pieces of timber parallel tc the wall-plates, resting on the ends of the tie-beams. The supporting frame-work alto. gether is called a carcass-roof. The outward thrust against the wall is counteracted by a horizontal tie-beam, and the sinking is pre vented by a vertical bar called the king-post, Sometimes the king-post is dispensed with, and its office performed by two similar posts, called queen-posts, at equal distances from the centre of the truss. In order to keep these in their right position, a short horizontal beam, called a cellar•beam, is inserted between their upper extremities, and another, termed a straining-sill, between their lower ends. shiom rafters are pieces occasionally added, in large roofs, to strengthen the principals. In curb roofs the upper rows of rafters are called curb-rafters, and the horizontal bars that re ceive the tipper ends of the lower rafters, and the feet of the curb-rafters, are known as curb-plates.