ROOF, the covering of a house or other building. The name, in its most extended sense, embraces the external covering itself, and the framework by which it is supported; but, as a term in carpentry, it is limited to the carcass-roof or framing.
The importance of this part of a building can hardly be overrated, since on its right construction depends not only the comfort of those for whose shelter it is designed, but &leo the safety and durability of the edifice itself. For the former of these purposes it is desirable that a roof should exclude extremes of heat and cold, and be impervious to rain or snow. For the latter, the exclusion of water is equally neces sary ; and it is essential that the framework be so disposed as to throw the least possible strain on the walls. By a judicious arrangement in this particular, a roof may not only be prevented from pressing on the walls in an injurious manner, but may be made to contribute greatly to the stability of the whole structure. In order to the due combi nation of the requisite qualities, an intimato acquaintance with the principles of mechanical philosophy is indispensable ; and a correct knowledge of the strength of different materials, when exposed to various kinds of strain, is necessary to the economical adjustment of the dimensions of the several parts of a roof. A roof of large span forms, indeed, one of the most interesting applications of the science of carpentry, theoretical or constructive.
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 extending 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 atone or brick, or to form a framework of timber to support the covering. The former plan is objectionable in the case of ordinary buildings (rein its expense and weight, and from the great solidity required in the walls, where they have to be used as the abutments of an arch. The principles on which such coverings of masonry are formed are explained under ARCH and Dow: ; and in this article the more usual kind of roof, that sus tained by a wooden framing, will be described. Such structures occa
sionally partake of the character of an arch or dome, but more usually consist of flat planes variously disposed. Roofs formed of one level plane, which are extensively used in eastern countries, are not adapted for buildings in which a Largo space has to be spanned over, nor to resist the penetration of water, and are therefore unsuitable for climates in which rain and snow are common. A simple inclined plane is well adapted to resist injury from weather; but, as it is scarcely more favourable to an economical disposition of the timbers than a flat roof, it is only suited for small buildings, and is meldoin used except as a lean-to. Another objection to its use on a large scale — — is the disproportionate height it re p droll on one able of the building. The best ti,eure for a simple roof is that formed of two Inclined planes, riming 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. By this form, supposing the same Slope to be maintained, one-half of the height of the single inclined plane is avoided ; and the length of the thnt*rs being diminished one-half, their scantling may be considerably nolueed. Fite 1 representa a plan with side and end views of such a Pouf, which is called a common or gable-rndol roof. Frequently four inclined pbuiea are used, disposed as shown in fig. 2, representing a hippest roof, which takes its name from the hips, or inclined ridges formed by the meeting of the sides and ende. Where a hipped roof covers a perfectly square building, the faces all meet in a point, and form • pyramid ; but when, as in the diagram, the plan of the reef is oblong, the planes rising from the nearest opposite walls meet in a ridge. Sometimes the inclined faces era nut continued upwards till they meet. but the roof is completed by a horizontal plane. Such a roof Is called a trsacakd, terrace, or cut roof, and may have two, three, or four inclined faces. Fie. 3 represents a truncated roof hipped at end, and terminating at the other in a vertical wall, like the gable s nded roof.