It is not usual to vary the scantling., or transverse dimensions of rafter:, in any considerable degree, on account of their various lengths ; nearly the same scantling being used in all buildings, and the required strength bring obtained by introducing intermediate supports between the wall-plates and ridge-piece where the size of the roof renders such necessary. This additional support is supplied by horizontal rectangular ars called purlins, placed under the rafters in such a manner as to divide their length into two or more equal parts, the ends of the pur lins being fixed to the sides of the bordering frame. Like the rafters, the purlins are not much varied in thickness according to the strain upon them, but they are in turn supported by a series of bars placed equidistant from each other, and parallel with the rafters, but with their upper face in the sane plane as the lower face of the purlins. These are called principal rafters, or, for brevity, principals, to distin guish them from the first described, or common rafters. Where it is desirable to save room by reducing the thickness of a roof, the purlins may, as shown in fig. 15, be notched into the principals and common rafters, but this practice is not to be recommended, as it weakens the timbers. Where principals are used, their lower ends are morticed into the ends of a tie-beam, which stretches across the building, and rests upon the wall-plates. This beam keeps the lower extremities of the principals from separating, and discharges the weight of the roof on the walls in a vertical direction, relieving them entirely from the lateral thrust of the rafters. The triangular frame formed by the two principals and a tie-beam, with any bars it may comprise for additional strength, is called a truss, and such frames being placed at regular intervals, the timber work between any two of them is called a bay of roofing. The lower extremities of the common rafters, being elevated by this arrangement above the wall-plates, are supported by pole plates, or pieces of timber parallel to the wall-plates, resting on the ends of the tie-beams. The supporting frame-work altogether is called a eareass-roof.
Fig. 7, which represents a small carcass-roof supported by four trusses, and having one pnrlin only between the wall-plato and ridge piece, may assist the reader in comprehending the arrangement of the parts enumerated ; and their names will be found more dis tinctly by referring to the representation of a more complicated truss at jig. II.
In fig. 7, the common rafters are represented on one half of the roof only, that the truest% may be more distinctly seen ; and the end walls are omitted for the same reason.
The proper construction of the trusses of a roof, with reference to the size of the building, and the weight of the covering, Is a matter requiring much scientific knowledge. For the want of this it is not unusual to encumber tru.aes with much more timber than is necessary or useful : and the disadvantage of this is not confined to the increased weight and cost of the roof, as superabundant timbers frequently occasion injurious strains, and the increased number of joints adds to the risk of derangement by the shrinking and warping common to all timber constructions. The general principles to be acted upon may be illustrated by a few diagrams ; but in the limited space devoted to this article no attempt can be made to describe all the modifications required by the ever-varying forms of buildings; in the design of which it is too common, instead of assigning its due importance to the roof, to treat it as an unsightly feature, to be concealed as much as possible from view.
In a roof formed as shown in fig. 8, consisting aimply of two inclined planes abutting on the walls, it is evident that the weight of the rafters a b and b c, as well as that of the covering sustained by them, will have a tendency to thrust out the walls. This tendency ordinary walls have not the strength to resist, and therefore it becomes needs airy to add the beam a c (jig. 9), which by receiving the outward thrust of the rafters, relieves the walls of lateral strain. if the tension of the ti,beam a c be sufficient to resist the extending force of the rafters without sensible elongation, the only effect that such a roof can have upon the walla is a vertical pressure on each, equal to half its weight ; and it cannot fall without the tie-beam, which acts the part of a cord or chain, being pulled asunder, or the rafters being crushed. If the materials were perfectly rigid, no additional parte would be required ; but as they are not so in practice, it becomes necessary, when the timbers are of considerable length, to provide means for counteracting their tendency to sinking, or sagging. By adding a bar shaped like bd (jig. 10), the centre of the tie-beam may be an/speeded from the crown of the roof. This piece is called a king-post, but the name is perhaps not a good one, as, though it appears like a post to support the ridge or crown of the roof, it is in reality a tie, supported by it, and sustain ing, instead of resting upon, the centre of the tie-beam. By cutting the king-post out of a piece of wood of larger scantling than the shank of the post itself, projections of the shape indicated iu the cut may be formed at its ends. These are called joggles, and those at the upper end form a wedge between the heads of the rafters, like the keystone of an arch. It is evident that a weight pressing on the projecting joggles at the base of the king-post will be by it transmitted to the crown of the roof. These therefore form fixed points, from which support may be obtained, by means of struts or braces, e and f, for the centre of each rafter. Where purlins are added, they rest on those points of the principal rafters that are thus supported by struts, as may be seen by reference to fib. 7. It may be observed that this truss consists of two pieces (the tie-beam and king-post) in a state of tension, and four (the two rafters and the two struts) in a state of compression; and that in every well-contrived truss, however the number of its component parts may be increased, every bar is in one or other of these atates. Those parts which are in a state of tension, acting merely as cords to bind the truss together, may be and some times are funned of ,lender rods of wrought-iron ; but the others, needing stiffness as well as cohesion, require bars of considerable sub stance, and are therefore mostly formed of wood or cast-iron. Some times the king-poet is dispensed with, and its oflico 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 hori zontal beam, called a collar-beam, is inserted between their upper extremities, and another, termed a straining sill, between their lower cuds. This arrangement is explained by jig. 11, which also shows the position of other parts of a truss. One side is represented as a gutter roof, and the other with eaves.