I6i ROOK. 



strength being obtained by introducing intermediate supports between 

 Jl-plates and ridge-piece where the size of the roof renders such 

 necessary. This additional support is supplied by horizontal rectangular 

 .lied 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 same plane as the lower face of the purlins. 

 These are called principal raflert, or, for brevity, principal*, to distin- 

 guish them from the first described, or common rafter*. Where it is 

 desirable to save room by reducing the thickness of a roof, the purlins 

 may, as shown in./iy. 15, be notched into the principals and common 

 rafters, but this practice is not to be recommended, as it weakens the 



Fl f . 7. 



ROOF. 



166 



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 truts, 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 pule- 

 platei, 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 careasi-roof. 



fig. 7, which represents a small carcass-roof supported by four 



trusses, and having one purlin only between the wall-plate and ridge- 

 piece, may asmst the reader in comprehending the arrangement of 

 the part* enumerated; and their name* will be found more dis- 

 tinctly by referring to the representation of a more complicated truss 



: n. 



' 7, the common rafters arc represented on one half of the 

 iily, that the trusses may be more distinctly seen ; and the end 

 wall* are omitted for the same reason. 



The proper construction of the trusses of a roof, with reference to 

 the size of the buil.ling, and the weight of the covering, U a matter 

 requiring much scientific knowledge. For the want of this it is not 

 unusual to encumber tru.-ses with much more timber than U necessary 

 or useful : and the disadvantage of this U not confined to the increased 

 weight and cost of the roof, aa 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 U 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 fg. 8, consisting simply of two inclined 



not a good one, as, though it appears like a post to support the ridge 

 or crown of the roof, it U in reality a tie, supported by it. and sustain 



planes abutting on the walls, it is evident that the weight of the 

 raften a 6 and 4 e, as well as that of the covering sustained by tin-in, 

 will have a tendency to thrust out the walls. This tendency ordinary 

 walls have not the strength to resist, and therefore it becomes neces- 

 sary to add the beam a c (Jig. in, which by receiving the outward thrust 

 of the rafters, relieves the walls of lateral strain. If the tension of the 

 tie-beam a c be xumcicnt to resist the extending force of the rafters 

 without sensible elongation, the only effect that such a roof can have 

 the walls is a vertical pressure on each, equal to half its weight ; 

 ami it cannot fall without the tie-beam, which acts the part of a cord 

 or chain, being pulled asunder, IT the rafters being crushed. If the 

 materials were perfectly rigid, no additional parts would be required ; 

 but aa they are not so in practice, it becomes necessary, when the 

 r< are of considerable length, to provide means for counteracting 

 tendency to sinking, or tugging. F.y adding a bar shaped like lid 

 "), the centre of the tie-beam may be suspended from the crown 

 This piece is called a king-poll, but the name is perhaps 



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 in the cut may be 

 formed at its ends. These are called jogr/les, 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 ttruti or braee*, e and/, 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 Jig. 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 states. Those parts which are in a state of tension, 

 acting merely as cords to bind the truss together, may be and some- 

 times are formed of slender 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-post is dispensed with, and its office performed by two 

 similar posts, called r/ueei<-pruts, 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 strainiuy fill, between their lower 

 cuds. This arrangement is explained by/'/- 11, which also shows the 

 position of other ]>art8 of a trills. One Bide is represented as a gutter- 



i I tlie other with 



The tiiisiliary or cudti'm ml . ;ire pieces occasionally added, 



in largi' trengtllQ tp principals ; ami they, with the collar- 



beam, &.C., form a complete truss within them. The trusses of truu- 

 cated roofs are formed in this manner, the collar beam forming, as it 

 were, the keystone of the arch, and being surmounted by a camber-beam, 

 the upper eilge of which is formed into two slightly inclined planes, to 

 give the necessary slope to the lead covering. In such a roof, pieces - 



