The covering of the roof' is sustained by one or several rows of pantile] timbers, each row being in a plane parallel to the covering. The force of the timbers, which would act laterally upon the walls, is generally restrained by tie-beams placed upon wall-plates on the top ot' the walls, and fixed to the lower ends of the rafters. in roofing, many ingenious contrivances may be resorted to, their application depending upon the pitch of the roof, the number of compartments into which it may be divided, or whether there are to be tie-beams or nut. It' an apartment is required to be coved into the roof. a longitudinal truss, supported at the ends, may lie placed in a vertical plane under the ridge, by which the rafters may be hung: for it is evident, that if the upper ends of the rafters were held in their situation, their lower ends would descend by their gravity, and would describe aces of circles in yertical p!anes. and in their descent would approach nearer together. and, consequently. instead of pushing out the wails. would have a tendeiwy to draw them towards each other, And it ..'. were placed transversely immediately under the lenp-ittelinal truss, and fixed to the opposite rafters, they would act as strait ieg- pieces. and pret nut the exterior sides of the root' fiont getting hoil,rt. It' the whole space within, under the rafters, mete required. that is, to hay e no intermediate wwk of trussing, the sides of the root' may be prevented loan descending by arching them with east iron, or trussing them with wood in the inclined planes of their sides; and to restrain the pressure of the raft(rs, which would be discharged at the extremities of ilk' blinding, a st rod (r Wail-plate, well connected i11 all its pa•ts. 11111,4 he Willa, acting as a tic, would prevent the lateral pressure forcing out the walls.
In this construction. as well as in the the rafters would have a tendency, from their gravity. to become hollow ; in this case straining beams should he introduced at a con venient height, wIfich would have a good effect in connter acting that tendency. If it be required to occupy very little space by the w cast-Iron arches, abutting upon k ach other, and screwed with their planes upon the upper sides of the rafters, will answer the purpose best.
'I he idea of trussing roof: upon these principles was discovered, many years ago, by llr, P. Nicholson. in con sequence of a dispute concerning a roof which had peen constructed upon a chapel. and which had pushed out the walls to such an alarming degree ns to threaten the demo lition of the whole fabric: Mr. Nicholson was chosen arbiter, but the principle which the architect adopted was so incompatible with the nature of the design, that, though chosen by the architect himself, he was under the dis• gre, able necessity of giving judgment in titvenr of the enlist floor. This roof was truncated, or flat, and the ceiling within eylindrieal, extending, horizontally the whole clear of the malls, and in height to the under sides of the camber-beams.
so that there were ne ties between opposite rafters. The principle consisted. therefore. of two sloping sides and a camber-beam. which were only tied together by angle-braces, and as the veiling came in contaet with the tueler side of the rafters and the under sides of the camber-beams, the braces were so that the middle of their under sides came in contact with the ceiling, and thus, to maintain the roof in its position, depended entirely upon the resistance of the walls, or upon the inflexibility of the timbers, or both ; all of which were or unusual strength.
The lesson which every architect ought to learn from this, is, alway s to emistruct, his roof in such a manner its to make it entirely dependent on itself.
In the year 1802, two years subsequent to the above dis pute. a model of a roof was exhibited helbire a numerous meeting of the Philosophical Sudety at Glasgow, wherein the timbers consisted simply of rafters abutting at the top upon a ridge-piece, leaving the whole space under the rafters clear. and, of course, forming a triangular hollow prism, with the two upper sides parallel to the inclined planes of the exterior. The wall-plates were unsupported, except at the four corners, which were sustained by uprights or posts; the pieces let in upon the upper sides of the rafters consisted of small arcs, alteost straight, timming on each inclined Oahe a parabolic curve. and extending from post to post. From the ridge-piece equal and very considerable w, ights were suspended, one from the 'fleeting of every pair of rafters, without producing any visible effect upon the wall-plates.
The firm of a parain)lic enrye is best adapted to that of equal weights suspended at equal distances. Instead of arcs, simple trusses may be used, and the rafters may bridge over them.
In many cases, where space is required, we cannot help thinking, that the disposition and fixing of the boarding in the form of a truss, is vastly superior to placing them with their joints parallel to the horizon, and would be a very proper substitute for arching or trussing the sides in all roofs of moderate dimensions. it must, however, he observed, that the meeting of every two hoards ought to be as nearly as possible upon the middle of a rafter, and not over the hollow. To which we may add, that as all the joists are abutting in such disposition, the boards forming trussed work may he male thicker, and let into the rafters, which will give greater security to the abutments ; but for this purpose they ought to be firmly fixed at their meeting, to prevent them from starting.
The principlo of arching the inclined sides of a roof, and making the wall-plates net as ties, is exhibited in the archi tectural plates of Nees' Cyclopedia, published in Ism.