The auxiliary or cushion rafters, sn, ma, are pieces occasionally added, in largo roofs, to strengthen the principals ; and they, with the collar beam, &c., form a complete truss within them. The trusses of trun cated roofs are formed in this manner, the collar-beam forming, as it were, tho keystone of the arch, and being surmounted by a eamber-beam, the upper edge of which is formed into two slightly inclined planes, to give the necessary slope to the lead covering. In such a roof, piecel of wood resembling ridge-pieces are inserted at the angles formed by the meeting of the rafters with the horizontal bans that support the fiat.
Is a representation of a very simple truss, from Nicholson's Carpenter and Joiner's Companion; which illustrates the',use of slender king-poste and queen-posts of wrought-iron, and shows how the stress of every part of the roof may be brought to bear on the ridge. The lower ends of the struts rest in stirrups attached to the vertical rods and the weight bearing on the strut a is imparted, through b and e, to the king-post. The tie-beam is suspended by bolts from each of the vertical rods, and the ends of the rafters are secured to the tie-beam by iron straps passing round them, and bolted to the beam at d, d. Trusses on the same principle may be made of timber only.
In curb roofs the upper rows of rafters nre called curb-rafters, and the horizontal bars that receive the upper ends of the lower rafters, and the feet of the curb-rafters, are known as curb-plata. The proper position of equilibrium for the rafters of a curb-roof may be ascertained by very simple means, within the reach of persons not possessed of euflieient mathematical knowledge for determining it by calculation. If the rafters are to be equally loaded, as in a roof entirely covered with one material, this position will be exactly the reverse of that which they would take by gravity, were they suspended in a chain or festoon, the joints being flexible. If they are framed together in this position of equilibrium, they will balance each other like the stones of an arch ; and the tie-beams, posts, and braces will have no other office to perform than that of resisting such irregular strains as might tend to alter their arrangement. The rafters thus suspended would fall into the position a bed c, fig. 13, a line drawn through the angles being a catenarian curve ; and a' b' c' d' r', in the same figure, represents the corresponding position in which they should be placed in an equally loaded roof. If the rafters b' c' and c'd' are to bear a greater weight form partitions between the bed-rooms, their posts and braces are so arranged as to leave one or more doorways for communication between them.
In roofs of very large span it is often desirable, in order to avoid running up to a great height, to form two or more ridges. When intermediate support can be obtained from partition walls, such con structions may be regarded as combinations of two or more distinct roofs placed side by side. fig. 15 is an example of a roof of large span without any intermediate support, and having a large available space between the tie and collar beams. It represents the form of the trusses, which were placed fifteen feet apart, of a roof of eighty feet span, erected over Drury-Lane Theatre in 1793.
It is sometimes necessary, in order to obtain additional height inside a building, to raise the above the level of the top of the walls. In small spans this may be done by the simple arrangement called the carpenter's boast (a, h. 16), in which a firm union is effected between than o' I; and d'r, they will, if proportionately loaded when suspended .n a curve, fall in such a way as to increase the angles a b c and c d c, and diminish bed, thereby indicating their proper position in the roof. When the roof is to be loaded unequally, and more on one side of the ridge than the other, as it would be if t' c' were to be covered with lead, and the other planes with elates, a corresponding weight added to the centre of gravity of 1, r will cause the bars to arrange themselves asabcdc,fig.14, the angles of which, being transferred to the roof,
give the position of equilib o'Vede'. This practical method of finding the proper angles of a curb-roof may be applied under all cir cumstances, the dimensions of the experimental bars being propor tionate to those of the rafters, and their centres of gravity being loaded according to the premure to be sustained by each plane of the roof. The great advantage of curb-roofs consists in the space they afford for chambers in the roof, such chambers being lighted by dormer windows in the lower inclined faces. When the trusties of the roof the beam and the rafters without the use of nails or pine. Such a roof can only press injuriously on the walls by the rafters sinking into a concave form, which however their lower ends are very liable to do. In such a case additional strength may ho obtained by inserting a longitudinal truss, as in n. h. 16, where c represents the end of the truss, which should be firmly built into the gables : d and c are side views of two longitudinal trusses suitable for such a situation, the first being stiffened by an arch of iron notched into the short vertical pieces, and the second formed of timber only. Similar trusses are occasionally introduced under the purlins. Roofs without ties may be greatly strengthened by the use of parabolic curves of iron, notched into the rafters of each inclined face, and abutting on the wall-plates, which in such a MSC are firmly bolted together. The timbeni of such a roof may be framed together in planes, each having a distinct ridge. piece, and the ridges being screwed or otherwise firmly connected together. The curves may be cast in short segments, as they are com pressed when in use, it being merely necessary to provide that the joints should always abut on a rafter. Tredgold, in his Elementary Principles of Carpentry,' recommends the use of similar curves, of either wood or Iron, in the trusses of an ordinary roof, by which the derangement often arising from the shrinking of the king-posts and queen-posts may be avoided. In this case the curves take the place of the principal rafters, and, if made of wood, may be constructed of posts by means of straps, so arranged as to allow the beam to be keyed up to its true position in case of the roof sinking. When this is not the case, the ties are sometimes drawn up into a slightly convex or cambered form, to meet the same contingency. Height may be gained inside a building by disposing the timbers as in fig. 20, the want of a short straight pieces, arranged as shown in fig. 17, and held together by bolts or wooden keys. When curved timber can be obtained it is to be preferred, as it reduces the number of joints. For small roofs timbers may be bent into the required form, as it is found that a piece of wood the thickness of which does not exceed nth part of its length, may be bent. into a curve rising one-eighth of its span without impairing its elasticity. Two such pieces may be laid together, and bent by twining a rope attached to their ends, as is done in tightening the frame of a bow or pit saw; and, being bolted together while curved, they will spring back but little when the rope is relaxed. Another mode of forming such a rib is to take a piece of wood whose thickness is about one-sixtieth of its length, and cut along the middle with a thin saw from each end, leaving about eight feet in the centre solid. The beam may then be bent, and bolted or pinned together as before described. In either case the rib should be bent about one-fourth more than it is intended to remain, to allow for springing back. A parabolic curve is the form most recommended; but a circular arc, rising half the height of the roof, will answer the purpose. Fig. IS continuous tie-beam being compensated for by an iron strap to unite the ties to the bottom of the king-post at a ; but it is evident that the safety of the plan must depend wholly on the straps, which alone counteract the outward thrust of the rafters.