Where a partition extends through two or more stories of a building, it should be as much as possible a continuous piece of framing, with strong sills at proper heights to support the floor joists. Where openings occur, as for folding doors, or where a partition rests on the ends of the sill only, it should be strongly trussed, so that it is as incapable of settlement as the walls themselves. From want of attention to these points, we frequently see in dwelling-houses floors which have sunk into curved lines, doors out of square, cracked ceilings and broken cornices, and gutters that only serve to conduct the roof water to the interior of the building, to the injury of ceilings and walls, and the great discomfort of its inmates. The above remarks will be better understood by a study of fig. 40, which is an example of a framed partition extending through three stories of a dwelling house. FLOORS. 104. The assemblage of timbers forming any naked flooring may be either single or double. Single flooring is formed with joists reaching from wall to wall, where they rest on plates of timber built into the brick-work, as in fig. 41. The floor boards are nailed over the upper edges of the joists, Fig. 41. JOIST A.WALL PLATE Single flooring. whose lower edges receive the lathing and plastering of the ceilings. Double floors are constructed with stout binding joists, a few feet apart, reaching from wall to wall, and sup porting ceiling joists which carry the ceiling; and bridging joists, on which are nailed the floor boards (fig. 42.) Fig. 42. BRIDGING JOIST BINDING JOIST CEILING JOST WALL PLATE Double flooring. In double-framed flooring, the binders, instead of resting in the walls, are supported on girders, as shown in fig. 43. Single flooring is, in many respects, inferior to double floorFig. 43. ing, being liable to sag, or deflect, so as to make the floor concave; and the vibration of the joists occasions injury to the ceilings, and also shakes the walls. In double flooring the stiffness of the binders and girders prevents both deflection and vibration, and the floors and ceilings hold their lines, that is, retain their intended form much better than in single flooring 105. The joists in a single floor are usually laid on a plate built into the wall, as shown in fig. 41; it is, however, preferable to rest the plate on projecting corbels, which prevents the wall being crippled in any way, by the insertion of the joists. The plates of basement floors are best supported on small piers carried up from the footings. This is an impor tant point to be attended to, as the introduction of timber into a wall is nowhere likely to be productive of such injurious effects as at the foundations, where, from damp and imperfect ventilation, all wood-work is liable to speedy decay. The ends of all girders should rest in recesses, formed as shown in figs. 38 and 39, and with a space for the free circulation of air round the timber, which is one of the best preventives of decay. The manner in which ceiling joists and bridging joists are framed to the binders, and these latter tenoned into the girders, is shown in figs. 44, 45, 46, and 47. a a, bridging joists; bb, ceiling joists; c, girder 106. Fire-proof floors are usually constructed with iron girders a short distance apart, which serve as abutments for a series of brick arches, on which either a wooden or plaster floor may be laid (see fig. 48). Fig. 48. I 107. Of late years many terraces and flat roofs have been constructed with two or more courses of plain tiles, set in cement, and breaking joint with each other, supported at short intervals by cast-iron bearers, as shown in fig. 49. Fig. 49. This mode of construction, although appearing very slight, possesses great strength, and is now very much used in and about London, and in some portions of the United States. ROOFING. 108. In roofs of the ordinary construction, the roof covering is laid upon rafters supported by horizontal purlins, which rest on upright trusses or frames of timber, placed on the walls at regular distances from each other. Upon the framing of the trusses depends the stability of the roof, the arrangement of the rafters and purlins being subordinate matters of detail. The timbering of a roof may be compared to that of a double-framed floor, the trusses of the former corresponding to the girders of the latter, the purlins to the binders, and the rafters to the joists. Timber roofs may be divided under two heads— 1st. Those which exert merely a vertical pressure on the walls on which they rest. 2d. Those in which advantage is taken of the strength of the walls to resist a side thrust, as in many of the Gothic open timbered roofs. 109. Trussed Roofs, exerting no Side Thrust on the Walls. In roofs of this kind each truss consists essentially of a pair of principal rafters or principals, and a horizontal tie beam, and in large roofs these are connected and strengthened by king and queen posts and struts (see figs. 51. and 52). Fig. 50 shows a very simple truss in which the tie is above the bottom of the feet of the principals, which is often done Fig. 50. RIDGE PURLIN COLLAR WALL in small roofs for the sake of obtaining height. The tie in this case is called a collar. The feet of both common and principal rafters rest on a wall plate. The purlins rest on the collar, and the common rafters but against a ridge running along the top of the roof. This kind of truss is only suited to very small spans, as there is a cross strain on that part of the principals below the collar, which is rendered harmless in a small span by the extra strength of the principals, but which in a large one would be very likely to thrust out the walls. 110. In roofs of larger span the tie beam is placed below the feet of the principals, which are tenoned into, and bolted to it. To keep the beam from sagging, or bending by its own weight, it is suspended from the head of the principals by a king post of wood or iron. The lower part of the king post affords abutments for struts supporting the principals immediately under the purlins, so that no cross strain is exerted on any of the timbers in the truss, but they all act in the direction of their length, the principals and struts being subjected to compression, and the king post and tie beam to tension. Fig. 51 shows a sketch of a king truss. The com |