968 COTTAGE, FARxM, AND VILLA ARCHITECTURE. 1708 P n ^ m .r^ ^ ' n 4 L ^L^ ^ L/' U ^ 1 tension rods of the main ribs. Fig. 1708 is an edge view, or bird's-eye plan, of the same. Fig. 1709 shows the extreme lower terminations of the prin- cipal rafters ; o, the rib ; p, the lower end of the tension bar ; n, the retaining pin passed through the projection attached to the rib ; and t, the tension rod of the main rib. The cast-iron gutter inside the parapet, when a parapet is used, is also shown. When there is no parapet, any kind of eave-shoot may be employed. Fig. 1710 shows a side view of the main centre joint: a is a vertical rod; and 6 b, the main diagonal stays to the bottom of the main ribs. Fig. 1711 is a plan from the top of the same. The same letters refer to the same parts in both figures. This completes the details of the trussing ; and I shall now show the application of the filling-in rafters. 1 940. Application of the Filling-in Rafters. Rafters are usually ap- plied either lying parallel to the principals, or at right angles to them. In the first case, purlins are necessary ; in the latter, each rafter is exposed to two strains (like a purlin) ; one vertically, and one parallel to the rake of the roof; and, as each rafter so placed is unaided by any other, they all soon swag. To remedy this defect, and save material, I propose placing my rafters dia- gonally ; and, for this purpose, certain sockets are cast in the sides of the vertical part of the main ribs ; which, when seen on edge, appear as in fig. 1712. The rafters are cut to the a. proper form, and driven into the sockets, n n. A hole is then bored right through both rafter ends, and through a hole cast in the iron, and an oak trenail or dowel (to keep the rafters from blowing off) is driven through, as shown by the dotted lines in the figure. The arrangement of the rafters is somewhat as in fig. 1 7 1 3, supposing the centre couple, or principal, a a, to be that in the centre of the length of tlie roof. The rafters, being thus arranged, either stout laths for slating (sawn to two inches and a half by three quarters of an inch) are to be spiked down upon them ; or they are to be sheeted over with thin boards, and covered with zinc or copper. Thus, it is seen, each rafter corroborates all the rest, both as regards the vertical and the diagonal strain ; so that no one of these wooden rafters can either swag, or bend aside, without bringing others with it. By this means much timber is saved; moreover, all wall-plates, foot rafters, rafter-plates, purlins, drc, are dispensed with. Tl'o ends of the couples rest on stone, built into the walls. There is no waste of timber in cutting the rafters diagonally ; as, when proper gauges are made, they will cut out of one another (by the property of the rectangle), like the 1710
