The purlins, eight in number, to run the whole length of roof, notching in the rib arches. The purlins to be placed in pairs and to have small cross struts either notched into them or securely nailed to prevent them from buckling or twisting.
All the horizontal timbers of roof, such as the purlins, poll plate, sill, and heads of skylight, to run 9 inches in end walls, and to be cogged on template. Each purlin, if not in one piece, to be properly scarfed. An oak wall-plate, 9 in. by 6 in., was laid the whole length and width of the building, running 6 in. in the wall at angles, where it was pinned and lapped. The plate in the arch over the entrance formed the upper part of the railing in the gallery.
This plate served as the abutment for twenty-four oak braces or struts, each 7 in. by 4 in., placed in the lower portions of the roof on each side, each strut to be sub-tenoned either into purlin or cross piece between rib, and the whole to be securely fixed.
The framing to support curb or sill of skylights to be in one piece, to run over the wood arches, and to be securely fixed to purlin.
Each pair of circular ribs moneyed out 22l. 4s. 6d. The more modern French style of forming this kind of roof would have been by bent ribs composed of three ten-inch planks, 12 inches by 3, cut true at the saw-mill, jointed with glue, planed all round, chamfered to edges, with 20 half-inch bolts. These would have cost only 13l. 16s. 8d. each, but they would have caused considerable lateral pressure against the side walls.
The roof of the riding-house is correctly shown in the small view, p. 389, which serves also to show the section. Fig. 1 of the accompanying cut shows one
of the circular ribs, fig. 2 the section of the pair joined together, and fig. 3 the section of the more modern French method of bent ribs. A roof in this latter construction was put up by Mr. Charles Fowler, architect, at the sale-room, St. Paul’s Churchyard. The circular ribs of the roof were formed in three thicknesses of 1¼ deal, footed into iron sockets or corbels let into stone templates. As a precaution until the perfect set and settlement of the work, three of the roof-frames had iron tie-rods, which were removed when all fear of lateral thrust was over. A print of the room was given in the Builder.
The first construction described could be much improved, strengthened, and lightened by introducing an iron bar in lieu of the oak rib; and this has been done in several instances, resulting in the roofs standing well.
The chief portion of the bricks used in the construction of the riding-house were provided from the estate, and were carted on the ground for the use of the builder. His account came to 920l.