The Figure above on the left-hand, shows how the timbers join at the top of the queen-posts, and that on the right how the timbers join at the end of the tie-beam.
The roof over the church of St. Martin-in-the-fields, designed by Gibbs, is rather curious. The entire breadth of the building is 69 feet, which is divided into 3 parts, the nave measuring 40 feet, and the remaining 29 feet being divided between the two aisles. The central portion is covered by a truss, which is supported at either end by an upright tim ber-post above the columns, and at two intermediate points by braces from these posts to the tie-beam, which is sus pended from the principals at three points by a king-post and two queen-posts; struts are carried from the foot of the king post to the head of the queen-posts, and from the feet of the latter to the principals. Above the aisles, hammer-beams are supported at one end on upright posts in the wall ; and at the other, on the post above the columns, and two interme diate bearings are afforded by struts from the same posts. The main principals have a continuous bearing on the rafters of the truss over the roof, another on a post which rests upon the hammer-beams, and at the extremities are strapped to the hammer-beams. From the last-named post there is a brace to the top of post over column. The scantlings of the timber are rather full, and are as follow : Main principal, 13 inches by 10 inches at bottom, and 11 inches by 10 inches at top.
Straining-brace or principal of nave-truss, 14 inches by 10 inches at bottom, and 11 inches by 10 inches at top. King-post, 9 inches by 9 inches.
Strut on king-post, 7 inches by '71 inches.
Queen-post, S inches by 91 inches.
Strut on queen-post, 7 inches by 71 inches.
Tic-beam, 14 inches by 91 inches.
Post over column, 14 inches by 91 inches.
Brace below tie-beam, 7 inches by 7 inches.
I lammer-beam, 14 inches by 9- inches.
Brace from uprights to hammer-beam, S inches by S inches. Post 9 hammer-beam, S inches by 9 inches.
Perlin rafters, 4 inches by 6 inches.
The roof the basilica of St. Paolo fuori le mura, executed in the fifteenth century, has double trusses, each consisting of two similar frames nearly 15 inches apart, and placed at intervals of 101 feet from each other. The tie-beam, which
is in two lengths scarfed together and secured by three iron straps, is suspended at three points by a king and two queen posts, the latter being separated by a straining-beam, and having auxiliary rafters abutting against their heids. The king-post is of curious construction, and consists of three pieces for the double truss ; that piece against which the prin cipals abut., is very short, only extending as low as the strain ing-beam. Between the trusses another piece is placed, and sustained by a strong key of wood passing through it and the two upper short pieces ; and this piece, in its turn, sustains the tie-beams by means of another strong key. The scant lings are : Tie-beams, 221 inches by 15 inches.
Principal rafters, 211 inches by 15 inches.
Auxiliary rafters, 131 inches by 131- inches. Straining-beam, 15 inches by 121 inches.
Purlins, inches by 8?, inches.
Common rafters 51- inches by 4k- inches.
The purlins are 5 feet 7 inches apart, and the rafters 81 inches.
The span of the roof is 78 feet.
The roof over the Passengers' Shed of the Croydon Rail way, at London Bridge, is of simple construction, consisting only of tie-beam, principals, and iron suspending-rods, with timber-struts between each two, the tie being suspended at eleven points.
The tie-beams, principals, and struts being framed toge ther, the suspending-rods are introduced, and screwed up by nuts upon their lower ends, until the whole is firmly united. The tie-beams camber 6 inches in the centre. There are 14 of these trusses, 12 feet 6 inches apart, connected by the ridge-piece and purlins. These extend to within 25 feet of either end, where a half-truss of similar construction is framed into the centre of each extreme truss at right angles to the same, to form the hip, with ridge-pieces as usual. The whole is covered in with lk-inch rough boarding, upon which zinc is laid and joined. The under side of the roll that covers the joint is grooved to admit the edges of the zinc, and the angle next it rounded, to allow the zinc to expand or con tract. The rolls are painted, and are sometimes covered with zinc or lead.