ROOF 421 in exposed situations, as in constructing light- bouses, or tall chimneys, or houses in unpro- tected locations, a large allowance should be made ; in the midst of cities, in sheltered valleys, and localities in which heavy gales are unknown, comparatively little attention is given by builders to this force. A usual al- lowance is 40 Ibs. per square foot of vertical surface. In roof framing, the simplest form FIG. 1. King-post Truss. of truss is that known as the "king-post truss,". consisting of a triangular frame (fig. 1), in which the middle portion of the triangular piece, or tie beam, a a, is secured to the junc- tion of the inclined timbers, or main rafters, b ft, by a vertical post called the " king post," e. Several of these frames are placed paral- lel with each other to support the roof cover- ing, and are connected by longitudinal pieces called " purlins," e e. Over the latter, and parallel with the main rafters, are the common and lighter rafters, //, distributed at short in- tervals ; upon these the boarding is laid, and over all the slate, tin, or other covering. The FIG. 2. Queen-post Truss.
- ' queen-post truss " (fig. 2) consists of a hori-
zontal tie beam, D, and main rafters, e e ; the latter do not meet, but abut against a horizon- tal " straining beam," H, which is also held at each end, and at the proper height above the tie beam, by queen posts, B B. In Gothic ar- chitecture the members of the roof trusses are usually exposed to view from below, and are FIG. 8. Complete Roof. frequently carved and ornamented. The com- plete roof is shown in fig. 3, in which a is the tie beam, 6 b are the main rafters, c is one of two short rafters abutting against the straining beam d ; e and / are king and queen posts in pairs ; g g are purlins supporting the common rafters, A. The tie beam is sometimes omit- ted, and the thrust of the rafters is received by the side walls, which are often buttressed to enable them to sustain this thrust safely. Koof trusses are often given peculiar forms to suit special designs. The large spans which are sometimes built in wood are often con- structed of built arched beams. One of the largest ever designed, but which was never constructed, was planned for the great impe- rial riding house at Moscow in 1790 ; this was to have been a single built beam, in the form of an arch of 285 ft. span, with a tie beam and intermediate trusses. A somewhat-similar structure has been used in the United States, first by Wernwag and Burr, and in Germany, by Wiebeking, for spans of 200 ft. and upward. In France, Philibert de Lorme first used, and Col. Erny of the army engineers afterward constructed, roof trusses in which planks are bent and built into an arched beam; trusses of this form have been designed for roofs up to 828 ft. span. Built arches of polygonal form are frequently used ; where the struts and ties of the trusses, and the points of support for the superstructure, or incumbent load, are properly placed at the angles of the polygon, this is a stronger form than the arched. The roof of Westminster hall, London, somewhat resembles Erny's construction. Domed roofs have been frequently built with similar fra- ming ; that of the halle au He at Paris, by De Lorme, was 129 ft. in diameter, and the arched beams were a foot thick. All roofs of large span are now built with iron trusses. One of the earliest iron roofs was that of the British house of parliament ; one of its trusses, FIG. 4. Eoof of Parliament House. composed of wrought and cast iron, is shown in fig. 4. The struts, a a, are of cast iron ; the ties or suspension rods, & &, are of wrought iron ; m and n are the chords or tie rods ; e e and // are stay rods. The roof of the St. Pan- eras station, of the Midland railway in London, is 690 ft. long, 240 ft. wide, and contains 1,100
