Roofs

roof, ft, trusses, rafters, dome, principal, beam, top, concrete and fig

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The "king-post truss" may be used for spans up to 3o ft. and is constructed as shown in figs. i and 2. It has a central post sustaining the "tie-beam" in the centre with struts projecting from its base to support the principal rafters at a point where the weight from the purlins renders strutting necessary. The members are connected by wrought-iron straps and bolts; a strap connects the king-post and tie-beam and is often fitted with a gib-and-cotter arrangement (iron folding wedges) which allows the whole truss to be tightened up should any settlement or shrinkage occur. "Queen-post trusses" have two queen-posts sup porting the tie beam (fig. 3). Trusses of this type are suitable for spans up to 45 ft. The joints between the members are made in a similar manner to those of the king-post principal with wrought-iron straps. There are two purlins on each slope, one supported at the top of each "queen," the other about half-way down the principal rafter. A stout straining beam connects the heads of the queens. In fig. 4, a and b are details at the foot of the queen-post, and c at the head. In roofs of larger span, up to 6o ft., the tie-beam requires to be upheld at more than two points, and additional posts called "princesses" are introduced for this purpose. This also entails extra struts and purlins. In such large spans the straining beam often becomes of such a length as to require support and this is effected by continuing the principal rafters up to the ridge and introducing a short king-post to sustain the beam in the middle of its length.

Open Timber Roofs.

Open timber roofs of various types but principally of "hammer-beam" construction were used in the middle ages where stone vaulting was not employed. Many of these old roofs still exist in good preservation and exhibit the great skill of the mediaeval carpenters who designed and erected them. Such forms are still used, chiefly for ecclesiastical build ings and the roofs over large halls. In the best periods of Gothic architecture the pitch of these roofs was made very steep, some times as much as 6o° with the horizon. In the hammer-beam type of roof the tie-beam at the foot of the rafters is omitted, a collar connecting the principal rafters at a point about half way in their length, and the lower portion of the rafters con sisting of struts and braces rigidly connected to relieve thrust upon the walls serving as abutments. There are two kinds of hammer-beam, the arched and the bracketed. The "hammer beam" projects from the top of the wall and is bracketed from a corbel projecting from the wall some distance below. This form of roof has a style and dignity appropriate to the period in which it was developed.

The Mansard Roof

(fig. 5) is a useful form of construction which obtains its name from Francois Mansart (q.v.). This kind of roof has been largely used in France and other European coun tries, and in America in the old colonial days. It is well adapted to some styles of architecture, but requires careful proportion ing of the slopes for a satisfactory effect. In America the term "Mansard Roof" (q.v.) is reserved for roofs of double pitch slop ing up from all 4 sides of building; the term "gambrel roof" (q.v.) being applied to double pitched roofs ending in gables. By the use of a Mansard roof extra rooms can be obtained without adding an additional storey to the building. Fig. 5, A, B, C, D and E show various sectional forms. A similar type of curb roof is often used having a flat lead or zinc-covered top in place of the pitched slate or tile-covered top of the ordinary Mansard roof.

Composite Roof Trusses

of wood and iron are frequently used for all classes of buildings, and have proved very satisfactory.

They are built upon the same principles as wooden roof trusses. The struts—that is, those members subjected to compressional stress—are of wood, and iron bars or rods are used for the ties, which have to withstand tensile forces. When shrinkage occurs, as usually happens in large trusses, the joints of the framing are tightened up by the bolts attached to the tie-rods. Figs. 6, 7 and 8 are the sections and plan of a simple method of constructing the roof for a small domestic building with plaster ceilings to the top rooms. It is a couple close roof with the addition of a collar and struts, and king-rod to every fourth rafter. Trimming is necessary for openings and where portions of the structure, such as chimney stacks, cut into the roof. A dragon tie is framed to the wall-plates at the hipped angles to receive the thrust from the hip rafters.

Steel Roof Trusses.

Because of their adaptability in form and detail, speed of fabrication and general economy, roof prin cipals of steel are now the most usual kind of roof support where trussed framing is required. They are particularly suitable for commercial buildings, and, while not artistic if judged by com parison with the traditional treatment of timber, are capable of port cantilever trusses, the main girders being placed to form the ridges of the slopes and the cantilevers attached at each side as shown in fig. i o. The gutters are central between the trusses and the girder spacing up to 3o ft. span. Clear bays are thus provided 15o ft. X 3o ft.

Domical Roofs.

Domes may be framed up with wood rafters cut to shape. For small spans this construction is satisfactory, but domes of considerable size are now framed in steel or in reinforced concrete. The outer dome of St. Paul's cathedral in London is of lead-covered wood, framed upon and supported by a conical structure of brickwork which is raised above the inner dome of brick. Concrete is a very suitable material for use in the construction of domes, with iron or steel reinforcement in the shape of bars, mesh, or perforated plates. One of the best modern examples of concrete vaulting and domical roofing with out metal reinforcement occurs in the Roman Catholic cathedral at Westminster, a remarkable building designed by Mr. J. F. Bentley. The circle developed by the pendentives of a nave dome is 6o ft. in diameter. The thickness of the dome at the springing is 3 ft. gradually reduced to 13 in. at the crown; the curve of equilibrium is theref ore well within the material. The domes were turned on closely boarded centering in a series of super imposed rings of concrete, averaging 4 ft. in width, and the con crete was not reinforced. The independent external cohering of the domes is formed of 3 in. artificial stone slabs cast to the curve. They rest on radiating ribs 5 in. deep of similar material fixed on the concrete and rebated to receive the slabs; thus an air space of 2 in. is left between the inner shell and the outer covering, the object being to render the temperature of the interior more uniform. At the springing and at the crown the spaces between the ribs are left open for ventilation. The sanc tuary dome differs in several respects from those of the nave. Unlike the latter, which seem to rest on the flat roofing of the church, the dome of the sanctuary emerges gradually out of the substructure, the supporting walls on the north and south being acceptable aesthetic form based on the economic suitability for their particular purpose.

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