Mansard roof. The Mansard roof (fig. 5) is a useful form of construction | top rooms. It is a simple construction of the couple close which obtains its name from François Mansard, a distinguished order with the addition of a collar and struts and king-rod French architect who lived in the 17th century. This to every fourth rafter. Trimming is necessary for openings kind of roof has been largely used, especially in France and where portions of the structure, such as chimney stacks, and other European countries, as well as in America cut into the roof. The trimming rafters are made an inch in the old colonial days. It adapts itself well to some styles thicker than the others. The dragon tie is framed in connexion of architecture, but should be very carefully applied, since it with the wall-plate at the hipped corners to take the thrust of the hip rafters. FIG. 5.-Mansard Roof Truss: detail of outline as A; other outlines noqu dari at B, C, D and Exe 300 allow is apt to appear ungainly in some situations. By the use of a Mansard roof extra rooms can be obtained at a small expense without adding an additional storey to the building proper. The outward thrust upon the supporting walls is not so great as with an ordinary pitched roof, the load coming practically vertically upon them. There is no recognized rule for the proportion or pitch of a roof of this description, which should be designed to suit the particular building it is intended to cover. Fig. 5, A, B, C, D and E show various 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 types of 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 any shrinkage occurs to loosen the joints of the framing, as usually happens in large trusses, the tie-rods are tightened up by the bolts attached to them. Figs. 6, 7 and 8 are the sections and plan of a simple method of constructing the roof for an ordinary domestic building with plaster ceilings to the Hanou bas SECTION ON A Jeningn FIGS. 6 and 7.-Roof for Domestic Building. to the smallest size compatible with safety. In this way any unnecessary surplus of material is avoided, and so is the heavy, overwhelming effect noticeable in many roofs of large span. There is an entire absence of long wide plates and webs; the various members are composed wholly of flat bars and angle irons riveted together, and plates are introduced only where required to cover joints. Some notes on its size and construction will be interesting. The dimensions of the great hall are 440 ft. long by 250 ft. wide, the height to the crown of the roof being about 100 ft. The main ribs of the roof have a clear span of 170 ft. and are placed 34 ft. apart. They are of boxgirder form and measure 7 ft. deep and 2 ft. wide. The gallery around the hall is 40 ft. wide on three sides and 26 ft. wide on the remaining side. It is covered by a lean-to roof which abuts against the curved ribs on the north and south sides, and is attached to horizontal members of the screens on the east and west sides. The brick walls of the building are not called upon to resist any portion of the thrust from the roof, as the side frames through which the gallery floor passes form a self-contained system of steelwork in which the thrust is ultimately conveyed to the ground. The screens which close the semicircular ends of the roof are of vertical ridge and furrow construction, as can be clearly seen in the illustrations, this form offering great resistance to wind pressure while at the same time requiring a minimum amount of material. Of the two illustrations, fig. 11 is a detailed cross-section showing fully the method of construction of the foot of the main rib and column, and the arrangement of the side frames above referred to is shown in fig. 12, which is a complete cross-section view, and will convey to the reader some idea of the vast size of the building and its general proportions. The following five roofs are examples of large span: Crystal Palace (104 ft.); Olympia, London (170 ft.); St Enoch station, Glasgow (198 ft.); Central station, Manchester (210 ft.); St Pancras station, London (240 ft.). Domical roofs. Domes may be framed up with wood rafters cut to shape. For small spans this construction is satisfactory, but when the dome is of considerable size it is often framed in steel as being stronger and more rigid than wood, and therefore not exerting so great a thrust upon the supporting walls. 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, and may be employed simply or with iron or steel reinforcement in the shape of wires, bars or perforated plates. One of the best modern examples of concrete vaulting and domical roofing without metal reinforcement occurs in the Roman Catholic cathedral at Westminster, a remarkable building designed by Mr J. F. Bentley. A few details of the roofs will be interesting. The circle developed by the pendentives of the nave domes is 60 ft. in diameter. The thickness of the domes at the springing is 3 ft. gradually reduced to 13 in. at the crown; the curve of equilibrium is therefore well within the material. The domes were turned on closely boarded centring in a series of superimposed rings of concrete averaging 4 ft. in width. The concrete is not reinforced in any way. The independent external covering 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 sanctuary 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 kept down so as to give greater elegance to the eastern turrets. The apsidal termination of the choir in the east is covered in with a concrete vault surmounted by a timber roof, in striking contrast to the domes covering the other portions of the structure. Fig. 13 is a section through the nave showing how the domes are buttressed, fig. 14 is a section through the sanctuary dome, and figs. 15 and 16 a section and part plan of the vaulting of the choir with its wood span roof above the concrete vault. Covering Materials for Roofs.-There are a large number of different roofcovering materials in common use, of which short descriptions, giving the principal characteristics, may be useful. The nature of the material employed as the outer covering affects the details of roof construction very considerably. A light covering such as felt or corrugated iron can be safely laid upon a much lighter timber framing than is necessary for a heavy covering of tiles or slates. C. -boo To ΤΟΥ bisi ged DETAIL qsi blyade AT C. Roofing felt is an inexpen BIR MIsive fabric of animal or vege table fibre treated it Felt. capable of resisting the weather. It is largely used as a roofing material for temporary buildings. When exposed to the weather it should be treated with an application of a compound of tar and slaked lime well boiled and applied hot, the surface being sprinkled with sand before it becomes hard. Felt is also used on permanent buildings as a good non-conductor of heat under slating and other roof covering materials. In this case it is not tarred and sanded. It is supplied in rolls containing from 25 to 35 yds. 30 in. wide. The sheets should be laid with a lap of 2 in. at the joints and secured to the boarding beneath by largeheaded clout- nails driven in about 2 in. apart. Corrugated iron. Being Corrugated iron is supplied either black or galvanized. It is especially suited for the roofs of outbuildings and buildings of a more or less temporary character. to a large extent self-supporting, it requires a specially deMe signed roof framework of light construction. If, as is usually the case, the sheets are laid the cathe corrugations running with the slope of the roof, they can be fixed directly on purlins spaced 5 ft. to 10 ft. apart according to the stiffness and length of the sheets. In Polant zine covering 6 diar HALF ELEVATION П 212 |