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ROOFS

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Originally appearing in Volume V23, Page 703 of the 1911 Encyclopedia Britannica.
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ROOFS. A roof is a construction placed as a covering over the upper portion of a building to exclude the weather and preserve the contents dry and uninjured. Roofs are designed to throw off rain and snow, and their slope or " pitch," as it is generally termed, is governed to a great extent by the climate, as well as by the material used and manner of laying. The pitch may vary from an almost horizontal surface (as largely adopted in dry countries and also in temperate climates for roofs of metal or asphalt) to the steeply pitched roofs required for the ordinary flat tiles which to be weatherproof must be laid at an angle of from 450 to 8o° with the horizon. Besides serving the useful purpose of protection against inclement weather the roof, both externally and internally, may be designed to form an architectural feature in keeping with the character of he building.a time the ridge instead of remaining level takes on a wavy out-line, due to the fact that some of the timbers have settled slightly owing to decay or other causes, whilst others have remained firm in their places. The lower ends of the rafters should pitch on a wood plate bedded on the top of the wall; this, as described under CARPENTRY, assists in spreading the weight over a large area of the wall, and provides good fixing for the timbers. The simple " couple roof " consists merely of two sets of rafters pitched from plates on the walls on either side of the building and sloping upwards to rest against a common ridge-piece. There are no ties between the feet of the rafters, which therefore exert a considerable thrust against the supporting walls. On account of this and of the lack of rigidity of the framing this form of roof should only be used to cover small spans of ro to 12 ft. Generally the ends of the rafters are connected by ceiling joists which form a level ceiling and at the same time prevent any outward thrust on the supports. When used for spans between 12 ft. and 18 ft. a binder supported by an iron or wood " king " tie every 5 or 6 ft. should be run along across the centres of the ceiling joists and the latter spiked to it. Such roofs with the wood tie across the feet of the rafters are termed " couple close roofs." When the ties are fixed about half-way up the rafters it is called a collar roof," and may be used for spans up to 16 ft. These are the type of roof commonly used in ordinary dwelling-houses where the framing, usually of rough northern pine or spruce, is generally hidden from view by the ceilings. The spans usually are not great, and extra support is obtained at various points from partitions and cross walls. Where the span is large, that is, above 20 ft. without intermediate support, it is necessary to employ roofs with " principals " and " purlins," sometimes called " double rafter roofs." Principals are strong trusses of timber rigidly framed together and placed at intervals of about to ft. to support the weight of the roof covering. Purlins—stout timbers running longitudinally—are fixed on the principal rafters with intervals of about 8 ft., and on these the common rafters are fastened. Principals, or " roof trusses " as they are more often called, are framed together in various ways, and the members may be entirely of wood or reinforced by ties of iron rods or bars; the latter are called " composite trusses." The " king-post truss " may be used for spans up to 30 ft. and is constructed as shown in figs. r 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 their centres at a point where the weight. of the purlins renders strutting necessary. The members are connected by wrought-iron straps and bolts; the strap connects the king-post and tie-beam and is often fitted with a gib-and-cotter arrangement (really a pair of iron folding wedges) which allows the whole truss to be tightened up should any settlement or shrinkage occur. " Queen-post trusses " have, in place of the king-post dividing the tie-beam into two, two queen-posts supporting it at two points (fig. 3). The joints between the members are made in a similar manner to those of the king-post principal with wrought-iron straps. The purlins are two in number on each slope, one supported at the top of each " queen," the other half-way between that point and the wall-plate and resting upon the principal rafter at a point where strutted from the base of the queen-post. 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. Trusses of this type are suitable for spans up to 45 ft. In roofs of a larger span than this and 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 con- tinuing the principal Ridge rafters up to the ridge and introducing a short king-post to sustain the beam in the middle of its length. Open timber roofs of various types but principally Open of " ham- timber mer - beam " roofs. construction were used in the middle ages where stone vaulting was not em- ployed. Many of these old roofs still exist in good preservation and exhibit the great skill of the medieval carpenters who designed and erected them. Such forms are still used, chiefly for ecclesiastical buildings and the roofs over large halls. In the best periods of Gothic architecture the pitch of these roofs was made very steep, sometimes 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 being thrown across connecting the principal rafters at a point about half-way in their length, the lower portion of the principal consisting of a number of struts and braces rigidly connected in such a manner as to throw as little thrust as possible upon the walls serving as abutments. There are two kinds of • hammer beams, the arched and the bracketed; the chief examples are Westminster Hall and Middle Temple Hall (Plate I. figs. 24 and 25). 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 of its own, and gives greater height in the upper part of the building as well as being more ornamental and lighter in effect than tie-beam trusses, which have a rather heavy effect. b. Vertical section through queen-post. c. Detail of queen-post truss at head; purlin and wrought-iron straps are omitted for the sake of clearness. The Mansard roof (fig. 5) is a useful form of construction which obtains its name from Francois Mansard, a distinguished French architect who lived in the 17th century. This kind of roof has been largely used, especially in France and other European countries, as well as in America in the old colonial days. It adapts itself well to some styles of architecture, but should be very carefully applied, since it at B, C, D and E. 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, q and 8 are the sections and plan of a simple method of constructing the roof for an ordinary domestic building with plaster ceilings to thetop rooms. It is a simple construction of the couple close order 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. The trimming rafters are made an inch thicker than the others. The dragon tie is framed in connexion with the wall-plate at the hipped corners to take the thrust of the hip rafters. Steel and iron trusses in many cases follow the wood models already described. The struts and principal rafters are usually of T section, the tensional members being rods or flat bars. Flat plates and bolts or rivets are used to form the connexions between the members, and a means is provided in the tie-rod for tightening up the truss should any of the members " give " slightly under their load. Large trusses for very wide spans are specially designed for their work and may be of many different types of design. Big roofs on the tie-rod principle are now being discarded as being more liable to failure, through deterioration or defect, than those built on the girder principle in one form or another. Fig. 9 is a queen-rod roof principal for a span of 50 ft., and shows the sizes of the different members, a line diagram of the truss and large details of the joints. Fig. ro in a similar manner shows the roof at Cardiff railway station, which has a span of 43 ft. The steel roof covering the great hall at Olympia, London, is an example of a carefully designed and well-built roof which combines with strength an extremely light and elegant appearance. This is due to the fact that every member of the roof is adapted to meet the particular stresses found by calculation to affect it. By careful study of conditions the sections of steelwork used for the various members have been reduced SEC"I'I01~1 c SB . Fics. 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 Mansard roof. Iron roofs. 1jIiltliIFIIi. T A. Angle tie. B. Boarding. B.B. Barge board. C. Collar. C.J. Ceiling joist. C. R. Common rafter. D. Drip. D.P. Dragon-piece. F. Flue. G. Gutter. G.B. Gutter bearer. H.R. Hip rafter. a DL.,".?^l . V.B. Jack rafter. . King-bolt. P. Purlin. P.W. Parapet wall. P.E. Projecting eaves. R. Ridge. S. Strut. T. Trimmer. T.F. Tilting fillet. T.R. Trimming rafter. V. Valley. W.P. Wall-plate. 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 too ft. The main ribs of the roof have a clear span of 170 ft. and are placed 34 ft. apart. They are of box-girder 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 bricks 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. II 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 pro-portions. 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.). 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 Domtcal in steel as being stronger and more rigid than wood, roots. 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 6o 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 70I Roofing felt is an inexpensive fabric of animal or vegetable fibre treated Felt. with asphalt to make it capable of resisting the weather. It is largely used as a roofing material for temporary buildings. When ex-posed 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. C.. I 30 in. wide. The sheets should be laid with a lap of 2 in. at the joints and secured to the N boarding beneath by large- headed clout-nails driven in about 2 in. apart. Corrugated iron is supplied either black or galvanized. It is especially suited Con for the roofs of out- buildings and build- lrrug on ated ings of a more or less temporary character. Being to a large extent self-supporting, it requires a specially de-signed roof framework of light construction. If, as is usually the case, the sheets are laid with the corrugations running with the slope of the roof, they can be fixed directly on purlins spaced 5 ft. to io ft. apart according to the stiffness and length of the sheets. In 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 sub-structure, 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 r6 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 roof-covering 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 corru- gated iron can be safely laid upon a much Il~'LHII~~ OF SHOE_9. lighter timber framing than is necessary for a heavy covering of tiles or slates. necessary point of fixing. Sheets are usually attached to timber framework with galvanized screws, or nails with domed washers placed under their heads. Fixing to a steel frame-work is effected by means of galvanized hooked bolts clipping the purlins passed through the sheet and held tight by nuts i--32 di..- - a~r-y9r~roa.'r~~' Column, Olympia. e I pure air zinc coating of the galvanized sheets is durable for many years, but in large cities and manufacturing towns its life is short unless protected by painting. In such districts it has often been found that plain ungalvanized sheets well coated with paint will last longer than those galvanized, for the latter are attacked by corrosive influences through minute flaws in the zinc coating developed in the process of corrugation or resulting from some defect in the coating. The stock sizes of corrugated sheets vary from 5 ft. to io ft. long, and from 2 ft. to 2 ft. 9 in. wide with corrugations measuring 3 in. to 5 in. from centre to centre. For roofing purposes the sheets are supplied in several thicknesses ranging from No. 16 to No. 22 Standard Wire Gauge. No. 16 is for exceptionally strong work, No. 18 and No. 20 are used for good-class work, and No. 22 for the roofs of temporary buildings. The sheets when laid should lap about 3 in. at their sides and from 3 in. to 6 in. at the ends. Riveting is the best method of connecting the sheets, al-though galvanized bolts, which are not so satisfactory, are frequently employed. The joints should be made along the raised corrugations to lessen the risk of leakage. Holes can be punched during the erection of the roof ; their positions should first be deter-mined by placing the sheets in position and marking the 1 on the outside. Sheets corrugated in the Italian pattern have raised half-rounds every 15 in. or so, the portions between being flat. Such sheets have a very neat appearance and give a better effect in some positions than the ordinary corrugations. e loot cs rain r~ [~ z q Ii -47 z ,alit •i : •1 i .gi. .q +::.•. r ~.. 1.OM: =`• ...N~,~• N• e...a •,N H . . - N~~ Hi •44 11OXCE - • C^1+Xf I~ s • ~ X.1= Ground levelJloor L. J Zinc in sheets is a material largely used as a roof covering, and if care be taken to ensure metal of good quality, it proves itself light, ZJnc. strong and durable, as well as inexpensive. Zinc is stronger weight for weight than lead, slate, tile and glass, but weaker than copper, wrought-iron and steel, although with the exception of the two last mentioned it is not so durable when exposed to the weather. It is not liable to easy breakage as are slate, tile and glass. It is usually supplied in flat sheets, although it can also be had in the corrugated form similar to corrugated sheet-iron. When exposed, a thin coating of oxide is formed on the surface whichthe life of the roof and should always be used, as the edges of the boarding upon which it is laid are, when the latter warps, apt to cut the sheets. It also forms a cushion protecting the zinc if there is traffic across the roof. Sheet-lead forms a much heavier roof covering than zinc, but it lasts a great deal longer and more easily withstands the attacks of impure air. Lead must be laid on a close boarding, for Lead. its great ductility prevents it from spanning even the smallest spaces without bending and giving way. This characteristic of the metal, however, conduces largely to its usefulness, and enables it to be dressed and bossed into awkward corners without the necessity of jointing. The coefficient of expansion for lead is nearly as great as that for zinc and much higher than in the case of iron, and this fact requires precautions similar to those affecting zinc to be taken when laying the roofing. The manner of laying is with rolls and drips as in the case of zinc, the details of the work differing somewhat to suit the character of the material (see figs. 19, 20 and 21). Allowances must be made for expansion PLAN' AT }1- dral: diagonal section through Cathedral: choir-vaulting. sanctuary dome. protects the metal beneath from any further change, and obviates the necessity of painting. In laying the sheets, the use of solder and nails should be avoided entirely except for fixing clips and tacks which do not interfere with the free expansion and con-traction of the sheets. The reason for this is that zinc expands freely, and sheets laid with soldered seams or fixed with nails are liable to buckle and probably break away owing to movements set up by changes of temperature. The usual sizes of zinc sheets are 7 ft. or 8 ft. long by 3 ft. wide. The thickness and weights of zinc are shown in the following table, which compares the Vieille Montagne Gauge with the Old Belgian Gauge and the British Imperial Standard Wire Gauge. O.B.G. S.W.G. V.M.G. approximately. approximately. Weight per sq. ft. to 9 25 I1t oz. II lo 24 13~ „ 12 II 23 15 „ 13 I2 22 17 14 13 21 181 „ 15 14 20 2I4 ,, 16 15 19 24i ,, The best method of laying a zinc flat roof is with the aid of wood " rolls " of about 2 in. X2 in. in section, splayed at sides and spaced 2 ft. 8 in. apart and fixed to the roof boarding with zinc nails. Iron nails should not be used as this metal affects the zinc. The sheets of zinc are laid between the rolls with their sides bent up 11 in. or 2 in. against them, and held firmly in position by clips of zinc attached to the rolls. A cap of the same metal is then slipped over each roll and fastened down by tacks about 3 in. long soldered inside it so as to hook under the same clips that hold the sheet down. Drips of about 22 in. are made in the slope at intervals of 6 ft. or 7 ft.—that is, the length of a sheet—and special care must be taken at these points to keep the work waterproof. The lower sheet is bent up the face of the drip and under the projecting portion of the upper sheet, which is finished with a roll edge to turn off the water. The end of the roll has a specially folded cap which also finishes with a curved or beaded water check, and this in conjunction with the saddle piece of the roll beneath forms a weather-proof joint (figs. 17 and 18). The fall between the drips is usually made about IZ in., 17 18
End of Article: ROOFS
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