Roofs

From Britannica 11th Edition (1911)

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 45° to 80° 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 outline, 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 io 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 I o 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. 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 their centres at a point where the weight of the purlins renders strutting necessary. The members are connected by wroughtiron 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 kingpost 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 queenpost. 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 60 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 ade¢ ' ° " rafters up to the ridge ids 2"x 1 0 and introducing a short king-post to sustain the beam in the middle of its length.

Open timber roofs of various types but principally of " ham mer - beam " construction were used in the middle ages where stone vault ing was not em ployed. Many of these old roofs still exist in good pre servation and exhibit the great skill of the medieval carpenters who designed and erected them. Such forms are still used, chiefly for ecclesi astical 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 60° 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.

'9' ' ?

FIG. 3. - Queen-post Roof Truss; half elevation, 38' o" span.

FIG. 4. - a. Detail of queen-post truss at b. 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.

io ' 'bcta.. 6.8. W l b- J° Roof Of Olympia. London.

,-Ti 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 E FIG. 5. - Mansard Roof Truss: detail of outline as A; other outlines 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 leador zinc-covered top in place of the pitched slateor 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 top 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. io 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 4 () 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 J.R. Jack rafter.

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 ioo 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 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. i i 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 (2ro 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 in steel as being stronger and more rigid than wood, cal 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 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 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 is supplied either black or galvanized. It is especially suited Cori for the roofs of cut- ra buildings and build- iron. ings of a more or less temporary character. Being to a large extent self-supporting, it requires a specially designed 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 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. i 5 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.

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 framework is effected by means of galvanized hooked bolts clipping the purlins passed through the sheet and held tight by nuts FIG. 13. - Westminster Cathedral: section through nave.

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.

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 10 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 goodclass 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, although 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 determined by placing the sheets in position and marking the ?1?1 II?1911? iti: ? ? 'r:`iz`? ?:? I? i"? ? ? ,:????- ?

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' ! - - .z_. ... _ ?R?I 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, strong and durable, as well as inexpensive. Zinc is Z 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 which FIG. 14. - Westminster CatheFIGS. 15 and 16. - Westminster 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 contraction 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.

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 i z 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 I Z in., the 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.

	O.B.G. S.W.G.
V.M.G.	approximately. approximately.	Weight per sq. ft.
IO	9 25	III oz.
II	10	24	132 „
12	II	23	15 „
13	12	22	.17 „
14	13	21	184 „
15	14	20	214 „
16	15	19	244,,

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 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 19 20 21 FIGS. 19, 20 and 21. - Details of Lead Flats.

and contraction, and the use of nails and solder avoided as far as possible. Contact with iron sets up corrosion in lead, and when nails are necessary they should be of copper; screws should be of. brass. Lead is supplied in rolls of 25 to 35 ft. long and 6 ft. to 7 ft. 6 in. wide. That in general use varies from one-fourteenth to oneseventh of an inch in thickness. The weights most suitable for employment in roofing work are 7 or 8 lb per square foot for flats and gutters, 6 lb for ridges and hips, and 5 lb for flashings.

As a roof covering copper is lighter, stronger and more durable than either zinc or lead. It expands and contracts much less than these metals, and although not so strong as wrought-iron Copper, and steel it is much more durable. From a structural point of view these qualities enable it to be classed as the best available metal for roof covering, although its heat-conducting properties require it to be well insulated by layers of felt and other non-conducting material placed beneath the metal. On exposure to the air copper develops a feature of great beauty in the coating of green carbonate which forms upon its surface protecting it from further decomposition. Perhaps the chief disadvantage in the use of copper lies in its first cost, but against this must be set the almost imperishable nature of the metal and the fact that by reason of its light weight less substantial framework is required for its support. Copper roofing should be laid in a similar manner to zinc, with wood rolls at intervals of about 2 ft. 4 in. It is, however, often laid with welted seams. The general stock sizes of sheets are from 4 ft. to 5 ft. 3 in. long and 2 ft. to 3 ft. 6 in. wide. The thickness almost invariably used is known as 24 S.W.G. and weighs 16 oz. per square foot. Thinner metal would suffice, but owing to the increased cost of rolling very little would be gained by adopting the thinner gauges.

In the United States of America " tin " roofs are quite commonly used. Sheets of wrought-iron coated either with tin or zinc are used of a size usually 14 in. by 20 in., though they may i be had double this size. Preparation for laying is made by fixing an insulating foundation of somewhat stout paper.

or felt; this must be dry, else it is apt to spoil the impermeable covering laid upon it by causing it to rust. Junctions between the sheets are made by welted seams in which the four edges of the sheets are turned over so as to lock together, thus forming one large sheet of tin covering the roof. In high-class work of a permanent nature the seams in addition are soldered, rosin only being used as a flux. Each sheet also is secured to the roof with two or three tin cleats. The life of such roofs may be practically doubled by the application of a good coat of paint, which, however, adds considerably to the cost.

Slate is a strong and very impermeable material, and these qualities and the fact that it is easily split into thin plates suitable for laying, as well as its low cost, cause it to be by far the most generally used of all materials for roof covering.

Some of the best known varieties of slates, classed according to their colour, are as follows: Blue.. North Wales (Penrhyn, Festiniog, Dinorwic, &c.), France, Norway, Germany.

but where necessary it may be less, the least permissible fall being about I in 80. Felt laid beneath zinc has the effect of lengthening said L irs,ll.2,:dwr_ Hollow Lend roll, North Wales (Bangor, Penrhyn, Dinorwic), Newfoundland, Germany.

South Wales (Precelly), Cumberland, Westmorland, Lancashire, Ireland, Newfoundland, Norway, United States and Germany.

Slates are cut to many different sizes varying in length from To in. to 36 in. and in width from 5 in. to 24 in. There are perhaps thirty or more recognized sizes, each distinguished by a different name. In common practice those generally used are " large ladies," 16 in. by 8 in.; " countesses," 20 in. by io in.; and " duchesses," 24 in. by 12 in. Generally speaking, the rule governing the use of the different sorts is that the steeper the pitch the smaller the slate, and vice versa. Buildings in very exposed positions naturally require steeply pitched roofs.

Some of the technical terms used by the slater are as follows: - Bed, the under surface of a slab when laid.

Gauge, the distance between the lines of nailing. This depends on the length of the slate and equals half the length of the slate after the lap plus an inch for the nail-hole has been deducted. This is for slates nailed near the top edge; for those fixed near the middle the gauge would be half an inch more, as no allowance for nail-holes is required.

Margin, the width of the exposed portion of each course which equals the width apart of the nailing.

Lap, the lap of the tail of one course of slates over the head of the second course below it. The lap is made from 22 in. to 4 in. (usually 3 in.), and for this distance there are three thicknesses of slate, namely, the tail of the top course, the middle of the next and the head of the third course.

Slates may be fixed by nailing at the head (see fig. 22) or at about the middle. The latter method is the stronger, as the levering effect of the wind cannot attain so great a strength. There is a small economy effected by centre nailing, as the margin is slightly larger and fewer slates are required to cover a given space; longer nails, however, are required, for as slates are laid at an angle with the pitch of the roof their centres cannot be made to approach so near FIG. 22. - Detail of a Slated Roof.

to the slating battens or boarding as the head, which lies close on the surface to which it is fixed. Another point worth noticing is that the nail-holes in the centre nailed slating are only covered by 3 in. of the tail (the amount of the " lap ") of the course of slates above, and rain is very liable to be forced under by the wind and cause the wood battens or other woodwork to rot. Head-nailed slates, on the other hand, have their holes covered by two layers of slate, and are removed from exposure by the length of the gauge plus the lap, which in the case of " countess " slating equals it in.

" Open slating " is an economical method of laying slates that is often adopted for the roofs of sheds and temporary buildings. The slates in the same course are not laid edge to edge as in close slating, but at a distance of two or more inches apart. This forms a roof covering light in weight and inexpensive, which, although not strictly weather-proof, is sufficiently so for the buildings upon which it is used.

Slates are laid upon open battens fixed upon the rafters or upon close boarding or upon battens fixed upon boarding. The battens are in. or I in. thick and 12 in. to 3 in. wide, and are spaced to suit the gauge of the slates. When close boarding is used it is often covered with inodorous asphalted felt. While taking these precautions to make the roof sound and tight it should be borne in mind that slate is liable to decay if not ventilated, and to effect this the battens are sometimes fixed vertically, ridge ventilators introduced and air inlets arranged at the eaves. The bed of slates laid without provision for the admission of air will be found on removal after some time have rotted so as to scale off and easily crumble into powder.

The nails used in slating are a very important item, and the durability of the work depends to a large extent upon them. They should have large flat heads. The most satisfactory are those made of a composition of copper and zinc, but others of copper, zinc, galvanized iron and plain iron are used. Those of copper are most durable, but are soft and expensive. Zinc nails are soft and not very durable; they will last about twenty years. Iron nails even if galvanized are objectionable in permanent work, though they may be used for temporary roofs. When the plain-iron nails are employed they should be heated and plunged in boiled linseed oil. The pitch of a roof intended for slating should not incline less than 25° with the horizontal, while 30° is a safer angle to adopt.

Tiles for roofing purposes are made from clay and burned in a manner similar to bricks. The clay from which they are made is, however, of a specially tenacious nature and prepared with great care so as to obtain a result as strong and as w nearly non-porous as possible. Tiles are obtainable in many different colours, and some of these have a very beautiful effect when fixed and improve with age. They comprise a large number of tints from yellowish red, red and brown to dark blue. As with bricks the quality depends to a large extent upon the burning; underburnt tiles are weak and porous, liable to early decay, while overburning, though improving the tiles as regards durability, will cause them to warp and will spoil colour. The usual shape is the " plain tile," but they are made in various other shapes with a view both to easier fixing and lighter weight, and to ornamental effect. There are also several patented forms on the market for which the makers claim special advantages. The ordinary tiles are slightly curved in shape to enable them to lie close one upon the other. Some of them have small " nibs " moulded on at the head by which they may be hung upon the battens and nailing avoided (see fig. 23). Nail-holes are provided, and upon steep slopes it is ad visable to make use of them. Others are made without the nibs, and are fixed either by nailing to the battens or boarding or hung by means of oaken pegs wedged in the holes to the bat tens, the pegs in the latter case acting in the same way as the above-mentioned nibs. Plain tiles are of rec tangular form, the standard dimensions are 102 in. long by 62 in. wide. They are usually in. thick and weigh about 22 lb each.

There are many forms of ornamental tiles, which are plain tiles having their tails cut to various shapes instead of moulded square. A number of patented forms of tiles also are on the market, some of which possess considerable merit. Pantiles are suitable for temporary and inferior buildings such as sheds and outhouses. They are laid on a different principle from plain tiles, merely overlapping each other at the edges, and this necessitates bedding in mortar and pointing inside and sometimes outside with mortar or cement. This pointing plays an important part in keeping the interior of the building free from the penetration of wind and water. Pantiles are generally made to measure 132 in. long by in. wide, and weigh from 5 lb to lb each. Moulded on at the head of each tile is a small projecting nib which serves for the purpose of hanging the tile to the lath or batten. They are laid with a lap of 32 in., 22 in. or 2 in., giving a gauge (and margin) of 10 in., II in. and 12 in., respectively. The side lap is generally 12 in., leaving a width of 8 in. exposed face. There are many other forms based upon the shape of the pantile, some of which are patented and claim to have advantages which the original form does not possess. Among such are " corrugated tiles," of the ordinary shape or with angular flutes, and also the Italian pattern " double roll tiles," " Foster's lock-wing tiles." Poole's bonding roll tiles are a development of the Italian pattern tile.

Glass as a roof covering and the different methods of fixing it are dealt with in the article Glazing.

There are many other materials used for roof covering besides those already described, many of them of considerable value. Some have in the past enjoyed considerable vogue, but have m practically died out of use owing to the development and cheapening of other forms of roofing. Among these may be included thatch and wood shingles, the use of which in these days is practically reduced to special cases. Other little used roofing materials are those of recent invention, some of which perhaps stq?

have a great future before them. Plates of asbestos used as slates or tiles make a light, strong and fireproof covering. Large terracotta tiles or slabs are much used in the United States of America. A good form of flat roof is that in which concrete is used as a foundation for a waterproof layer of asphalt, laid to slight falls to allow the water to run off easily. This is the usual method adopted when a roof garden is required. Shingles or thatch look extremely well on a roof, but their use is debarred in a great many districts owing to the danger of fire. Galvanized iron tiles, zinc tiles and copper tiles may be employed on small areas with good effect. " Willesden paper," often used as an insulating layer beneath slates and tiles, is also at times used as a roof covering. It is cardboard chemically treated to render it tough, waterproof and fire-resisting.

The weights of some of the various materials used in the construction and covering of roofs are given in the following table. The Weight. weights which are approximate are for a square foot of roofing. The roof trusses are taken to be spaced i o ft. apart and include the necessary purlins.

Zinc 12 to 16 gauge laid complete including rolls Copper 25 to 19 gauge laid complete including rolls Lead weighing 6 lb per square foot laid complete including rolls .

Wind pressure is usually calculated at 22 to 25 lb on a roof with pitch of 30°, and 27 to 30 lb on a roof of 45° pitch.

From these particulars it is easy to calculate the weight of a square (ioo superficial ft.) of roofing material, this being the usual standard of measurement for many roofing materials.

The London Building Act of 1894 and its amendments set forth with regard to roofs erected in the London district that every structure on a roof is to be covered with slate, tile, metal Regis or other incombustible material, except wooden cornices and barge boards to dormers not exceeding 12 in. in depth, and doors and windows and their frames. Every dwelling-house or factory above 30 ft. in height and having a parapet must have means of access to the roof. The pitch of the roofs of warehouse buildings must not exceed 47°, and those of other buildings must not exceed 75°, but towers, turrets and spires are excepted. In domestic buildings not more than two storeys are to be formed in the roof, and if the floor is more than 60 ft. above the street level fireproof materials must be used throughout and a sufficient means of escape provided. The building by-laws of the municipality of Johannesburg contain several clauses affecting the designing of roofs and their method of construction. In the designing of buildings roof-slopes must be within a line drawn and produced from the ground level at the opposite side of the street to the top of the eaves, gutter or parapet. No roof in the municipal fire limit may be constructed of thatch, reed or other inflammable material. Without the fire area they may be so constructed if the building stands at least 20 ft. from the boundary of its site. Roofs having a pitch of less than 221° must be constructed to bear safely a load of at least 28 lb per square foot of surface. Roofs of steeper pitch must be able to support a live load of 21 lb per square foot. The framing of Mansard or other roofs of more than 60° pitch on a building exceeding 45 ft. high must be constructed of approved fireproof material at least 2 in. thick. No roofs except those of towers, turrets or spires shall exceed 70° pitch for a Mansard or 60° for an ordinary roof. Every fireproof roof, in addition to a door or scuttle for access from below, must have a skylight or skylights with metallic framing, having an area equal to at least one-sixtieth the area of the roof. Skylights placed over rooms or areas to which the public have access must be protected by wire netting below or be glazed with wire-wove glass.

The Building and Health Laws and Regulations and Amendments of 1905 affecting the city of New York are based, so far as the construction of roofs goes, upon the same lines as those of London, the principal exceptions being that they give very full working details, under part 24, as to the strengths of materials required to be used and the wind pressure to be provided against. In part 17 they provide that where a building exceeds three storeys or 40 ft. in height and the roof has a pitch of over 60°, it shall be constructed of iron rafters and be lathed with iron or steel on the inside and plastered or be filled in with fireproof material not less than 3 in. thick and covered with metal, slate or tile. The provision as to access to roof and fire escapes therefrom adopted by the London County Council in 1907 under the London Building Act Amendment Act 1905 were in operation in New York in 1899. LITERATURE. - The principal reference books on this subject are the following: - Thomas Tredgold, Elementary Principles of Carpentry; J. Newland, Carpenter and Joiner's Assistant; G. L. Sutcliffe, The Modern Carpenter, Joiner and Cabinet Maker; J. Griffiths, Trusses in Wood and Iron; F. Bond, Gothic Architecture; J. Gwilt, Encyclopaedia of Architecture; F. E. Kidder, Trussed Roofs and Roof Trusses; J. Brandon, Analysis of Gothic Architecture; A. Pugin, Ornamental Gables; M. Emy, L'Art de la charpenterie; Viollet le Duc, Dictionnaire; J. K. Colling, Details of Gothic Architecture. (J. BT.)