Page:EB1911 - Volume 23.djvu/741

Rh 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 weights which are approximate are for a square foot of

roofing. The roof trusses are taken to be spaced 10 ft. apart and include the necessary purlins.

King-post wood truss 20 ft. to 30 ft. span. . 2 to 2% lb Queen-post,, 30 ft. to 50 ft. .. . 3 to 3%, , Wood rafters ...... . 2%, ,

Ceiling joists and ceiling. . . II, ,

%-in. boarding for roof covering. . 2%, ,

1-in., ,,, .... . 3%, ,

1%-in., ,,, ...., 4%, ,

2%-in. XI in. slate battens for 8%-in. gauge. 1, ,, Felt ......... . =}, ,

Thatch ........ 6%, ,

Slates (ordinary laid with 3-in. lap). 9,, Tiles, plain Hat ........ 18, ,

Pantiles .......... II, ,

Zinc 12 to 16 gauge laid complete including rolls 1% to 1%, , Copper 25 to 19 gauge laid complete including rolls 1% to 2% Lead weighing 6 Ib per square foot laid complete including rolls ........ 6%

Corrugated iron 20 S.W. gauge ..... 2

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 (IO0 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 wit slate, tile, metal or other incombustible material, except wooden cornices ms' 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 exce ted. 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 22%° 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. N0 roofs exce t those of towers, turrets or spires shall exceed 70° pitch for a R/Iansard 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 bein that they give very full working details, under part 2, as to the strengths of materials required to be used and the wind pressure to be provided against. In part I7 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 rovision 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.

.—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.

ROOK (O.E. Hróc, Icel. Hrókr, Swed. Råka, Du. Roek, Gael. Rocas), the Corvus frugilegus of ornithology, and throughout a great part of Europe the commonest and best-known of the crow-tribe, belonging to the Passerine family Corvidae. Besides its pre-eminently gregarious habits, which did not escape the notice of Virgil (Georg. i. 382) and are so unlike those of nearly every other member of the Corvidae, the rook is at once distinguished from the rest by commonly losing at an early age the feathers from its face, leaving a bare, scabrous and greyish-white skin that is sufficiently visible at some distance. In the comparatively rare cases in which these feathers persist, the rook may be readily known from the black form of crow (q.v.) by the rich purple gloss of its black plumage, especially on the head and neck, the feathers of which are soft and not pointed. In a general way the appearance and manners of the rook are well known, and particularly its habit of forming communities in the breeding-season, which it possesses in a measure beyond that of any other land bird of the northern hemisphere. Yet each of these communities, or rookeries, seems to have some custom intrinsically its own. In a general way the least-known parts of the rook's mode of life are facts relating to its migration and geographical distribution. Though the great majority of rooks in Britain are sedentary or only change their abode to a very limited extent, it is now certain that a very considerable number arrive in or towards autumn, not necessarily to abide, but merely to pass onward, like most other kinds of birds, to winter farther southwards; and, at the same season or even a little earlier, it cannot be doubted that a large proportion of the young of the year migrate in the same direction. As a species the rook on the European continent only resides during the whole year throughout the middle tract of its ordinary range. Farther to the northward, as in Sweden and northern Russia, it is a regular summer immigrant, while farther to the southward, as in southern France, Spain and most parts of Italy, it is, on the contrary, a regular winter-immigrant. The same is found to be the case in Asia, where it extends eastward as far as the upper Irtish and the Ob. It breeds throughout Turkestan, in the cold weather visiting Afghanistan, Cashmere and the Punjab, and Sir Oliver St John found a rookery of considerable size at Casbin in Persia. In Palestine and in lower Egypt it is only a. winter-visitant, and H. B. Tristram noticed that it congregates in great numbers about the mosque of Omar in Jerusalem. The same writer (Proc. Zool. Soc., 1864, p. 444; Ibis, 1866, pp. 68, 69) considered the Palestine rook entitled to specific distinction as Corvus Agricola. The rook of China has also been described as a distinct species, C. pastinator (Proc. Zool. Soc., 1845, p. 1) from having the feathers of its face only partially deciduous.

 ROOKE, SIR GEORGE (1650–1709), English naval commander, was born near Canterbury in 1650. Entering the navy as a volunteer, he served in the Dutch Wars and became post-captain in 1673. After the Revolution of 1688, he commanded