Page:The American Cyclopædia (1879) Volume XIV.djvu/440

 420 ROOF by Guillaume Golletet (Paris, 1854), and Es- tai sur Ronsard et a reforme litteraire, by G. Scheffler, Dresden, 1874). ROOF, the covering of a building. Roofs are very various in form, material, and con- struction. The rude dwellings of barbarous tribes are usually covered with the branches of trees. In tropical climates, the long and broad leaves of palms serve well for this pur- pose. In Europe the huts and cottages of the poorer classes are frequently covered with a thick layer of skilfully arranged straw, called thatch, supported by a simple framework. The flat roofs of the dwellings in Palestine and other eastern countries are often made of strong timber and covered with earth ; tem- ples and palaces are built of stone and roofed with slabs of marble. Tiles (sheets of earth- enware) have been used for coverings of roofs from very early times, and are still extensive- ly employed in Europe. Slate, cut into rec- tangular shape and nailed upon the boarding which covers the framing of the roof, is much used on the larger classes of dwellings and on public buildings. When the inclination of the roof is sufficient, shingles flat rectangular pieces of wood, either pine or cedar, varying in thickness from three eighths of an inch at one end to an eighth or less at the other are generally used on the roofs of wooden build- ings throughout the United States. Roofs of slight inclination are frequently covered with sheet tin or sheet zinc. A layer of heavy pa- per, covered with pitch or with mineral or ve- getable tar, is becoming common as a roofing material ; this is often protected from the sun and from wear by a coating of gravel. Many other forms of mineral roofing, as asbestus and various complex preparations, have been intro- duced, and some of them appear to find favor with architects and builders, as being very cheap, readily applied, and moderately durable substitutes for the better known materials. In some cases public buildings, railway stations, and similar edifices, are covered with glass; this was done on the most extensive scale in the so-called " crystal palaces " in which inter- national exhibitions were held in London in 1851, and in New York in 1853. The inclina- tion of roofs varies from 60 degrees in exam- ples of Gothic architecture, and 25 or 30 de- grees in the steeper roofs adopted in climates where snow falls heavily, to 10 degrees in warm climates where exposed to heavy rain, and to a perfectly horizontal position in dry countries. In carpentry, the framing which supports the outside covering is called the roof ; this framing is generally of timber, but iron roofs are becoming common, and are now almost universally adopted for buildings of great width, when no support can be conve- niently obtained between the side walls. This roof often serves not only to support the ex- terior covering, but to bind the walls together and sustain the upper floors. The design and construction of such roofs, when of great span, demands the highest skill of the architect and the builder. When a roof consists of two in- clined planes meeting along a line extending over the middle of the building, this junction is called the " ridge," and the end is said to be "gabled." When several inclined planes of roofing meet, forming a pyramid or a frustum of a pyramid, the roof is called a " hip roof." When the roof rises steeply from each side wall, meeting other portions extending with less inclination to the middle line of the build- ing, there forming a ridge, the roof is called a "curb roof," or "gambrel roof." The Man- sard consists of a nearly flat roof, extending back from the front of the building, and a sec- ond part at the front rising from the wall nearly vertically, permitting the construction of windows, as in the walls of the building. This system of roof construction, introduced by the celebrated architect Mansard, has the advantage also of permitting the upper rooms to be made with vertical side walls, and of giving thus greater capacity, better shape, and superior convenience, while giving at the same time a fine architectural effect which cannot be secured with the ordinary forms of roof. When the Mansard roof is built in wood, it is a source of danger to otherwise comparatively fire-proof buildings. If built, as is now com- mon, in iron, this serious demerit is avoided. The strength of roof framing is determined by the character of the covering, the weight of snow to be expected, the pressure of the wind, and by its own location and inclination. Tin roofing weighs } to 1J lb. per square foot, in- cluding the boarding ; zinc weighs from 1J to 2 His. ; slate from 5 to 10 Ibs. ; tiles from 6 to 7 Ibs. as a minimum, though more usually 10 to 15 Ibs. ; while exceptionally heavy roofs, covered with flagging, weigh 25 Ibs. or more. Snow, when freshly fallen, weighs from 5 to 12 Ibs. per cubic foot ; if wet, much more. Flat roofs laden with snow, in the United States, are sometimes subjected to a pressure of 50 Ibs. per square foot. In Europe an allowance of 6 Ibs. is usually made for snow ; in the United States 15 or 20 Ibs. is thought a better allow- ance. The pressure of the wind against a flat vertical surface is equal to about 55 Ibs. as a ' maximum per square foot ; in a tornado, the velocity being known, the formula for its pres- sure is very nearly P='005V a, or j^ of the square of the velocity. P is the pressure in pounds on the square foot. The pressure on a cylindrical tower is half that on a turret of square section, having a thickness equal to the diameter of the tower. On any inclined surface the pressure may be estimated as, according to Hutton, P=P sin a 1 '** 008 "- 1, where a is the angle made by the surface with the direction of the wind. As it frequently acts wholly on one side of the structure, this is often a serious addition to the load borne. The location of the structure with reference to surrounding objects will modify the allowance to be taken for the pressure of the wind. When building