Roof, the covering of a building. Roofs are very various in form, material, and.construction. 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 purpose. 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; temples and palaces are built of stone and roofed with slabs of marble. Tiles (sheets of earthenware) have been used for coverings of roofs from very early times, and are still extensively employed in Europe. Slate, cut into rectangular 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 buildings throughout the United States. Roofs of slight inclination are frequently covered with sheet tin or sheet zinc.
A layer of heavy paper, covered with pitch or with mineral or vegetable 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 introduced, 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 international exhibitions were held in London in 1851, and in New York in 1853. - The inclination of roofs varies from 60 degrees in examples of Gothic architecture, and 25 or 30 degrees 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 conveniently obtained between the side walls. This roof often serves not only to support the exterior 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 inclined 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 building, there forming a ridge, the roof is called a "curb roof," or "gambrel roof." The Mansard consists of a nearly flat roof, extending back from the front of the building, and a second 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 common, 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 3/4 to 1 1/4 lb. per square foot, including the boarding; zinc weighs from 1 1/4 to 2 lbs.; slate from 5 to 10 lbs.; tiles from 6 to 7 lbs. as a minimum, though more usually 10 to 15 lbs.; while exceptionally heavy roofs, covered with flagging, weigh 25 lbs. or more. Snow, when freshly fallen, weighs from 5 to 12 lbs. per cubic foot; if wet, much more. Flat roofs laden with snow, in the United States, are sometimes subjected to a pressure of 50 lbs. per square foot.
In Europe an allowance of 6 lbs. is usually made for snow; in the United States 15 or 20 lbs. is thought a better allowance. The pressure of the wind against a flat vertical surface is equal to about 55 lbs. as a maximum per square foot; in a tornado, the velocity being known, the formula for its pressure is very nearly P=-005V2, or 1/200 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 a1.84cos a-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 in exposed situations, as in constructing lighthouses, or tall chimneys, or houses in unprotected locations, a large allowance should be made; in the midst of cities, in sheltered valleys, and localities in which heavy gales are unknown, comparatively little attention is given by builders to this force.
A usual allowance is 40 lbs. per square foot of vertical surface. - In roof framing, the simplest form of truss is that known as the "king-post truss," consisting of a triangular frame (fig. 1), in which the middle portion of the triangular piece, or tie beam, a a, is secured to the junction of the inclined timbers, or main rafters, b b, by a vertical post called the " king post," c. Several of these frames are placed parallel with each other to support the roof covering, and are connected by longitudinal pieces called "purlins," e e. Over the latter, and parallel with the main rafters, are the common and lighter rafters, f f, distributed at short intervals; upon these the boarding is laid, and over all the slate, tin, or other covering. The "queen-post truss" (fig. 2) consists of a horizontal tie beam, D, and main rafters, e e; the latter do not meet, but abut against a horizontal "straining beam," H, which is also held at each end, and at the proper height above the tie beam, by queen posts, B B. In Gothic architecture the members of the roof trusses are usually exposed to view from below, and are frequently carved and ornamented.
The complete roof is shown in fig. 3, in which a is the tie beam, b b are the main rafters, c is one of two short rafters abutting against the straining beam d; e and f are king and queen posts in pairs; g g are purlins supporting the common rafters, h. The tie beam is sometimes omitted, and the thrust of the rafters is received by the side walls, which are often buttressed to enable them to sustain this thrust safely. Roof trusses are often given peculiar forms to suit special designs. The large spans which are sometimes built in wood are often constructed of built arched beams. One of the largest ever designed, but which was never constructed, was planned for the great imperial riding house at Moscow in 1790; this was to have been a single built beam, in the form of an arch of 285 ft. span, with a tie beam and intermediate trusses. A somewhat similar structure has been used in the United States, first by Wernwag and Burr, and in Germany, by Wiebeking, for spans of 200 ft. and upward. In France, Philibert de Lorme first used, and Col. Erny of the army engineers afterward constructed, roof trusses in which planks are bent and built into an arched beam; trusses of this form have been designed for roofs up to 328 ft. span.
Built arches of polygonal form are frequently used; where the struts and ties of the trusses, and the points of support for the superstructure, or incumbent load, are properly placed at the angles of the polygon, this is a stronger form than the arched. The roof of Westminster hall, London, somewhat resembles Erny's construction. Domed roofs have been frequently built with similar framing; that of the halle au blé at Paris, by De Lorme, was 129 ft. in diameter, and the arched beams were a foot thick. All roofs of large span are now built with iron trusses. One of the earliest iron roofs was that of the British house of parliament; one of its trusses, composed of wrought and cast iron, is shown in fig. 4. The struts, a a, are of cast iron; the ties or suspension rods, b b, are of wrought iron; m and n are the chords or tie rods; e e and f f are stay rods. The roof of the St. Pan-eras station, of the Midland railway in London, is 690 ft. long, 240 ft. wide, and contains 1,100 tons of iron. The roof of the Grand Central station in New York is 652 ft. long and 200 ft. wide, and consists of 32 wrought-iron trusses, rising in semicircular arches 94 ft. above the rails; the covering is of iron and glass; the trusses weigh 40 tons each; the roof contains 80,000 sq. ft. of glass, and covers nearly three acres. - See Rondelet, L'Art de bâtir (Paris, 1812-'17); Ardant, Etudes sur lea charpentes à grande portée (Paris, 1837); Erny, Traité de la charpenterie (Paris, 1840); Ritter, Dach-und Brückenconstruction (Hanover, 1869); Unwin, "Iron Bridges and Roofs" (London, 1869); Tredgold, "Carpentry" (3d ed., London, 1870); Matheson, "Bridges and Roof Structures" (London, 1871); Bow, "Economics of Construction in relation to Framed Structures" (London, 1873); Stoney, "Theory of Strains in Girders" (revised ed., London, 1873); D. H. Mahan, "Civil Engineering" (revised ed., New York, 1873); and De Volson Wood, "Bridges and Roofs" (New York, 1873), and "Resistance of Materials" (revised ed., New York, 1875).
Fig. 1. - King-post Truss.
Fig. 2. - Queen-post Truss.
Fig. 3. - Complete Roof.
Fig. 4. - Roof of Parliament House.