Tiles for roofing purposes are made from clay and baked in a kiln, like 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 strong and non-porous covering. Tiles are obtain able in many colours, some having a beautiful effect when fixed and many improving with age. They comprise tints from yellow ish 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 warp ing and variation of colour. Variation of colour is now delib erately obtained, and artistic effects are secured by sand facing, artificial rustication and by burning to metallic surfaces. 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 the direction of length to enable them to lie closely at their lower edges. Some of them have small "nibs" at the head by which they may be hung upon the battens without nails. Nail-holes are provided, and it is advisable to nail every 4th or 5th course, at least. 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 battens, 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 '01 in. long by 61 in. wide. They are usually 1 in. thick and weigh about 21 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, if laid dry and on flat slopes, but if laid upon boards and felt and bedded and pointed in mortar, pantiles may prove an excellent covering. They are laid on a different principle from plain tiles, merely over lapping 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 131 in. long by 91 in. wide, and weigh from 5 lb. to 5 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 31 in., 21 in. or '1 in., giving a gauge (and margin) of io in., I1 in. and 12 in., respectively. The side lap is generally II 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. French and Belgian tiles of the "Marseille" pattern are economical and therefore popular for housing schemes and cheap building.
Wood shingles are common roofing materials throughout the United States and Canada, and were at one time in general use in Europe. The present customary usage is to make them either of cedar or cypress, varying from 18 to 24 inches long, and from 6 inches to a foot or more wide. The butts at the thick end are from a half to three-quarters of an inch thick; the upper end tapers to not over one-eighth of an inch. They are usually laid from four and one-half to five and one-half inches to the weather. Owing to the fire risk, wood shingle roofs are gradually passing out of use, and in many localities are prohibited.
The 20th century has seen the development of many specialized roof materials, either with an asbestos or a bituminous felt base, designed for sloping roofs and intended to give an aesthetic effect similar to that of either shingles or slate. In the better and heavier types the units come separately as individual shingles; in the lighter and cheaper classes, the shingle shapes are cut upon strips.
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 practically died out of use owing to the development and cheap ening of other forms of roofing. Among these may be included thatch and wood shingles, the use of which in these days is prac tically reduced to special cases. Other roofing materials are those of recent invention, some of which may have a great future, depending upon the development, reliability and aesthetic value of the products. Sheets of asbestos-cement used as slates or tiles make a light, strong and fireproof covering. Large terra-cotta 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 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. The London County Council (General Powers) Act, 1909, re quires provision for a normal wind pressure of 28 lb. per sq.ft. if the inclination is 2o° or more. Flat roofs must be designed to carry 56 lb. per sq.ft. in addition to their own weight. From the above particulars it is easy to calculate the weight of the roofing material and the wind pressure on the roof.