TABLE LXXXIV.-WEIGHTS PER SQUARE OF ZINC Covering.*

244. Lead Covering.-Lead sheets are laid upon rolls somewhat in the same manner as zinc sheets, but with close boarding underneath. Lead, as used for roofs, is first cast into small sheets, and then rolled out to the size and thickness required. The different thicknesses of the sheets are known by their weight per square foot; thus we have 4 lbs. lead, 6 lbs. lead, 8 lbs. lead, and so on, a square foot weighing 4 lbs., 6 lbs., and 8 lbs. respectively. A square foot of lead inch in thickness weighs about 7 lbs. The strength of lead sheets usual for roof coverings is 6 lbs. and 8 lbs., and for flashings 5 lbs. and 6 lbs. Lead covering is more expensive than zinc, but it lasts much longer.

245. Felt Covering.-Felt is a cheap form of roof-covering, and may be easily renewed from time to time, it being laid on boarding. Each roll of felt for roofing purposes contains about 25 yards, 32 inches wide, and about inch in thickness. It is made from hair, wool, or vegetable fibre by compressing and saturating these materials with asphalt, bitumen, or ordinary tar. Good felt is impervious to rain or snow, and will last a considerable time under most conditions of climate. For good permanent roofs it is only used as an inner lining; the outer covering being slates, corrugated iron, or zinc.

246. Glass. Nearly all roofs of large structures contain glass as part of their covering, and in some cases it forms the entire covering. The glass usually runs in widths longitudinally with the roof, and joins on at its sides to the other covering.

The old-fashioned, and perhaps the best, method of glazing is with timber sash-bars and putty. The sash-bars, which may be made of wood or iron, are usually placed from 12 to 20 inches apart, and supported at intervals of from 6 to 8 feet. It is easier to make the covering water-tight by using wood sash-bars; those made of iron do not expand and contract equally with the glass, and consequently the putty is liable to get cracked, thereby * Matheson-Works in Iron, p. 212.

allowing the water to percolate through. When made of wrought iron, the sash-bars may be ordinary bars of T-section from 1 to 2 inches deep and from 1 to 2 inches across the flange. A very useful form is that shown in section in fig. 204; the upper flange forms a good protection for the putty. Cast iron is sometimes used either in single bars or in the form of a frame.

Fig. 204.

Several varieties of glass are used for glazing purposes. When a good deal of light is required it should be clear and transparent; but for ordinary roofs, such as those that cover warehouses and railway stations, a much coarser kind is employed.

The width of glass sheets for this purpose varies between 12 and 20 inches, and they are made in lengths up to 6 feet, the thickness varying between and inch. What is known as "patent rolled rough plate" is most suitable for roofs.

The price of glass varies with the thickness. Panes of ordinary size inch thick cost about threepence per square foot, and those inch about fivepence; the fluted varieties being about three halfpence per foot more. Glazing costs from a penny to twopence per square foot, depending on the height from the ground and other circumstances.

247. Ventilation of Buildings.-The usual method of ventilating a building through the roof is by means of a lantern or similar contrivance. A lantern may run the whole length of the roof or extend only over a portion of it. It is formed by raising the covering at the ridge for a certain width; a space is thus created at each side of the ridge, which allows the egress and ingress of air. In order to prevent rain or snow being driven through the ventilating openings, louvre blades are fixed to upright standards, called louvre standards; these blades, which are usually made of wood, are arranged at an angle, one lapping over another, so that, while allowing a free passage for air, they prevent rain being blown through. With fixed louvre blades it is impossible to prevent snow being blown in; this difficulty may be got over by arranging the blades so that they may revolve on a horizontal axis, they can thus be opened or closed at will. The blades are sometimes made of iron, zinc, glass, &c., as well as wood. In roofs of large span and where a great deal of ventilation is necessary, such as in railway stations, it is advisable to have similar ventilating openings down the sides of the roof as well as at the ridge.

248. Timber Roofs.-There is not the same objection to timber roofs that there is to timber bridges. They are better protected from the weather, and are consequently more durable, and for small spans they are not likely to be superseded by iron roofs.

The following tables* give the size of the scantlings generally used for the different spans named; the covering being slates, and the timber Baltic pine, or other equally strong:

TABLE LXXXV.-SCANTLINGS OF TIMBER FOR DIFFERENT SPANS FROM 20 TO 30 FEET; THE TRUSSES BEING 10 FEET APART.

The form of truss is shown in skeleton outline in fig. 168.

TABLE LXXXVI.-SCANTLINGS FOR ROOFS FROM 30 TO 46 FEET SPAN. TRUSSES 10 FEET APART.

Form of truss as shown in skeleton outline in fig. 175.

TABLE LXXXVII.-SCANTLINGS FOR ROOFS FROM 46 TO 60 FEET SPAN. TRUSSES 10 FEET APART.

These trusses have a horizontal straining-beam between apex and tie-beam.

249. Causes which Influence the Deflection of Girders. When a girder is loaded it becomes deflected, or cambered in a downward direction. If the limit of elasticity of the metal be not exceeded, the girder will practically regain its original form when the load is removed; when this limit is exceeded the girder becomes permanently deflected, or takes what is known as a permanent

set.

It is possible to calculate beforehand what will be the deflection of a girder with a given load.

The amount of the deflection depends mainly on the following:

1. The length and depth of the girder;

2. The stress per unit of area on the flanges.

The deflection arises from the top flange being compressed or shortened, and the bottom flange extended. In scientifically

constructed girders the sectional areas of the flanges at different sections are in proportion to the stresses at these sections, so that the unit stress on each flange is uniform throughout its length. The amount of deflection is practically independent of any change of form which may take place in the web, and is not affected by the kind of web; a continuous plate and a lattice web giving similar results under similar conditions of loading. When a girder is loaded, the unit flange-stress may be determined; and knowing this, and also the modulus of elasticity of the material, the amount of compression in the top flange and of extension in the bottom flange may be calculated. Having determined these changes of length, the deflection may be found by means of a simple equation, which we will now investigate.

Fig. 205.

250. Rules for finding the amount of Deflection. Fig. 205 represents a girder supported at its extremities and loaded; when the unit stress is constant throughout the entire length of each flange, the curve of deflection will be the arc of a circle.

Let O represent the centre of the circle,
afb-l-length of top flange,
cgd=4=length of bottom flange,
ef=D= the central deflection,
fg=d=depth of the girder,

O a=r= radius of curvature of the top flange.

Since in loaded girders the deflection is small compared with the radius, O e may be taken equal to r; also a eb is nearly equal to l. Making these substitutions, we get (Euc., Book III., prop. 35)

Substituting this value of r in equation (1) we get

(2).

(3).