By far the most important roofing material, and the one in most general use at the present time, is slate. Slate is an argillaceous sedimentary rock which, after being deposited as clay at a very early geological epoch, has been subjected to enormous mechanical pressure, the result of which has been that the beds have been squeezed together and the material rendered very dense and compact, while the original lines of stratification have been almost obliterated, and the particles have been rearranged in fresh planes perpendicular to the direction in which the pressure was exerted; along these planes the rock splits with great ease. The strength of the material has thus no connection with its natural bed, even when the latter can be discovered. This splitting or fissilo property makes slate eminently useful as a building material, as, notwithstanding the fact that it is one of the hardest and densest of rocks, it can be obtained in such thin sheets that the weight of a superficial foot is very small indeed, and consequently, when used for covering roofs, a heavy supporting framework is not required.

Slate absorbs a scarcely perceptible quantity of water, and it is very hard and close-grained and smooth on surface; it can be laid safely at as low a pitch as 22 1/2°. In consequence of this, the general introduction of slate as a roofing material has had a prejudicial effect upon the architectural character of buildings. The bold, high-pitched, lichen-covered roofs of the middle ages - which, with their warm tints, form so picturesque a feature of many an old-fashioned English country town - have given place to the flat, dull, slated roofs.

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The best roofing slate is obtained from North Wales, chiefly in the neighbourhood of Llanberis, where there are numerous quarries, those at Penrhyn being the largest; the slates from this district generally go by the name of Bangor. At Ffestiniog and in the neighbourhood there are also numerous quarries, the slates from which are generally designated Portmadoc slates, as they are shipped from this port. The colour varies from green to purple and black, and a good effect can be obtained in buildings by using alternate bands of different colours in the roof. Good slates are also obtained from Cornwall, where the quarries have been worked for a long period; and from the Lake district, those which coma from the neighbourhood of Maryport being of a bright sea-green colour. As a general rule, the finer the grain of the slate and the cleaner and smoother the surface with which it splits, the better it will be. When the surface is coarse and uneven, it is probable that the slates have been obtained from a bed where the pure rock was in close proximity to, and partly mixed with, some foreign substance, such as sandstone; and such slates would be likely to absorb more water than the finegrained varieties.

The large demand for roofing slates has led to the opening of many new quarries during the last few years, the slates from which are of varying degrees of excellence. Non-absorbtion of water is, of course, the most valuable characteristic; an easy test of this can be applied by carefully weighing one or two specimens when dry, and then steeping them in water for a few hours and weighing them again, when the difference in weight will of course represent the quantity of water absorbed. The light-blue coloured slates are generally superior to the blue-black varieties.

The chemical analysis of an average specimen of slate may be taken as

Silica.................. 54.75 per cent.

Alumina ................ 22.90 „

Iron oxide................ 9.66 „

Magnesia ................ 1 .90 „

Potash and soda.............. 5. 14 „

Water.................. 5.45 „

Its specific gravity is 2 • 8.

Pooling slates are sorted into various sizes, which are sold under the following names: -

Queens, the size of which is 36 in. by 24 in. Imperials „ „ 30 in. by 24 in.

Princesses „ „ 24 14 in.

Duchesses „ „ 24 in. by 12 in.

Countesses, 6ize of which is 20 in. by 10 in. Viscountesses „ „ 18 in. by 9 in.

Ladies „ „ 16 in. by 8 in.

Doubles „ „ 13 in. by 6 in.

Of these the Duchess and Countess slates are the most extensively used.

Slates should always be laid with a certain lap, that is, each course should overhang the next but one below it to a certain extent, and this should not be less than 2 in.

Thus there will be a certain width of slate in each course exposed, and this is called the gauge, its width diminishing as the lap increases. The gauge for any kind of slating is found by deducting the lap from the length of the slate, and then halving the remainder : thus, if Countess slates are laid with a 3-in. lap, the gauge will be 20-3 = 8 1/2 in.

Each course of slates "breaks joint" with the one below it. The average weight of ordinary slating may be taken as 7 cwt. per square of 100 superficial ft. The valleys of slated roofs are generally laid with lead turned up under the slates, and the hip rafters are either covered with lead - which is the best plan - or with thick saddle-back slates finished at top with an ornamental roll. Some years ago a system was introduced called patent slating. This consisted in laying large thick slates so that they butted against one another in a vertical line, the joints coming exactly over a rafter to which the slates were firmly screwed. The successive courses overlapped one another a few inches, and when the roof was covered, thin fillets of slate about 3 in. wide were bedded in putty over the vertical joints, tightly screwed down to the rafters, and pointed carefully all round. Slating could be laid in this manner to a very low pitch, and yet remain water-tight for a considerable time, but it was found that a slight settlement of the roof was sufficient to injure it, and as the putty also soon perished, the system was abandoned, and it is now never adopted.

It gave, however, a distinctly ornamental appearance to a roof.

The usual method of laying slates is illustrated in Fig. 1354*, which represents 3 courses, the distance between a and b being the lap, and that between b and c the gauge. Slates are sometimes laid on close boarding nailed to the rafters, in which case it is usual to put a layer of sheathing felt on the boarding before laying the slates; but they oftener rest on battens or slate laths, as in Fig. 1355, placed at a distance apart equal to the gauge to be given to the slates. It is usual to fasten slates by 1 or 2 zinc or copper nails, according to size, passed through holes punched near the centro and top of the slates. Slating is measured by the square of 100 ft. super, 12 in. extra being allowed for eaves, hips, valleys, and irregular angles; circular slating is 1/3 extra. A good slate should emit a char ringing note when struck, and feel hard and rough to the touch. The "back" of a slate is its upper surface; the "bed," its under side; the "head," its upper edge; the " tail," its lower edge.

Fig. 1356, is a slater's pick hammer; Fig. 1357, a lath hammer; Fig. 1358, a cutting iron for reducing the dimensions of slates; Fig. 1359, a slater's axe having a cutting edge and a pick at the back.

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The effect of wind pressure on slated and tiled roofs is important. The cause of slates being blown off is not quite so simple as might at first be imagined. When loose ridge tiles or broken slates occur, these are blown off as a matter of course, but it frequently happens that roofs which were perfectly sound are seriously damaged. As the direction in which the force of the wind acts is horizontal, there would seem to be no tendency to rip up or break off materials lying so closely upon one another as slates. The true explanation is probably this : any exceptionally strong gust of wind is succeeded by a momentary vacuum, and as under ordinary circumstances the atmospheric pressure inside and outside a roof is equal, it follows that during the brief continuance of the vacuum outside the pressure inside is considerably in excess, and the weakest points of the roof covering will have a tendency to be pushed outwards. Now in roofs covered with battens, to which the top only of the slates are nailed, the lower portions of the slates would be much less able to resist any such outward pressure, and would be forced upwards, and if at such a moment another gust of wind were to occur it would find its way under the slate and break it.

If the rafters were covered with close boarding instead of battens this would offer a far more even resistance, and for this reason close boarded roofs are much less liable to damage from wind. The whole question of the action of wind upon sloping roofs has not received the attention from professional men that its importance demands.

An architect says he has been in the habit for many years of bedding his roofing slates in hydraulic cement, instead of having them nailed on dry in the usual way, which leaves them subject to be rattled by the wind, and to be broken by any accidental pressure. The cement soon sets and hardens, so that the roof becomes like a solid wall. The extra cost is 10 or 15 per cent., and he thinks it good economy, considering only its permanency, and the saving in repairs; but, besides this, it affords great safety against fire, for slate laid in the usual way will not protect the wood underneath from the heat of a fire at a short distance.