Into the details of construction of ordinary pitched roofs I do not propose to enter. One or two points, however, may be briefly alluded to. Care must be taken that the ends of roof-timbers do not enter into smoke-flues; neglect of this may result in the destruction of the house by fire. The ribs (or purlins) and other woodwork must be amply strong enough; sagged roof-timbers mean broken slates and tiles, leaks, and insecurity. The space between the top of the wall and the upper sidc of the common rafters must be thoroughly filled with bricks and mortar-, before the roof-boarding is laid or the slating begun; this is known as beam-filling, and is often left by the bricklayer to be done after the completion of the roof, - which means that it will be only half-done.

Pitched roofs are usually covered with slates or tiles, but sometimes thin flag-atones (known locally as "gray slates" to distinguish them from Welsh elates, which are known as "blue"), thatch, copper, zinc, lead, wire-wove roofing, and sheet-iron (often corrugated and galvanized), are used. Temporary buildings may be covered with tarred felt on boards. Slates and tiles, however, are the chief roofing-materials, and to these we must confine our attention.

Slates are either stone slates or clay slates.

Stone slates are thin slabs of stone, which have been split along the planes of bedding. Many of these slates are very "beddy", and may be cloven into sheets less than an eighth of an inch in thickness. Such .slates should be rejected, as frost may split them. Stone slates are somewhat absorbent, and water does not pass very freely from them; roofs covered with them should therefore be of quicker pitch, say not less than 1 in l¾, or 2/7 of the span. These slates vary connderably in size and thickness, and must be carefully sorted into groups, each group containing slates of approximately uniform height and thickness. On account of their irregularity a greater lap is necessary than with clay slates; a 4-inch lap is commonly specified. The slates are laid on battens, being hung to them by one or two wood pegs driven through holes in the head of each slate, and the joints beneath are afterwards pointed with good hair-mortar, an operation known as pointing or torching.

Stone slates are heavy, somewhat absorbent but durable, and are warmer than clay slates, but they are not much used nowadays except for farm-buildings, and for repairs and additions to existing houses.

There are two principal districts in which clay slates are produced in these islands, namely, North Wales and the English Lake District; but other quarries are worked in Leicestershire, Devonshire, and Cornwall in England, Perthshire and Argyleshire in Scotland, and in Wicklow and Kilkenny in Ireland. Slate is one of the densest and strongest rocks, weighing from 170 to l80 lbs. per cubic foot, and having a crushing strength of 9 tons and upwards per square inch. Good slate is clean, non-absorbent, and practically unaffected by atmospheric agencies.

Welsh slates are of two geological formations, the Cainbrian and the lower Silurian. The former gives the well-known and excellent Penrhyn and Velinheli slates, and other slates shipped at Bangor and Carnarvon, while from the Lower Silurian formation are obtained the Ffestiniog slates (shipped at portmadoc), the Llangollen slates, and others. The colour of Welsh slates varies considerably - red and purple Bangor, gray and purple Penrvhn and Velinheli, green, blue, dark-blue, and even black. Some of the black slates are of wretched quality, and will break after a few years' exposure; many roofs covered with them have had to be stripped and covered with new slates. Perhaps it would be too broad a generalization to say that all dark or black slates are bad, but they are certainly not above suspicion. A good slate will ring clearly when struck, will not be friable at the edges or holes, will not splinter easily, will be free from dark veins, non-absorbent, straight, and of uniform thickness.

Welsh slates are almost invariably sold in regular sizes, the extremes being 36 inches x 24 inches and 10 inches x 5 inches. The smallest slates undoubtedly make the prettiest roofs, as anyone who has observed the slated roofs and spires of the Belgian Ardennes will testify; hut they are seldom used in this country. except for turrets, dormers, and summer-houses. The size most commonly used is 20 inches x 10 inches.

There are generally two or three qualities of Welsh slates: firsts are thin, straight, and of uniform thickness; seconds are thicker and less uniform; thirds are still more irregular. The smooth regularity of a roof covered With first Welsh slates is far from pleasing; such slates, moreover, are so thin that they are easily broken by anyone walking over them. Seconds slates, from which the most irregular have been discarded, make stronger and more pleasing work.

The slate quarries of the English Lake District have during recent years largely increased their output These slates - which are commonly known as Westmoreland slates, although they may be quarried in that county or in Lancashire or Cumberland - are stronger and thicker than ordinary Welsh slates, more durable and of better appearance. Some of them, as the Elterwater, Tilberthwaite, Coniston, and Langdale slates, are of various shades of green, while others, such as those obtained near Ulverston, are blue. They are more expensive than Welsh slates, but are worth the extra cost; the green slates are much in favour now for the roofs of buildings constructed of red brick or terra-cotta, the contrast of colour being decidedly pleasing. For stone buildings also the green roof is welcome; a stone building with a blue roof has a cold, dull appearance.

Westmoreland slates are usually sold by the ton, and not, like Welsh slates, by the "thousand". And the slates in each consignment vary in size, and must be sorted (as already explained in the case of stone slates) before being laid. The extreme dimensions and "qualities" of Westmoreland slates are not uniform throughout the district, but the following table will be useful as an example; it refers to the Elterwater green slates.