This has been shown by a partial failure which has taker place. The wall plates having become rotten in consequence of the gutters being stripped of their lead, the weight has been thrown on the pseudo struts, which have bent under the pressure, and forced out the upper portion of the walls. 125. The roof of Westminster Hall (fig. 68) is one of the finest examples now existing of open timbered roofs. The Fig. 68. 34 peculiar feature of this roof is an arched rib in three thick nesses, something on the principle of Philibert de Lorme ; but it is so slight, compared with the great span, that it is probable, in designing the roof, the architect took full advantage of the support afforded by the thickness of the walls and the buttresses; if, indeed, the latter were not added at the time the present roof was erected, in 1395. It has been ascertained that the weight of the roof rests on the top of the walls, the lower part of the arched rib only serving to distribute the thrust, and to assist in preventing the hammer beams from sliding on the walls. 126. The medieval architects generally employed oak in the construction of their large roofs, the timbers being morticed and pinned together, as shown in fig. 62. This system of construction is impossible in fir and other soft woods, in which the fibres have little lateral cohesion, as the timber would split with the strain; and therefore, in modern practice, it is usual to secure the connections with iron straps or bolts passing round or through the whole thickness of the timbers. ROOF COVERINGS. 127. The different varieties of roof coverings principally used may be classed under three heads: stone, wood, and metal. Of the first class, the best kind is slate, which is used either sawn into slabs or split into thin laminæ. The different sizes of roofing slate in common use are given in the description of Slaters' Work. In many parts of England, thin slabs of stone are used in the same way as roofing slate. In the Weald of Sussex the stone found in the locality is much used for this purpose, but it makes a heavy covering, and requires strong timbers to support it. 128. Tiles are of two kinds: plain tiles, which are quite flat; and pantiles, which are of a curved shape, and lap over each other at the sides. Each tile has a projecting ear on its upper edge, by which it is kept in its place. Sometimes plain tiles are pierced with two holes, through which oak pins are thrust for the same purpose. 129. Wooden coverings are little used at the present day, except for temporary purposes; shingles of split oak were formerly much used, and may still be seen on the roofs of some country churches. Cedar shingles are much used. 130. Metallic Coverings.-The metals used for roof coverings are lead, zine, copper, and iron. 131. Lead is one of the most valuable materials for this purpose on account of its malleability and durability, the action of the atmosphere having no injurious effect upon it. Lead is used for covering roofs in sheets weighing from 4 to 8 lbs. per sup. foot. 132. Copper is used for covering roofs in thin sheets. weighing about 16 oz. per sup. foot, and from its lightness and hardness has some advantages over lead; but the expense of the metal effectually precludes its general adoption. 133. Zinc has of late years superseded both lead and copper to a considerable extent as roof coverings. It is used in sheets weighing from 12 oz. to 20 oz. per sup. foot. It is considered an inferior material to those just named; but its lightness and cheapness are great recommendations, and the manufacture has been much improved since its first introduction. 134. Cast iron, coated with zinc to preserve it from rusting, is now much used in a variety of forms. We have already mentioned its adoption for covering the roofs of the New Houses of Parliament. 135. All metallic coverings are subject to contraction and expansion with the changes of the temperature, and great care is requisite in joining the sheets to make them lap over each other, so as to make the joints water-tight, without preventing the play of the metal. The following table of the comparative weights of different roof coverings may be useful : 136. The arrangements for distributing a supply of water over the different parts of a building will depend very materially on the nature of the supply, whether constant or intermittent. The most common method of supply from water-works is by pipes which communicate with private cisterns, into which the water is turned at stated intervals. A cistern, in a dwelling-house, is always more or less an evil; it takes up a great deal of space, costs a great deal of money in the first instance, and often causes inconvenience, from leakage, from the bursting of the service pipes in frosty weather, and from the liability of the self-acting cock to get out of order. Fig. 68 shows the ordinary arrangements of a cistern for a dwelling-house. The common material for the cistern itself is wood lined with sheet lead; but slate cisterns have been much used of late. Large cisterns or tanks for the supply of breweries, manufactories, &c., are usually made of castiron plates, screwed together by means of flanges all round their edges. The service or feed pipe for a cistern, in the case of an |