The tiles after being laid should be torched or tiered, that is pointed from the inside with hair mortar. The Verges (see Fig. 100) should be pointed in cement, and the ridges, finials, etc., set in cement. In very exposed places each tile may be bedded in hydraulic mortar or cement upon those below it.
Pan Tiles form a covering not so warm as one of plain tiles, and liable to injury from gusts of wind.
The tiles are about 14 inches long by 9 inches straight across the width. Each is hung on to the laths or battens, b b, by a nib which projects from the upper edge of the back of the tile, shown in section at x x x. It should be remarked that this projection is not continuous throughout the width of the tile, but is only about one inch wide.
1 Or Hanging Tiling.
The tiles have a lap of 3 inches to 4 inches, and the joints on the under sides are pointed with hair mortar.
Pan tiles are well adapted for roofing over workshops where large furnaces are used, as they withstand the heat, and the interstices between them afford plenty of ventilation.
Half round or concave tiles set in mortar, and nailed to the woodwork, are used for the ridges, hips, and valleys. For common work sometimes the tiles themselves are used - the smaller curved portion being cut off, - but special tiles are generally made for the purpose.
In exposed situations, and where much ventilation is not required, the tiles are bedded on each other in mortar, and the space between the ridge tiles and those in the ridge courses at the top of the slopes are filled in with pieces of flat tiles bedded in mortar.
Glass Tiles of this form are made, and may be introduced among the others where light is required. Double Roll Tiles are similar to the above, but have a double wave in their width. Corrugated Tiles are similar in general form to pan tiles, but they are bent into several narrow curved or sometimes angular corrugations, instead of only two broad ones.
Italian Tiles are shown in section and elevation in Figs. 110, 111, from which the construction of the tiles is obvious.
These tiles present a handsome appearance, which leads to their use in some cases; but they are not well adapted to the British climate, as they cause the snow to lodge, and, when it thaws, the water frequently gets through the roof.
Fig. 110. Elevation.
Fig. 111. Section.
Taylor's Patent Tiling is somewhat similar in principle to the Italian tiling just described.
In this case, however, the Tipper or capping tiles are exactly like the lower or channel tiles, so that every tile can be used in either position.
Fig. 112 shows the general appearance of this kind of tiling, which is very picturesque.
Fig. 113 gives an upper and lower view of the tiles.
Fig. 114 shows a few channel tiles, T T, with one capping tile, U, in position.
The tiles are hung on battens 2¼ inches wide and 1 inch thick, laid to about a 10-inch gauge. The channel tiles are first laid in rows along the slope of the roof from eaves to ridge ; the narrow end of each tile is pushed into the wide end of the one below until the splay, s, fits firmly into the undercut in the shield, A, of the lower tile.
There are notches in the sides of the tiles, as shown at n n Fig. 113; each channel tile is secured by wedge-shaped nails l driven in alongside, so as to hold the tile down by these notches as at x x.
After the channel tiles are all fixed, the capping tiles are put on. These tiles are turned over, and so placed as to cover the intervals between the channel tiles. They are pushed downwards until the little blocks or cogs, c c, rest upon the nail-heads, x x, which secure the channel tiles below.
The under side of the corners of the joints between the tiles is pointed with cement mortar.
Foster's Lock Wing Roofing Tiles are illustrated in Fig. 115,2 which explains itself. It is claimed for these tiles that they are cheaper than the commonest tiles made, can be hung quickly and without skilled labour, require no pointing, and cannot be blown off the roof, as the stronger the pressure is underneath, the tighter the lock.
1 Or side keys.
2 From the Patentee's Circulars.
Poole's Patent Bonding Boll Roofing Tiles are shewn in Fig. 116 1 which requires no description.
Thatch is made of wheaten straw2 laid on laths nailed (8 inches apart) to rafters, frequently of a very rough kind. This covering keeps a building warm in winter and cool in summer, but it is very subject to destruction by fire or decay, and generally forms a refuge to insects and vermin.
The pitch should he 45°. If it is less the rain will not run off freely ; if more, the straw slips down.
About 3½ cwt. of wheat straw is required per square. It will last in England from fifteen to twenty years, - oat straw about eight years.
Corrugated Iron plates are much used.3 They are made in sheets varying in size from 6 feet x 2 feet to 8 feet x 3 feet, and in thickness from 1/45 to 1/16 inch, that is, from No. 24 to No. 16 Birmingham wire gauge. The sheets of medium thickness (for example, of No. 2 0 gauge with 5-inch flutes) require to be supported only at intervals of from 6 to 8 feet, and the roof may thus be cheapened by omitting the intervening purlins. Ordinary corrugated iron is so laid that the flutes run down parallel to the slope of the roof. The sheets overlap at the sides, and should be screwed at the top and bottom edges to the roof timbers. The screws should be on the ridges of the corrugations, so that all wet may at once be thrown off them.
Corrugated iron is sometimes laid with the flutes horizontal, so that the sheets span the interval between the principals, and all rafters and purlins can be dispensed with. In such a case the flutes should be of a peculiar angular shape,1 so as to throw off the water.
1 From the Patentee's Circulars.
2 Reeds make the best thatched roofs, but the use of reeds for roofing has nearly died out. 3 See Part iii., p. 288.
Corrugated iron is frequently used not merely as a covering, but to form the roof itself, the sheets being riveted together and bent into an arched form.
Lead is not adapted, as a covering for pitched roofs, owing to its expansion and contraction, by virtue of which it will crawl down a roof. During a warm day it expands, the expansion being assisted downwards by the action of gravity ; in the cool night it contracts, the contraction being diminished by the force of gravity acting downwards : the consequence is it contracts each night less than it expanded during the day, and in time gains a considerable distance.2
Copper is sometimes used in sheets weighing about 16 ounces per foot superficial. They should be laid on boards in the same manner as those of lead. The coating of oxide formed by the action of the air preserves the surface to a certain extent, but the first cost of this metal is so great as to prevent its being much used.
The method of laying zinc in this country has been greatly improved through the exertions of the Vielle Montagne Zinc Company, from whose beautifully illustrated pamphlet3 on the subject the figures and most of the information here given have been extracted.
There are several methods of laying zinc on roofs; in all of them the object should be to avoid soldered and rigid connection, and to arrange the joints so that they may be water-tight, but may still allow free play for contraction and expansion of the metal under changes of temperature.
1 See Part III. p. 288.
2 The lead on the moderately-inclined roof of Bristol Cathedral crawled down 18 inches in two years - Tyndall, Heat as a Mode of Motion.
3 Published by their manufactui'ing agents, Messrs. F. Braby and Company.
Fig. 117. Scale, ½ incli = l foot.