This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
If all tiles were of the brilliant hue that is occasionally met with in country districts, it is probable that their use as a roofing material would have remained restricted. But lately manufacturers have succeeded in producing a warm reddish-brown tint closely approximating to that of old weathered tiles, than which nothing can be more pleasing. Tiles are made from clay in much the same manner as bricks, but as they are thinner, they require a tougher clay, and considerably more care in their manufacture. The clay is dug and weathered, and then ground in a pug mill, so that it acquires great consistency. It is then formed into the required shape by being pressed into an oak mould, plated with iron. The clay is kept from sticking to the mould by dusting the latter with sand or fine coaldust. The tiles are then dried in the open air, and afterwards baked in a kiln, constructed on much the same principle as a brick kiln, but enclosed in a conical building called a dome. The period of burning depends upon the quality of the clay and the colour to which the tiles are to be burnt. This general description applies to nearly all roofing tiles, but of course the moulding processes vary according to the kind of tile that is being made.
Till quite recently, pantiles and plain tiles were the only kinds made, the former having a double curved surface, on the one side convex and the other concave, which shape is given to the plastic clay by hand. A small hollow in the mould gives the projecting nib by which pantiles are hung on the laths of a roof. The ordinary size of pantiles is 14 1/2 by 10 1/2 in. and they are generally laid to a gauge of 10 or 11 in. These tiles are only used for an inferior class of building, as it is difficult to keep a roof covered with them watertight for a length of time, because they can never be made to fit closely over each other at their lower edges, and therefore the rain has a tendency to drift up underneath them. They partially overlap laterally, but not sufficiently to make a watertight joint, and they are consequently frequently pointed with mortar, which forms a thick and ugly joint, and soon perishes. The weight per square is about 10 cwt. The Bridgwater tiles are very similar to pantiles, but wider, and formed with a double roll.
Plain tiles are oblong in shape, with a very slightly curved surface, and they are either moulded with nibs for hanging on the laths, or are formed with two holes for nails or pegs. Their size is 10 1/2 by 6 1/4 in. Many excellent specimens of plain tiles have been shown at the various building exhibitions which have been held lately, but it would be difficult to find any superior to the best Broseley tiles, which are well burnt, even in texture, and of a very pleasant tint. Plain tiles, of the size mentioned above, should be laid to a 4i-in. gauge, and they are either hung to the laths by their nibs or by pegs driven through the holes in their surface. Some builders are fond of bedding the heading joints in mortar, but it is very doubtful if this is a good plan. Of course it makes the roof somewhat tighter at first, but experience shows that tile roofs almost always fail in consequence of the laths becoming decayed and allowing the tiles to slip, and the presence of the mortar accelerates this decay.
It is, moreover, certain that if mortar be used the tiler will be disposed to depend upon it for keeping the tiles in position, and will not devote so much care to the proper hanging and pegging of each tile, and as all roofs are subject to slight movements duo to changes of temperature and to varying wind pressures - have in fact a certain amount of "spring" - this will instantly act upon the mortar joint and will tend to disturb it, and in a very short time the mortar begins to fall away and helps to block up the roof gutters. In some country districts, it is the practice to lay the tiles on a bed of hay laid over the laths, and this plan appears to answer very well if proper care be taken, and it adds to the warmth of a building in winter. It is not desirable to give tiled roofs a less pitch than 45°, and 50° is preferable.
With ordinary plain tiles, those in any one course do not overlap laterally; consequently each course must overlap to a certain extent the next but one below it, or the rain would enter between the joints. Of late years many attempts have been made to obviate this necessity, which is the cause of the great weight of this kind of roofing, - nearly a ton per 100 sq. ft. If tiles can be moulded so that they will fit into one another, and form a watertight joint laterally, the successive courses need only overlap sufficiently to prevent the rain driving upwards, and this can be prevented by forming a groove at the upper edge of one tile into which a corresponding projection on the lower edge of the next tile would fit. This method has been adopted with considerable success in Phillips' patent lock-jaw roofing tiles, which interlock with one another on all 4 sides, and form such closely-fitting joints that nothing can penetrate them; and the patentee claims that by exerting great pressure on the clay during the process of manufacture he is able to ensure uniformity and perfect fit, without which the tiles would of course be practically useless.
These tiles are of two different kinds: the "single grip," and the "double grip." The latter are suitable for the most exposed situations, and will stand the roughest usage. Half tiles are made for hanging next to a gable in alternate courses, in order to secure a perfect bond, in the same way that closers are used in brickwork. No mortar is required with them, nor any special skill in laying them. The difference between these tiles and the ordinary kind in the weight per square, 5 1/2 cwt., and the number required, 150, is very striking, the weight being less than one-third that of ordinary tiles. Taylor's patent tiles are moulded with a different kind of lateral overlapping arrangement. All these patent tiles give a decidedly ornamental appearance to roofs, as they break up the plain surface into a series of elevations and depressions.
It is curious to notice how closely some of the new patent systems of tiling resemble those in use among the Romans and in the early part of the middle ages. Tiles were used at a very early period for roof coverings, and were first made with rims on each side, and under the rims were notches forming a lap laterally; and hollow tiles, similar to common hip and ridge tiles, were laid over the vertical joints, themselves overlapping each other. An improvement was effected by making the tiles trapezoidal in shape, instead of rectangular, and thus the narrow end of one tile was pushed down till it closely fitted between the rims of the one below it; the notches under the rims were then discontinued, but the vertical joints were covered as before.
In the thirteenth and fourteenth centuries, in the old French province of Champagne, tiling was carried to very great perfection; and it is probable that no better tiles have ever been made than those which can be still seen in many buildings in Troyes and its neighbourhood. The tiles are very like the modern Broselys, but were made with one nib for hanging on the laths and one hole for nailing; and to show the extreme care which was taken with them, the positions of the nib and hole were reversed in each alternate course of tiles. This was done in order that, as the alternate courses were laid "breaking joint," the nail-hole should always come over the centre line of each rafter, and the nib always midway between the rafters. The rafters were of course fixed in the proper position for the tiles, which were laid to a gauge of about 4 1/2 in.; and as the length was 12 3/4 in., there was always a lap of nearly 4 in. In some of the tiles of this period, the exposed portion of each tile was glazed, and thus rendered non-porous. This would be an excellent plan to adopt with modern tiles, but it would render them too expensive for general use.
There is, however, another peculiarity in the best of these old French tiles which might easily be adopted by modern manufacturers, and which would be a great improvement, and that is the chamfering of the lower edge of the tile. The mould could easily be made of the shape requisite to form this chamfer, which would greatly diminish the risk of the tiles being ripped up by the action of a strong wind. The tiles of this period are frequently found in as good condition now as when first burnt.
The great objection to all tiles is their porosity, which causes them to absorb a considerable quantity of water, and this tends to rot the woodwork underneath. This wooden substructure consists in the case of both slates and tiles, either of fillets of wood nailed on the rafters at intervals corresponding to the gauge required, or of close boarding similarly nailed to the rafters. For slates, these fillets are generally 2 1/2 in. or 3 in. wide and 1 in. thick, called slating battens, and the slates are nailed to them by nails - 2 to each slate - passing through small holes pierced in the slate itself. For tiles, which are much heavier, stouter fillets are required, and the tiles are hung on to these by pegs passing through the holes in the tiles, or by the projecting nibs which have been already described. Close boarding is far preferable for either kind of covering, as it keeps the roof tighter and warmer, and in case it should be necessary for workmen to pass over the roof for any purpose after its completion there is much less danger of the slates or tiles being broken than with battens.
The risk of damage from high winds is also much less.