The use of tiles for roofing, although by no means a modern practice, has of late years become more general in this country. Tiles are made in a variety of shapes, the general principle, in all cases, being that of a slab of baked clay, moulded with an interlocking roll or rim. Tiles are laid on battens or on boarding, as described for slates; the difference being, that instead of a double lap for tightness, the tightness of tiling depends upon the fitting of the tiles into each other. Sonic of the modern forms of tiles are shown in Fig. 157. With tiles should be used copper or zinc for valleys and flashings, but ridges and hips are generally formed by tiles of special patterns. Plain tiles have long been used in England and are in most cases found to be in as good condition as when new. Tiles may be used on roofs which have a pitch of more than 22 1/2 degrees, but will need a free use of elastic cement, especially in valleys, hips, and ridges. In appearance, tiles should be of even color, free from fire checks, but well burned and non-absorbent, of uniform size and without blisters, cracks or warped surfaces.