Willesden paper is another extremely light, durable, and waterproof roofing material, which differs essentially from the 2 preceding substances in needing to be fixed to rafters or scantling, and requiring no boarding on the roof. It is a kind of cardboard treated with cuprammonium solution, and has become a recognized commercial article. It is made in rolls of continuous length, 54 in. wide, consequently, when fixing the full width of the card (to avoid cutting to waste), the rafters should be spaced out 2 ft. 1 in. apart from centre to centre, so that the edge of one sheet of card laid vertically from eaves to ridge will overlap the edge of the adjoining sheet 4 in. on every alternate rafter. By far the most important and generally used roofing material in this country is slate. Its splitting or fissile property makes it eminently useful as a roofing material, as, notwithstanding the fact that it is one of the hardest and densest of rocks, it can be obtained in such thin sheets that the weight of a superficial foot is very small indeed, and consequently, when used for covering roofs, a heavy supporting framework is not required. Slate absorbs a scarcely perceptible quantity of water, and it is very hard and close-grained and smooth on the surface; it can be laid safely at as low a pitch as 22½°. In consequence of this, the general introduction of slate as a roofing material has had a prejudicial effect upon the architectural character of buildings. The bold, high-pitched, lichen-covered roofs of the middle ages--which, with their warm tints, form so picturesque a feature of many an old-fashioned English country town--have given place to the flat, dull, slated roofs. The best roofing slate is obtained from North Wales, chiefly in the neighbourhood of Llanberis. Non-absorption of water is, of course, the most valuable characteristic; an easy test of this can be applied by carefully weighing one or two specimens when dry, and then steeping them in water for a few hours and weighing them again, when the difference in weight will of course represent the quantity of water absorbed. The light-blue coloured slates are generally superior to the blue-black varieties. (J. Slater.) Some architects bed the roofing slates in hydraulic cement, instead of having them nailed on dry in the usual way, which leaves them subject to be rattled by the wind, and to be broken by any accidental pressure. The cement soon sets and hardens, so that the roof becomes like a solid wall. The extra cost is 10 or 15 per cent., and it is good economy, considering only its permanency, and the saving in repairs; but, besides this, it affords great safety against fire, for slate laid in the usual way will not protect the wood underneath from the heat of a fire at a short distance.