There are two types of asphalt shingle units. One consists of a unit of twin shingles, so arranged that the butt ends which show to the weather appear as two individual shingles, and the other consists of one shingle unit. Both types are usually laid 4 inches to the weather and nailed with 1-inch galvanized nails No. 10 wire with ⅜-inch heads. At the eaves should be nailed a galvanized-metal drip edge, and over this a double course of shingles for the first course. Hips and ridges are finished with what appears to be a Boston hip, but the shingles are bent over the hip line. The valleys and gutters are best when they are lined with strips of ready roofing similar to the shingles themselves. Asphalt shingles which come in long rolls or units of four or five are laid in a similar manner, except that, due to their continuous length, they are unable to expand without bulging up on the roof. TIN ROOFS Flat roofs, with an incline of about ½ inch to the foot, should be covered with the flat-seam roof. The standing seam may be used on roofs with a pitch not less than 2 inches to the foot. The tin is laid upon the sheathing-boards without an intermediate layer of building-paper; in fact, tar paper should never be used. In cities building codes often require that tin roofs should be laid upon roofing felt ¹/₁₆ inch thick, placed over the sheathing-boards, but this is a fire precaution against burning brands which may drop upon the roof, for this felt cushion gives an air insulation, preventing the quick ignition of the decking below the tin. [Illustration: Tin Roofs] In laying the flat-seam roof a number of sheets are fastened together to form a long strip of tin. The edges are bent over ½ inch, so that they can be interlocked with the next strip. The tin is fastened to the roof with tin cleats that lock into the seams of the sheets and are fastened at the other end with two 1-inch barbed-wire nails. These cleats are spaced about 8 inches apart. All the seams are flattened down, and solder well sweated into them, rosin being the only flux used. Tin, approximately in thickness 30-gauge, U. S. Standard, is called IC, and recommended for the roof proper, while valleys and gutters should be lined with IX tin, approximately 27-gauge. It should be painted on both sides, before laying, with pure linseed-oil and red lead, or red oxide, Venetian red, or metallic brown. Two coats should be given to the exposed side and a third coat about a year later. Before the second coat is applied the first should have dried for at least two weeks.