Advice on installing a new roof Shingles must always be nailed to 1 X 4 in.(2.5 X 10 cm) wooden laths. The distance between the centers of the laths must be equal to the width of the exposure. To align the shingles, tack a board temporarily to the roof or draw a straight chalk line. In areas with harsh climatic conditions, the shingles must be fixed to walkboard that is covered with a vapor barrier and insulated on the underside by foam or a rigid insulator. Place two nails, 3/4 in. (2 cm) from each edge of the shingle and 1/2 in. (4 cm) above the exposure. Never have the joints of two shingles in the line if they are seperated by only one course of shingles. Keep at least a 1 1/2 in. (4 cm) space between the joints in successive courses. Knots and other inperfections in the shingles must be treated as edges of shingles. Joints must be placed in alternating rows, at least 1 1/2 in. (4 cm) from the start of the inperfections. To facilitate the flow of the water into the eaves, let the first row of shingles protude at least by1 1/2 in. (4cm) above the first roof board. For the ridge of the roof, choose shingles of the same width, between 4 to 5 in. (10 and 13 cm) and install them by alternating from one side to the other, from one row to the next. The first row of shingles and the ridge of the roof must be two shingles thick. For roofs with slapes of less than 45% , the metal base, which is installed under the shingles, must cover at least 10 in. (25 cm) of the ridge or the valley. For roofs with slapes of more than 45% , the metal base must cover at least 7 in. (18 cm) of the ridge or the valley.