This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
Fig. 12 shows a trussed rafter suitable for a flat pitch roof of from 30 to 40 ft. span, the rafters being set at from the tower, and at a point b draw c b and b d, each equal to one-half the diameter of the tower on the sheathing line, plus the thickness of the upper edge of the siding; from c or d draw a line towards a, having the same inclination as the back of the siding on the wall; the point where the line intersects a b, as at a, will be the vertex of the cone, and a c will be its slope length, and also the radius for describing the curve c e d. As the radius will be too long to lay down full size, any chord of the curve, as c e, may be used, from which offsets h, i, and j may be measured and laid down, so that the points can be fixed on the floor. A few wire nails being driven to mark the points, a flexible strip run around them will give a satisfactory curve, and the lower edge of the siding strip will be cut as at c' e' d' in (d), the curve of which is shown greater than it would be in practice, in order to emphasize it. The butt joints of the siding, as at c' d', being radial lines, must be cut square to the curve c' e' d'.
2' to 3' centers. The rafters and joists are 2 in. X 8 in., and of a good quality of spruce or hemlock. The lattice braces are 1 in. X 8 in., and are placed in pairs, one on each side of the main members, to which they are spiked. The spiking should be well done, especially near the supports. The tie-member is spliced at the center of the span by two 1" X 8" fish-plates, well spiked to the ceiling joist; two iron dogs, well driven in, further strengthen the splice. The roof is covered with 1 1/4" X 6" tongued-and-grooved surfaced spruce, then with a layer of felt, upon which tin is laid. All other necessary details of construction are shown in Fig. 12.