Small conical roofs, such as are frequently used on small circular towers, are best framed as shown in Fig. 85. The plate should be cut out of wide planks, and always made of two thickness put together so as to break joint; when well spiked together, they thus form a continuous ring to resist the outward thrust of the rafters. The rafters should be spaced at even distances apart, about 3 feet at the bottom, and circular ribs cut in between about every 20 inches to 2 feet on the slant. If the diameter is 10 feet or more only part of the rafters need be carried to the peak, the others being terminated under one of the circular ribs, as shown at A. For small roofs it is necessary to cut the boards to a wedge shape and put them on vertically, nailing to the circular ribs. Larger roofs may be covered by using ¾-inch boards and bending them around the roof, at an angle of 45°, with the plate. In this way the boards do not have to he cut to a pattern, and the ribs or sweeps are not required, except for securing the upper ends of the short rafters, as the boards are nailed to the rafters.
Bell-shaped Roofs. - If the vertical section of the roof is bell shaped, instead of conical, furring pieces cut to the desired curve should be spiked to the top of straight rafters, as shown in Fig. 86, and circular ribs cut in between, as in conical roofs. If the roof is very high, or has a finial, the upper end of the rafters should cut against a centre pole, having as many sides as there are rafters. This pole stiffens the roof and forms a support for the finial. It is also well to brace the lower end of the pole by cross pieces spiked to the rafters and to the pole.