In locating the position of the rolls it is necessary to consider the general construction of the edifice, both above and below the dome. The ribs or rolls should not be put up to suit the standard stock sizes of copper sheets, but they must be located to harmonize with the leading architectural features of the composition.

Fig. 51.

For example, consider the dome of the cupola, or lantern, shown in Fig. 51. The ribs here divide the dome surface into equal sections, two sections being immediately over each window opening. Every second rib is directly in line with the center line of a column. When the sheets are drawn closely together at the top, as shown, the ribs are usually tapered, being wider at the bottom than at the top, but when there is considerable space between the top ends of the ribs, they are usually made the same width throughout their length.