A flat dome like a gasholder top (Fig. 319) can have the patterns for its sectors struck out by assuming that each ring or tier of plates forms part of a cone surface. Thus a pattern for the outer ring in Fig. 319 will be laid out by making radius A B equal to a b; A C equal to a c, and the length B F equal to b f. In the same way the next tier of plates can be dealt with.

Fig. 319.

A dome pattern of this character can also be made up by the strip method, as shown in the chapters on roofing work.