Traditionally, a cupola was used to vent hot air out of the attic. Today, it provides a nice finishing touch to a new home. It is not necessary to have a weathervane on top of a cupola but it is a nice addition depending on the design. A rule of thumb on how to measure for a cupola for your home is 1 inch of cupola to 1 foot of running roof line. Once that measurement is determined, you would go up one additional size. For example, a 24 foot garage=24 inch cupola plus 1 size up. Perhaps, purchase a 28 inch cupola. Weathervanes do not serve as a lightning rod.