Why do builders place a small tree on the top of a building as soon as
its top frame has been completed?



In ancient times, the success of building projects was believed to
depend less on the builders' skill level and more on the moods of the
gods. To appease the gods and drive away evil spirits, builders would
attach plants to the tops of their bulidings as soon as the frames were
completed; they were thought to be inhabited by "nice" spirits.

These plants were often decorated with ribbons and flowers, and were
supposed to lend their life-giving powers to the buildings and show the
buildings were as safe and solid as the ground on which the plants grew.

Today, builders still observe this superstition by placing a small tree
on the top of a frame of a new building. This custom is called "topping
out."