“Topping out”—putting the last structural steel beam or final roofing piece in place on a building—is a project milestone. To celebrate, many contractors place an evergreen on top of the building, which gets the Christmas touch during the holiday season.
It turns out that this construction celebration has ancient roots. The most widely cited story credits pre-Dark Age Scandinavian cultures, which reportedly placed a tree on top of a new building as an appeasement to spirts, a practice that eventually migrated—albeit with different symbolism—to America.
The evergreen usually stays on top until the building is finished.