It's an old custom. Very old.

The custom of placing a pine tree at the top of a new building stems from ancient superstitions. Some cultures feared evil spirits occupied new buildings; some religious groups still practice blessing rites for new homes and buildings today. The first evidence of tree topping occurred in Scandinavia in 700 A.D. and signaled that a completion party was about to begin.

Now, the "topping trees" celebrate the completion of the skeleton of a building; when one is hoisted to the top of a skyscraper, it signals that the building has reached its final height. For some builders the tree may represent that no one on the crew died during construction; it may also represent good luck and future prosperity for the building occupants.

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