How do you build a Cantilevered Deck on the second floor? Look carefully and you will see the floor joists are reversed in "run" into the house exactly twice as far as the deck is cantilevered, this gives it the necessary strength. Only with planning and design work that is thorough can this be done. It is impossible to add a cantilevered deck once the house is built. This is a great addition to any Vacation Home. Call us for details.