The team, which included 50 volunteers, built the 2,200sq m garden on top of the Queen Elizabeth Hall, part of the Southbank Centre arts venue in London.
Eden Project landscape architect Jane Knight and designer Paul Stone included an orchard, vegetable plots, a rosebud walk and herb garden. Knight, who worked with ex-prisoners and former homeless people, said: "The idea is to give a taste of British gardens and landscapes.
City Hall provided funding as part of scheme to encourage green roofs. The roof garden is one of three gardens featured in the celebrations, which run until 4 September.