From the hanging gardens of Babylon to today's high-tech green roofs, the idea of gardening in the sky is one that has survived and thrived over the centuries. Roof gardens can be as simple as a collection of containerized specimen plants on a conventional flat roof, to a system that requires a heavily buttressed, specially adapted roof surface, hundreds or thousands of square feet in size, that functions as a small, elevated park. The motivations for roof gardening are as varied as roof gardens themselves, encompassing aesthetic and ecological considerations.
Ancient Roof Gardens
The ancient Babylonian king, Nebuchadnezzar (605-562 B.C.) may or may not have created the first roof garden, but he certainly built one of the most celebrated--the hanging gardens of Babylon. A series of ascending terraces built on an arcaded structure, the hanging gardens were separated from the staterooms beneath them by layers of reeds, natural asphalt, brick tiles and lead. Michael Lancaster, writing in the "Oxford Companion to Gardens," describes plantings of flowering shrubs and trees, including cypress, cedar, mimosa and chestnut.
Roof Gardens Revived--The Renaissance
The Renaissance brought about a renewed appreciation of the ideas of the ancient world, especially Greece and Rome. Roof gardening became fashionable again as wealthy individuals like Florentine ruler Cosimo de Medici collected rare plants and installed them in ornate roof gardens. The practice of making gardens on top of various structures--from palaces to grottoes--spread, leading to the creation of some rooftop gardens in Northern Europe and England.
Innovations in Waterproofing
In the late 19th century, a German builder, Carol Rabbitz, built a roof garden on the flat roof of his house in Berlin. The waterproof barrier between the top floor and the house was made of vulcanized cement, which Rabbitz had invented. A model of the house was displayed at the Paris World Exhibition of 1867. A generation later, in the early 20th century, further advances in waterproofing technology, including the development of petroleum-based waterproofing products, made large-scale roof gardening more viable.
Architects and garden designers of the 20th century, ranging from Le Corbusier to Frank Lloyd Wright to Brazilian Roberto Burle Marx, embraced the idea of the roof garden. The famous Kensington Roof Garden, completed in 1938 atop a department store building in London, includes a series of themed gardens planted with perennials, roses, shrubs and trees The rooftop space is also home to water features, complete with flamingos and ducks. After a period of neglect, the Kensington Roof Garden is again open to the public.
Modern Green Roofs
Today, the idea of the roof garden has been transformed into the "green roof." In the past, the majority of roof gardens featured plantings consisting of containerized specimens. Green roofing systems, by contrast, are generally composed of specimen plants installed directly into a soil mixture that sits atop a waterproof barrier that protects the spaces below the garden. Green roofs can be either "extensive," with a thin layer of soil that supports a variety of meadow plants, or "intensive," requiring a thicker soil layer and buttressing of the roof. Intensive green roofs support perennials, shrubs and even trees.