As a true urbanite, roof gardens are a subject close to my heart. 
Here is the first of (what I envision to be) many posts on this subject matter.

 Illustration of ziggurat

The ancient history of aerial gardens began around 4000 B.C. as large temples were being built in Mesopotamian towns on top of mud-brick platforms. Over hundreds of years the temples were rebuilt on the remains of previous buildings, thus the platform grew in size.  As these “ziggurats” evolved in structure, multiple stepped stages were added and stairways spiraled up them on the outer edges. On each level of the ziggurat there was a terrace covered in baked brick. According to British archeologist Sir Leonard Wooley, “at landings on these stepped towers, plantings of trees and shrubs on flat terraces softened the climb and provided relief from the blazing heat of Babylonia.”  The most well known of these is the ziggurat of Nanna in the ancient city of Ur and  “Etemenanki”,  which translated from Sumerian is “house on the foundation of heaven on earth”.  As modern students of history and the bible may be aware of, “Etemenanki” is more commonly referred to as “the Tower-of-Babel.”

 Brueghel, Peter,  The Tower-of-Babel

One of the original seven wonders of the world, the Hanging Gardens of Babylon were created from emotional and spiritual reasoning. …built by Nebuchadnezzar around 600 B.C.E.  He built it to please his homesick wife, Amyitis, who was from distant Media. Amyitis found the flat and sun-baked environment of Mesopotamia depressing.  She longed for the trees, meadows and fragrant plants of her homeland.  Nebuchadnezzar, in the hope of appeasing her, decided to build a “recreated homeland” -- an artificial mountain with rooftop gardens (above grade), an “evolved ziggurat”.

(top) Hanging Gardens- Assyrian interpretation (bottom) Hanging Gardens ©Briwn Brothers

The Greek geographer Strabo, described the gardens in 1 BCE as “consisting of vaulted terraces raised above one another and resting on cube-shaped pillars.  These were hollow and filled with earth to allow trees of the largest sizes to be planted.”   Diodorus Siculus, a Greek historian of the same era believed “ the garden was 100 feet square and built up in tiers so that it resembled a theatre.  Vaults had been constructed under the ascending terraces which carried the entire weight of the planted garden… the highest gallery contained conduits for the water which was raised by pumps in great abundance from the Euphrates River.”2 Though historians question  the existence of these roof gardens  (although except in 1899 archeologist Robert Koldewey believed he had discovered the site at which it was created in southern Iraq near modern day Baghdad), one can conclude by the survival of this oral history that if the Hanging Gardens of Babylon were a myth, ancient civilizations still conceived of this concept and dreamed of creating this oasis.

Even today there is an innate feeling, an awareness that harkens from the same inner place and thoughts of the ancient Mesopotamians and Incan civilizations of creating a sanctuary at a higher elevation, rising above the rest of the world to a place closer to the sun and the heavens.

Many urban gardens can be inward looking, almost “cloister-ish”.  A garden in the rear of an urban brownstone is no doubt a cherished piece of property.  An oasis surrounded by fifty-foot high buildings is inward looking, reminiscent of the enclosed garden of the Middle Ages.  But, look aloft, to the top of the buildings… the roof garden is outward looking, a sanctuary high up on top of a building, sometimes with an endless panorama, a bright, beautiful, and open sky above it.  Most appropriately, it fits today’s city dweller with their overscheduled, time challenged lives.  For many, traveling to a city park takes a 1/2 hour or longer to embrace nature; walking up a flight of stairs or out their side door to a shared or private roof garden is but seconds away and “immediately gratifying.” A place to look at and admire the blue skies at day and heavens at night. It is a place to relax and re-energize, a place to reflect and even to pray. We enjoy company and serve them meals below the heavens; we even light candles and torches for ambiance at dusk.  How different is this than the Mesopotamian ziggurat, perhaps the first roof garden and cosmic axis?
Maybe the roof garden is a holy place.  Maybe it hasn’t changed much--its essence is arguably the same as it was 6,000 years ago.  People escalated themselves, or surrounded themselves on this (mostly) raised platform to reach another plateau, physically higher and spiritually greater.  One could argue that there isn’t any difference between the priests of Ur in Mesopotamia ascending the ziggurat to its apexical temple and the urban dweller that uses his/her roof garden to unwind and meditate.  Roof gardens can be intensely private spaces, essentially... sanctuaries.  

Frederick Law Olmstead is paraphrased by historian Elizabeth Rogers that the “creation of scenery evokes a poetic mood lifting one out of everyday care and ennobling the spirit with intimations of the divine.”

Gardens are an ethereal world – they should be calming spaces and transport you to another state of mind...

“Let me recommend
What to do
When your heart is heavy or blue.
Get to steppin.’
Climb those stairs
To that ballroom in the air.
Does anyone wanna go
waltzing in the garden?
Does anyone wanna go dance up on the roof?”

  -  Al Jarreau, Larry Williams, Andrew Ford, “Roof Garden”, Reprise Records ©1981