For centuries people have had roof gardens and green roofs. 3,000 years ago in Egypt trees were planted on the stone terraces of temples and plants grown on the roofs of houses. In Mesopotamia the Hanging Gardens of Babylon were famous and roof gardens common. For hundreds of years people in Northern Europe covered their roofs with living grasses. Today there are many new techniques and different materials used in building green roofs so the process can appear complicated. However the same basic principles apply now as before. The roof needs to be protected from water and from plant roots: excess water must be drained away; and the plants given a suitable substrate in which to grow.
For a green roof, or indeed any flat concrete roof, balcony waterproofing is the first step and is more important now than in the past. When water permeates concrete it can cause various problems, from small surface cracks to those requiring extensive concrete repairs. Materials such as bitumen torch on membranes are commonly used to waterproof green roofs. In ancient Mesopotamia however they used a layer of reeds sealed with natural asphalt and for the roof gardens of the Emperor Tiberius in Rome a hardened mix of finely crushed bricks and lime.
Next there is the root barrier, although this can be incorporated into the bitumen membrane. In Mesopotamia, two courses of bricks with a sheet of lead on top stopped most roots, for a while. In ancient Rome it was ceramic plates with an air space beneath. That air space also let excess water drain away. A steep pitch ensured good drainage of the grass roofs of Northern Europe. On flatter roofs however a drainage layer is needed through which water can flow freely and above that, a filter confining the substrate.
Historically garden soil was used, but the growing media now is a mix of organic and inorganic material which weighs less than soil but retains a lot of water. The depth of the substrate depends on the plants. Trees need 200mm or more; shallow rooted grasses and small succulents as little as 20mm
Many of the benefits of green roofs were also known to people in previous centuries. They provide wonderful insulation, significantly reducing the amount of energy needed to heat or cool buildings. They more than double the life of the roof, insulating and protecting the waterproofing membrane from weathering, UV light and mechanical damage. They afford additional space where land is scarce. Green roofs freshen the air, reduce glare and noise and delight the senses. However the ancient Egyptians had no thought of climate change, of the need to minimise their carbon footprints or reduce urban heat island affects; nor about increasing biodiversity, creating urban habitats, or even reducing storm water runoff. These are all more recent concepts which provide further reasons to ensure that, where possible, the roof over our head is green.