Can green roofs make our cities better?
Researchers call the Hanging Gardens of Babylon the first examples of green roofs. While there is no evidence of its exact location and very little literature on the structure, the most widely accepted theory is that King Nebuchadnezzar II built a series of raised, ascending terraces of varied species as a gift to his wife, who missed the forests and mountains of Persia, their local land. According to Wolf Schneider  the gardens were supported by brick vaults, and below were shaded halls cooled by artificial irrigation of the gardens, with a much milder temperature than the outside, in the plains of Mesopotamia (modern Iraq). Since then, examples of green roofs have appeared all over the world, from Rome to Scandinavia, in the most diverse climates and types.
Even so, placing plants on roofs is still viewed with suspicion by many, as they are considered costly and difficult to maintain. However, others argue that the high implementation costs are quickly offset by savings on air conditioning and, above all, that occupying the fifth facade of the building with vegetation is above all a rational solution. Regardless, the question remains how green roofs can really help with climate change.
A green roof consists of a roof sheet with a layer of soil and vegetation on top of a waterproofed surface, which can be flat or slightly sloping. The thickness of the soil layer affects the type of vegetation that can be planted, ranging from 15 centimeters to more than 1 meter. Constructively, underground, usually several layers of material are inserted, with variations and refinement in the details according to the responsible designer. A geotextile blanket is usually placed under the substrate, which allows water to pass through but retains the soil. Underneath is a drainage layer, where the water will remain and overflow drains are provided. Including an anti-root layer is important to prevent them from infiltrating the structure and causing more infiltrations. Below that, the somehow waterproofed sheet structure completes the system.
There are several suppliers and manufacturers of green roofs that can be considered for these projects. It is interesting to involve engineers and suppliers from the design process where possible so that they can advise on specifications for thermal performance, drainage, load and other factors. For example, there are prefabricated modular systems that already contain all layers and even prepared plants. Due to the significant increase in weight, the load-bearing structure of the building has to be calculated for this, which will result in more robust construction parts. In the case of a remodeling project, the additional load must be studied to avoid structural problems. Another element to consider is access to the roof for maintenance. This is an important factor, which often weighs heavily against many roof gardens. There are some that require very little maintenance, while others are more demanding, requiring more watering and pruning. Any garden roof requires routine cleaning and inspection of the parts to prevent future problems.
When properly built, designed and maintained, green roofs have more advantages than disadvantages. For the users of the building below, the vegetation reflects most of the direct sunlight rather than absorbing it, and the moisture present in the vegetation and substrate prevents heat gain from the structure, saving energy for cooling. In arid climates, the increase in thermal inertia due to the substrate will increase comfort and reduce indoor temperature variations. In addition, it is generally a potentially useful, extremely pleasant green space.
But the benefits aren’t just private. Especially in large and densely populated cities, installing green roofs can reduce a number of serious problems. According to the EPA (Environmental Protection Agency), the use of green roofs in cities or other built environments with limited vegetation can mitigate the heat island effect, especially during the day. The temperature of green roofs is between 16 and 22°C lower than conventional roofs and can lower the ambient temperature in the city by up to 2.7°C. In other words, they are highly efficient devices for reducing urban heat islands.
In New York City, Stuart Gaffin has investigated the possibilities of installing roof gardens on the roofs of buildings. According to this article he signed with other researchers, it is estimated that the potential area of the roof area in the city is equal to 20-30 times the land area of Central Park. According to him, as a protected and safe space, roofs have many advantages as places for urban vegetation. Currently, traditional roofing membranes only have adverse effects on the environment and are also degraded and neglected spaces. The studies include modules with the succulent plant species Sedum spurium, comparing the results to standard dark roofs. The maximum temperatures on roofs planted with Sedum were found to be 30 degrees Celsius lower than the temperatures on standard roofs. They also found that Sedum plants are low maintenance and thrive without additional water, greatly reducing the need for care and maintenance.
Another crucial point is that the green roof also helps to manage runoff and rainwater retention. In densely populated cities, soil sealing is a serious problem. First, because in heavy rain, the drainage rate of the urban drainage system can be insufficient, causing it to become congested and cause flooding. In addition, all the pollution and dirt in the streets goes straight into the system, which can pollute rivers and streams. Garden roofs work by dampening and filtering a large portion of these impurities. Data collected from a study by researchers at Pennsylvania State University’s Green Roofing Research Center show that green roofs capture up to 80% of rain during storms, compared to a typical 24% for standard roofs. As green roof plants mature and root systems grow, rainwater retention may actually increase. The roof’s growing medium and plant material also act as a filter, helping to neutralize acid rain and trap dust and airborne particles.
Two other interesting possibilities for green roofs are the possibility of space for urban agriculture and especially a new habitat for useful plants and animals in urban areas, contributing to greater biodiversity. Birds, bees and other insects are finding more space to live, improving the ecological balance in places as harsh as cities. Observing features is an important point to create a more balanced environment adapted to local conditions. if discussed in this article, on a green roof it is advisable to use regionally inspired natural strategies to maintain a green roof with vegetation, even in abnormal weather. In this way, the species is resilient and requires less water, fertilizer or pruning. Understanding the local climate and vegetation, along with the environmental constraints to implement the garden, is vital to its success. While installation involves a higher initial cost than a traditional roof, there are many economic and urban benefits that can offset this. If the proper precautions are taken, these can be very beneficial to the private and public spheres.
 From Babylon to Brasilia – Wolf Schneider. The city as the fate of man, from UR to Utopia. Bound, 16×23 cm, 339 pages, illustrated. Good Reading Publisher, no place and no date.