Green roofs, types, construction techniques and advantages

The spread of green roofs has increased the growing interest in sustainable architecture and green building. The green roof, green roof, or garden roof, if you prefer, can be of an intensive or extensive type. This classification depends on several factors: the amount of land, maintenance interventions, weight, and accessibility. Green roofs certainly bring social, economic, and environmental benefits: mitigation of the microclimate, energy saving, reduction of air and sound pollution, reduction of water flow rate, growth of biodiversity, better performance of the photovoltaic panels on the roof.

In times of climate change, there is also talk of green roofs, and more, and more often, there is news of projects by significant architectural firms that enrich their most recent creations with flat roofing solutions rich in greenery.

What is meant by a green roof (or garden roof)?

In truth, the Roof Garden construction technique has been used in Switzerland and Great Britain since the early 1980s. At the time, their use was justified with the needs of energy-saving and biodiversity conservation, but today their positive function is also extended to adaptation to climate change.

Basel, Sheffield, London, Copenhagen, Rotterdam, Amsterdam, Paris, Stuttgart, and Berlin are just some European cities that have long since started real intervention programs. Overseas, the experiences of Toronto, Chicago, and New York are also in evidence.

When it comes to green roofs, the message is easy to understand even for the general public because it is a technical solution appreciated as a partial antidote to climate change.

This alternative solution to traditional technologies is greening the roof slab on the extrados. The stratigraphy includes some essential elements for any solution and can be carried out on horizontal and sloping roofs, with a maximum slope of about 35%.

A green roof complies with all the characteristics required of any roof – structural, mechanical, and thermal – and adds agronomic and draining capacity. The advantages are many, both for the building and for the environment. As with all solutions, there are technical, typological, and economic aspects that must be considered.

An extensive green roof or intensive green roof?

A green roof is a roof covered with a layer of soil and plant species; there are two possible types: extensive and intensive green roofs.

An extensive green roof is defined when we are in the presence of species that require a layer of earth between 8 and 15 centimeters and reduced maintenance interventions (once or twice a year); their weight is generally less than 150 kg / m2.

The surfaces are accessible only for routine maintenance. This solution is typically used for vast and impractical surfaces, with an extensive green covering the roofs of entire warehouses, shopping centers, or other buildings whose top is not used for the permanence of people.

On the other hand, it is defined as intensive when there are species that require a layer of earth from 25 to 50 centimeters and constant maintenance interventions or, in any case, more than 4-5 times a year. The weight exceeds 150 kg / m2 but varies with the design solutions adopted.

The maintenance of green roofs

The maintenance required for a green roof is undoubtedly more significant than a traditional roof, even when it comes to low maintenance in the case of extensive systems. Installation and initial maintenance are essential for the proper development of the green design, but they are not complex activities.

If an intensive green roof is chosen, the traditional maintenance activities related to vegetation, as expected for any other garden, must be added. The primary purpose of all maintenance activities is to ensure order, usability, and well-being of the vegetation, but also to ensure that the main characteristics of the system, such as drainage and impermeability, are not affected.

What are the climatic benefits of green roofs?

In order: mitigation of the microclimate, energy saving, reduction of atmospheric pollution, noise pollution, the speed of water runoff, growth of nature and biodiversity, better performance of the photovoltaic panels on the roof, and finally social and economic benefits.

On a green roof, the maximum summer temperatures reach 25-30 degrees, against – according to data released by Ambiente Italia – about 80 of a traditional roof. The air temperature in the vicinity of the green roof can decrease by 2-5 degrees compared to a conventional top (the data comes from a study by American researchers Peng and Jim of the University of Hong Kong; NDA).

The spread of extensive green roofs can reduce the air temperature (at a man’s height) by about 0.4-0.7 degrees, while with intensive ones, a reduction of 0.5-1.7 degrees is possible. On an urban scale, on the other hand, a drop in ambient temperature of 0.3-3 degrees is estimated (the data comes from Lorraine Chow, environmental journalist of EcoWatch in New York; NDA).

A green roof, compared to a traditional non-green top, guarantees energy savings, both in summer and in winter. The savings – estimated at 10% – vary about the type of building, the climate, and the size and type of roofing.

If a green roof is integrated with photovoltaic systems, this choice allows the maximum temperatures to be kept around 30-35 degrees, thus improving the efficiency of the photovoltaic panels, which can produce about 5% more electricity than an installation—Photovoltaics on a traditional roof.

The vegetation on a garden roof performs another function: to retain harmful substances suspended in the air, which are thus absorbed through the photosynthesis process.

Creating a green roof improves the thermal performance of the top both in winter and in summer, with energy savings favored by the insulating and draining layer of the crop.

Another benefit is reducing noise pollution: the vegetation absorbs sound waves, for example, those produced by air and vehicular traffic, and reduces their propagation, with a drop of up to 3dB on roofs and up to 8-10dB in interior spaces. Below (the source is a 2016 study by the Dutch Deltares institute operating in the water management sector; NDA).

Another significant advantage is the increase in the duration of the structure since the waterproofing under the crop layer is protected from ultraviolet rays, and the system is not subject to significant thermal changes dampened by the plant layer.

A green roof retains rainwater and releases it more slowly to the sewage system, reducing the flow of rainwater to lighten the sizing of the drainage pipes, thus helping to reduce the risk of flooding in the event of heavy rain. According to data from Irsa of the CNR, the peaks of water runoff are estimated to be reduced by 60 to 80% in the presence of green roofs.

Furthermore, a garden roof is where plant and animal species can find an ideal habitat.

Finally, on a green roof, it is possible to create social relations and develop collective activities. It should also be remembered that green roofs extend the life of a shelter because the sheaths under the turf are protected from sudden temperature changes.

But the reasons for evaluating the installation of a garden roof are not all here; the benefits that can be obtained go beyond the building itself. The vegetation retains fine dust and pollutants, reducing the concentration of contaminants in the air.

The green roof is also a solution to reduce the heat island effect, as it performs a microclimatic mitigation action by returning the retained water to the air in the form of steam. In this way, the humidity of the air increases, which refreshes itself. Finally, the value of restoring critical green spaces in urbanized places, with the possibility of creating new usable surfaces.