Green roofs and walls: Solutions for making our territories future-proof ?
Today’s territories are under pressure as they are facing many sustainability challenges (social, environmental, and economic). Nevertheless, green space, having an important role in our territories for our wellbeing and for tackling climate change, became more and more scares in the past decades. It is here that green roofs and walls have a great, still often untapped, potential to contribute to the livability of our territories and thus, contribute to the development of smart, sustainable cities.
Indeed, they can contribute to the habitability of territories and, at the same time, to the development of sustainable and smart cities, since these techniques are capable of meeting some of the ecological, social and economic challenges that territories are dealing with
Green roofs and walls: What options for our territories?
We distinguish different types of green roofs and walls based on the type of vegetation, maintenance and system.
Can all roofs be green?
When we talk about green roofs we are referring to the installation of vegetation on the roof of a building.They can be found on the roof of a single-family home, or on a larger area such as the roof of a building or business. However, this system is mostly installed on so-called flat roofs. But can also be placed on sloping roofs as long as the slope is not steeper than 20°, as water would run off too quickly instead of infiltrate causing the substrate to slip off. The carrying capacity of the roof also determines what type of green roof is possible. In general, three different types of green roofs are determined by the maintenance requirements (see image 1).
Extensive green roofs: the most popular type
An extensive green roof is the simplest and least expensive one. It is therefore not surprising that this type is the most commonly used, especially when it comes to installation on existing buildings. For extensive green roofs, sedums, also known as succulents, are planted on the roof. These plants are used as they are very well in coping with droughts and do not require much maintenance.
This green roof consists of five layers, (1) the vegetation layer, (2) the substrate layer that the plants grow in, often consisting of lava rock, granules, and organic matter, (3) a root cloth to prevent roots from causing blockages or leaks, (4) a drainage layer to drain excess water and (5) a waterproof under layer. For an extensive green roof, the substrate layer is quite thin, ranging between 4 and 12 centimeters. The cost to install such a roof depends greatly on how accessible the roof is, the scale of the project, the shape, the height and the presence of obstacles (ex. windows, ventilation), but is usually between €40 and €110/m². The maintenance of an extensive green roof is comparable to a normal roof where you also have to clear the drains and remove dead leaves from surrounding trees every year. Although few weeds thrive on this type of roof, it may be necessary to remove them annually to maintain the desired natural type. Young plantings may require watering during drought conditions, although this vegetation can withstand drought relatively well. Some companies also offer maintenance contracts for green roofs.
As for the bearing capacity, you should take into account an average weight of 58 kg/m², depending on the substrate thickness. Also, the roofing should be of good quality when installing a green roof to avoid future leaks.
Semi-intensive green roofs: supporting local biodiversity
The main difference between an extensive and a semi-intensive gree roof is that the for semi-intensive green roof a thicker substrate layer is chosen, which makes it possible to work with herbs and grasses in addition to sedums. However, it is important to ensure that these species are drought-resistant, although we have seen that withered herbs shoot up again on their own in the autumn. One type of semi-intensive roof is the so-called ‘nature roof’ that mimics nature types from the environment by using native species to support local biodiversity.
The cost price of a semi-intensive roof is somewhat higher per square meter, as it amounts to approximately €70-145/m². The substrate layer is usually between 8 and 20 cm thick, making the weight up to 100 to 300 kg/m². A semi-intensive green roof is best maintained 1 to 3 times a year for the same reasons as an extensive roof plus the removal of dead grasses and herbs.
Intensive green roofs: a real rooftop garden
In an intensive green roof, the substrate layer is at least 15 cm thick. This type can be thought of as a garden and is therefore more likely to be installed by a garden contractor than a roofer. The roof can even contain bushes and trees, although this requires at least 50 cm of substrate and an irrigation system. It is often accessible and can have different functions such as for growing crops in the open air or in greenhouses, for a terrace, catering, sports and meeting.
Some elements to keep in mind for this type of green roof is the weight and its price. For these green roofs, the weight is often between 200 and 600kg/m² which makes it not so easy to add them to existing buildings. The price is also higher and starts from €100/m². Finally, the maintenance can be compared to that of any garden.
Solar panels and green roofs: a win-win solution
Solar panels are also very suited to be installed on flat roofs. This might create the idea that one has to choose between installing solar panels or a green roof. However, the combination of these two is possible and is even an excellent option. This is because solar panels perform best when the surrounding temperature is not too high (25°C on average). But on a standard black bitumen flat roof it can get as hot as 80°C in the summer. By combining solar panels and a green roof, the temperature can be lowered to 35°C, so the output of your solar panels will be higher. There are also other advantages. For example, the shadow of the panels creates a different micro-biotope on the roof that benefits fauna and flora. Also, the roofing is protected so that you don't quickly get into the situation where the solar panels have to be removed to renew the roofing.
Not only our roofs could be greened. There is also a lot of unexploited potential to green the facades of buildings. Below we discuss different types of green walls for outdoor facades. For green walls, a split is made between ground and non-ground types, depending on whether the roots are in the ground.
Ground-based green walls: the solution coming from the ground
For this type, the plant starts from the open ground and gets its nutrition and water there. This therefore requires a planting hole along the facade with sufficient fertile soil. In this category, a further distinction is made between plants that attach themselves directly to the facade, and plants that use a climbing aid.
Plants attached to walls
This type of green wall is created by plants that use their roots to grow against facades. Typical examples are ivy and vine (example see image 3). These are plants that grow very quickly so you will quickly have a full green wall, but as a result will need to do regular trimming to keep windows, doors and roofs clear. There are systems in place to fence these plants using an electric wire. This option is great for greening a blind wall cheaply and quickly.
Plants needing a climb aid
For this type of ground-based green wall, the plant grows a few centimeters in front of the wall along a steel wire or climbing frame. The advantage with this system is that it is easier to control the plant and it will not easily weed out. This system is sometimes even used to grow plants across the street, known as green slings. Which plant is suitable depends on how many hours of sun it gets and whether you prefer an evergreen. The downside is that in addition to purchasing the plant, you also need to purchase the climbing aid which costs around €7,5-180/m².
Living walls: an option that needs a lot of caring
Next to ground-bounded green wall, there are also so-called ‘living walls’, a type of vertical garden fed by an irrigation system that continuously pumps water and nutrients around (example: see image 4). Various grasses, flowers and herbs are anchored in this system attached to a facade.
Because rainwater does not infiltrate vertically, this requires a lot of water. Ideally, this is rainwater from the building's cistern. Care must also be taken that the pipes do not freeze in winter. In addition to its intensive water consumption, the energy required for such an installation (operation of the pump) and the origin of the substrate used make the sustainability of this system questionable. In addition, these are also expensive systems to purchase and maintain that are used primarily as eye-catchers.
Green roofs and wall: What are the benefits?
Now that we have discussed the different types of green roofs and walls, we would like to know the advantages of these for our territories.
We start with the benefits that help in reducing our climate impact. For example, these systems help reduce energy consumption. In limited measures, this greenery can contribute as an additional insulation layer of the building. In particular, it helps create a lower ambient temperature in the summer by evaporation and providing shade on materials that would otherwise retain heat. This lower ambient temperature reduces the need for indoor air conditioning. Vegetation on green roofs and walls also acts as an air filter for particulate matter and greenhouse gases, although this is limited for sedums. The effect is of course less than that of a tree, but it can be used in places where planting trees, for example, is not possible.
In terms of adaptation, green roofs and walls play an important role in counteracting the urban heat island effect caused by the large amount of surfacing with heat-absorbing materials. As a result, cities are on average 3°C warmer than the countryside. Due to the lower solar absorption (also known as albedo) of green roofs and walls, les heat is absorbed and only 20 to 30% of the solar radiation penetrates to the underlying layers. The evapotranspiration of the plants also cools the air, creating more pleasant temperatures locally. The magnitude of the effect depends on various factors, but would be between 0.5 and 2°C reduction for the environment.
Water is also an increasing problem for our territories as longer periods of drought occur and rainstorms become heavier, leading to flooding. Here green roofs in particular can play an important role. Depending on the thickness of the substrate layer, a green roof can buffer 15 to 150 liters of water per square meter. On average, a green roof evaporates 45 to 70% of the rainwater and allows the remaining water to gradually flow into the sewer system. However, in the event of heavy rain, the water may not have time to infiltrate the green roof. For this purpose, smart flow systems have been developed to take the weather into account and thus retain water during persistent droughts, while draining just before a rainstorm to buffer water optimally. Because other infiltration techniques such as infiltration pits or wadis are often not possible in urbanized settings, a green roof can offer a solution here.
In addition, green roofs and walls can also form an acoustic insulation to attenuate, for example, the sound of rain, hail or traffic. This reduction ranges from 10 to 46dB, depending on the type. These techniques also protect the roofing and facing bricks from UV radiation, which means they can last up to twice as long.
In terms of biodiversity, green roofs and walls are also the green stepping stones needed in cities to connect green spaces and increase habitat. This greenery is especially valuable when it connects to the local ecosystem. Sedum roofs therefore have limited value. Green roofs and walls also provide resting places, food and nesting material for the fauna of our territories. The value can be increased further by adding, for example, small elements such as water, gravel, and wood.
Finally, the economic benefits, in turn, are linked to the increased attractiveness of the property. This can increase the value by 4 to 8%. This is a result of the human need for nature to relax. Greenery in our environment provides many physical and mental benefits such as encouragement to exercise, better concentration, faster recovery of sickness, less stress, and more social interaction. It also results in more tourism and can be used for nature education. In this sense, green roofs and walls are an ideal solution for greening our territories without putting additional pressure on open space.
Green roofs and walls: some disadvantages
In addition to the many advantages, there are also disadvantages associated with the various types of green roofs and walls. A first disadvantage is the maintenance required for these installations. Because facades or roofs are not always easily accessible, this can be a difficult job.
Also, this greenery provides an extra weight that not all buildings are equipped for and thus the options are limited. Although green roofs protect the roof, if a leak occurs, it is less easy to repair.
As for climbing plants, we also often have a monoculture which is less interesting from an ecological perspective, although it is still much better than a stone facade.
Finally, the main disadvantage is that it requires an additional investment, the return on which is not as clear as with solar panels, for example. In addition, many of the benefits are also for the benefit of society and not purely (financially) for the investor.
Role of local governments
The question we try to answer here is how a local government can make our territories future-proof with more green roofs and walls. For this purpose we discuss a step-by-step plan with the aim of getting all stakeholders on board.
These steps, each with different measures (13 in total), are linked to the different groups of the adoption curve (see also Image 5). But even before the first group can be addressed, the local government must define its vision and mission (1) for the role of green spaces in its territory. This should be drawn up in consultation with all the actors involved and should consider the ecological, social and economic aspects of the project. Once the local government has a clear vision, implementation of the corresponding plan can begin.
- In phase 1 (for the innovators) it is a matter of adapting legislation and regulations (2+3) so that they promote rather than hinder green roofs and walls, both at the local level and higher levels of authorities. It is also necessary to bring different actors together to create a collective intelligence dynamics (4).
- In phase 2 (for the early adopters), it is important to inform and inspire (5) the various stakeholders. The many advantages of a green roof or green facade are often not known and too often the wrong choice is made. This informing has to be done internally as well as for citizens and companies. Providing information is especially important in the planning process for buildings or renovation because this is a crucial moment to start applying these techniques. Various media can be used, but events can also help to inform and inspire, for example by opening up green roofs, as happens in Rotterdam on the annual Green Roof Day. Setting a good example (6) yourself with your own buildings can also inspire others or by setting up pilot projects (7) as quick-wins.
- Phase 3 (for the early majority) aims to convince a large group that is already familiar with the techniques of green roofs and walls. For this, the local government can provide a financial push by offering subsidies (8) for the construction of green roofs and walls. This also recognizes the social value of these techniques. This subsidy can be especially useful to encourage citizens and companies to go for more qualitative systems with a greater positive impact. For example, the subsidy can be higher depending on the water storage. Also, the subsidy may require solid maintenance over a period of time. The subsidy can also be used, for example, to pay for the stability study (10) which is often also an early obstacle. These measures are especially interesting for existing buildings and small projects. For large projects, a municipal ordinance (9) can help make climate adaptive construction mandatory. This is already often done, for example, with regard to local rainwater infiltration, which must be sufficiently high.
- In phase 4 (for the late majority), the issue is to provide further financial encouragement for those for whom the investment is still too large. Thus, ways of co-financing (11) are explored for green roofs, just as is already done for solar panels. This relies on the idea that multiple stakeholders benefit from a green roof and would therefore be willing to contribute to the investment, rather than the owner alone. In return, there is then often a financial return in exchange for the cost savings. Group purchases (12) can also be useful to negotiate a lower price. However, care must be taken to ensure that the technology used is tailored to the site and that proper information is provided in advance. For green walls, this concept has often proven successful.
- Finally, phase 5 (for the laggards) aims to anchor these techniques in the long term. For this purpose, the elaboration of a green point system (13) for construction projects can be useful. In this point system, a score to be achieved for each new project is calculated depending on factors such as size. This score must then be achieved by implementing various measures of climate adaptive building that are defined in a list and yield a certain number of points. This allows creativity instead of mandating a green roof for every project and leaving no room for other functions of roofs. For example, the city of The Hague (Holland) developed in 2018 a "point system for green and environmentally friendly building", defined in 4 steps.
In conclusion, green roofs and walls have the potential to support our territories in their sustainable and smart transition as they allow to act, simultaneously, on the three pillars of sustainable development (environmental, social and economic).
However, it will be necessary to select the solution that best fits the building under construction in order to guarantee the resistance and longevity of the system, while optimizing its positive impact on the environment.
Finally, local governments have important levers to promote these techniques without requiring large budgets, but they will need to move forward in stages, considering the different phases of adoption.
- Final report of the internship of Benoit Ruysschaert 2021 for the City of Hasselt in the framework of a master at HEC Liège
Photo credits : Les Corpographes, Chuttersnap, John Moeses Bauan - Unsplash / Belinda Cave from Pixabay