How our cities can green up again
Since 2001, Prof. Dr.-Ing. Jörg Dettmar has headed the Department of Design and Open Space Planning at the Faculty of Architecture of the Technical University (TU) Darmstadt, conducting research and training architects in the Urban Design and Planning Group. We were able to talk with him about the greening of facades and roofs and we learned about its ecological importance and what can be done to make it even more popular with municipal and especially private builders.
Professor Dettmar, what exactly does your specialist group deal with?
Teaching is about looking at open space design mainly in cities, and building greening plays a role in that. Open spaces are all that is not occupied by buildings, such as roads, railways or the open spaces with parks, squares and pedestrian zones, as well as green corridors or similar areas that have primarily social and recreational functions and, in the context of urban planning and development, very important ecological functions in addition.
How has building greening evolved historically, and where are we today?
Actually, this is nothing new at all. Already in the ancient civilizations, in Mesopotamia, in Greece or with the Romans there were different forms of building greening especially on roof terraces. The famous hanging gardens of Babylon were nothing more than a green roof. These were mainly representative buildings.
In Europe, especially in Scandinavia and Iceland, green roofs have been very common for centuries, simply because there are many benefits associated with them, even for simple homes.
What are the advantages in detail?
Historically, it was initially mainly about waterproofing and insulation. If you put sod on the roof, as in Iceland and Norway, you get extra insulation in the winter time and you also have advantages in terms of durability of the roofing or the roof structure.
Nowadays, yes, people look at building greening differently, and more benefits or advantages are added …
First of all, I would differentiate. When we talk about greening buildings, we have to distinguish between green roofs and green facades.
In Germany, green roofs have actually been established since the 1970s in certain types of buildings, mainly office buildings. Germany could certainly be called the world champion when it comes to green roofs. In my estimation, there is no other country in the world that has such a high percentage of green roofs on commercial properties. It all started in the early 1970s when, in the context of the environmental movement, people became more sensitive to the fact that buildings seal soils and also take away habitats from plants and animals.
The question came up, why don’t we just pack a “green blanket” on top of the building and create a replacement habitat? This initially very simple approach was then gradually formalized in the so-called nature conservation compensation regulations. This stipulates that certain compensatory or replacement measures must be taken for interventions in nature, and this includes green roofs. For this reason, green roofs are often still stipulated in the development plans, which means that if you want to build a building here, it is difficult to get around it. For a long time, however, this only affected green roofs.
In recent years, this has changed because facade greening is also becoming increasingly interesting. This is mainly related to climatic aspects, because we know that in the course of climate change, the climatic situation in our cities will become even worse.
Can you elaborate on that?
We’re talking about heat, urban heat islands, and tropical nights in cities that have health implications. With nighttime temperatures above 20 degrees, it can become life-threatening for the elderly and those in poor health. What you can do about it on the ground always has a lot to do with “green”: You can plant more trees, create more green spaces, but you can’t do that easily in high-density urban neighborhoods. That’s why all that’s left are the buildings and the question: How do you get vegetation onto the building? The greening of the facades is particularly interesting because the roofs are usually relatively high up, and people on the first floor level do not have much of that. The facade greenery can be pulled down to the ground, and thus it has climatic effects and balancing functions where people mostly stay.
Can these effects be quantified?
Yes, there are now a number of measurements of the microclimatic effect that demonstrate a relevant reduction in surface temperature. On an exposed building facade with gray exterior plaster, temperatures of up to 80 degrees are reached in summer, while a comparable green facade only heats up to 30 or 40 degrees and thus hardly more than the surrounding air. This is a relevant difference.
In addition, there is the problem of increasingly frequent heavy rainfall events in cities and the lack of capacity to retain the water on site if possible. This is one of the most important reasons for green roofs today. To this end, it is important to know that even extensive green roofs are capable of retaining up to 50 percent of the annual precipitation, which means a massive relief for the sewage system – a very strong argument for green roofs.
We also know that urban vegetation elements are capable of filtering pollutants from the air. A great deal of research and testing is currently being done here to determine which greening elements are the best and most economical way to achieve this. More important than catching, however, is avoiding.
With all the ecological and climatic advantages there are also disadvantages, for example for allergy sufferers, by insects or for the building fabric?
Let’s start with the last one: Again and again we are confronted with the fear that building greening could damage buildings or rather their facades. However, this only applies if greening is not carried out professionally, for example through incorrect plant selection. Ivy can grow into joints and cause damage. Or wine, whose suckers attach to facades. At least on sensitive facade surfaces, these plants should not be used. However, there are designs with which such problems do not arise.
Roof and facade greening should always be carried out professionally. In the meantime, there are corresponding sets of rules that can be used to ensure that no damage occurs.
Allergies are indeed an issue. When greening hospital buildings, plant species known to potentially cause allergic reactions must be left out, but there are always alternatives.
This then probably also concerns the question of which insects settle on and on buildings …
The fear of this is widespread but poorly founded. I don’t know of any research that has really demonstrated an increased incidence of insects in greened buildings. Of course, these are habitats for spiders or insects, but also for those that keep annoying pests at bay. In general, we should actively do something for more biodiversity in the city.
What technologies are available today for greening facades and roofs, and which plant species are suitable?
Basically, a distinction is made between intensive and extensive green roofs. This is essentially determined by the respective substrate structure. After all, plants need substrates in which they can take root and grow. If this substrate structure is less than fifteen centimeters thick, it is called extensive greening. This does not need any further supply of water or fertilizers or the like, but lives essentially self-sufficient, except for necessary maintenance interventions.
Intensive greening, i.e. everything that is thicker than 15 centimeters, must be supplied with water and nutrients, i.e. more intensive horticultural care. Of course, essential is always the load-bearing capacity of the structure, which is located underneath. You can’t, without checking, just put tons of extra weight on a roof.
In facade greening, a distinction is made between ground-based and facade-based systems. In the former, the plants take root in the soil in place. This assumes that there is enough soil for growth. In densely populated urban areas with corresponding houses, narrow sidewalks and diverse piping systems in the ground, this is often difficult to ensure, which is why façade-bound systems have been developed, which are installed autonomously on the façade and then, however, must be technically supplied with water and nutrients.
So as a builder, you don’t necessarily have to reinvent the wheel …
Certainly not. Especially in the last ten to 15 years or so, a whole range of manufacturers have come onto the market offering systems for facade greening. These systems can be purchased virtually complete, including warranty.
Greening bus stop roofs seems to be pretty popular right now. What do you think that brings to the table?
For one thing, I think this has a very significant psychological effect, which is about nature in the city. The green on bus stops can still be perceived from the pedestrian perspective. In addition, even these small structures have limited climatic balancing effects. This does not immediately make the entire city cooler, but it is measurable in the immediate vicinity. Finally, however, a living space is also created here. As we discuss very intensively that biodiversity is decreasing in all corners, in the end every square inch of vegetation is a contribution to create habitats.
At the same time, buildings and supporting structures are also protected from UV radiation, among other …
That’s right. Provided that greenery is properly installed, layers of vegetation can buffer mechanical or radiation effects, giving buildings, in terms of facades or other structures, a longer life. The opposite would be the formerly popular bitumen roofs with tar paper, which heat up extremely in summer – they eventually become porous and have to be renovated every few years. However, green roofs also protect against heavy hail that can shatter roof tiles or tiles.
So the building greening proves to be very beneficial. How can their dissemination be further supported, for example through communication or other measures?
Communication is certainly important. But there is already a lot happening through public institutions – federal, state and local. There is an endless variety of brochures, of websites and planning tools. We at the TU Darmstadt are also working very intensively on this. The goal is for building greening to become a normal part of architects’ planning. This means designing buildings from the outset in such a way that greenery is seen as an integral part of facades or roofs. To this end, improved collaboration between architects and professional engineers is desirable, as well as between landscape architects and structural engineers.
One very important aspect, I think, is that we need to think about what cities should actually look like in the future. We have a wide variety of future fields that are driven by digitalization, new mobility structures, new forms of work, and of course the shape of the city will also change. In the process, we simply have to be careful that it doesn’t get too hot and that we get a better handle on the rainwater. Greening buildings is a very important building block in this respect.
The question is, after all, how do we perceive cities and whether we actually push this old division between city and countryside to the point where we only allow greenery into cities in a limited way, or whether perhaps we need to think further afield.
That probably costs money. What does the funding landscape currently look like, and what do you, as an expert in open space planning, think makes sense in order to further advance the greening of buildings?
Getting an overview of the currently available funding opportunities is not that easy. Many things in this area are regulated differently from municipality to municipality, sometimes supported by state programs. There are very spectacular things, such as in Paris, which launched an exorbitant facade greening program several years ago. Hamburg also has a current program to promote green roofs.
Support programs are not limited to large metropolitan areas, however; they also exist in numerous small towns. Of course, such programs help because they make the whole thing more attractive for private individuals and because they cushion the additional investment costs to some extent.
Private owners must be aware that the longer service life of green roofs or facades must also be taken into account, and that the life cycle costs that are repeatedly mentioned today do not make green structures look so bad. Nevertheless, there is still an additional expense, so there is no need to beat about the bush.
Plants are living things, and you have to take care of them. This means that every green roof and every green facade must be horticulturally maintained. The extensive green roofs less so – one or two inspection rounds a year are enough to make sure that nothing grows up there that might then cause damage. With extremely elaborate greenery that has the showmanship of hanging gardens, this becomes a horticultural freestyle. Greening is always an extra expense, but you also get an extra yield for it.
Green roofs and facades seem to be the current trend. Does this represent a change in thinking on the part of building owners, local authorities and transport operators?
In the context of global climate change, the issue is very virulent, and I hope that it doesn’t just remain a trend, but that it simply becomes a normal building block of how we organize cities in the future.
It is already present almost everywhere in the municipalities, and in many places people are thinking about how, beyond green roofs, green facades can also be legally stipulated, for example in development plans. This would make building greening regulated or required by law. An important contribution so that our cities can green up again.