Prof. Dr.-Ing. Jörg Dettmar from the Technical University of Darmstadt

How our cities can become green again

Since 2001, Prof. Dr.-Ing. Jörg Dettmar has been Head of the Department of Design and Open Space Planning at the Faculty of Architecture of the Technical University (TU) Darmstadt. He researches and trains architects in the Urban Development and Urban Planning group. We were able to talk to him about the greening of façades and roofs and find out what ecological significance it has and what can be done to make it even more popular with urban and, in particular, private developers.

Professor Dettmar, what exactly does your faculty do?

Teaching in our faculty is about grappling with the design of open spaces, mainly in cities, and the greening of buildings also plays a role in this. Open spaces are everything that is not occupied by buildings, such as roads, railways or open spaces with parks, squares, pedestrian zones, green corridors or similar areas, which have primarily social and recreational functions and, in the context of urban development, also possess very important ecological urban planning functions.

How has the greening of buildings developed historically, and where do we stand today?

Actually, this is nothing new. Already in the ancient advanced civilisations, in Mesopotamia, in Greece, and in the Roman Empire, there were different forms of building greenery, especially on roof terraces. The famous Hanging Gardens of Babylon were nothing other than a greened roof. They were mainly representative buildings.

In Europe, especially in Scandinavia and Iceland, greened roofs have been very common for centuries, simply because they offer many advantages, even for simple houses.

What are the advantages in detail?

Historically, the main focus was initially on sealing and insulation. If you put sods on the roof, as in Iceland and Norway, you get extra insulation in the wintertime, and you also have advantages in terms of the durability of the roofing or the roof construction.

These days, people building greening in a different light and there are even more benefits or advantages …

First of all, I would differentiate. When we talk about green buildings, we have to differentiate between green roofs and green façades.
In Germany, green roofs have actually been established in certain types of buildings, mainly office buildings, since the 1970s. As far as green roofs are concerned, Germany could certainly be called the world champion. In my estimation, there is no other country in the world that has such a high proportion of green roofs on commercial properties. And it all started when, in the early 1970s, in the context of the environmental movement, people became more sensitive to the fact that buildings impinge upon the ground surface and also take away the habitat of plants and animals.

The question came up, why don’t we just put a “green blanket” on top of the building and create a replacement habitat? This initially very simple approach was then gradually formalised in the so-called nature conservation compensation regulations. These stipulate that certain compensatory or replacement measures must be taken for interventions in nature, and these include green roofs. That is why green roofs are still often laid down in development plans today, which means that if you want to build a building here, it is difficult to get around that. For a long time, however, this only affected green roofs.
In recent years, this has changed because green façades are also becoming increasingly interesting. This has mainly to do with the climatic aspects because we know that the climatic situation in our cities will become even worse as a result of climate change.

Can you please explain this in more detail?

We are talking about heat, urban heat islands and tropical nights in the cities, which are relevant to health. Night-time temperatures above 20 degrees can be life-threatening for the elderly and people with weakened health. What one can do about it on-site always has a lot to do with “green”: one can plant more trees, create more green spaces, but that’s not easy in high-density urban areas. That’s why the only thing that really remains is the buildings and the question: how do you get vegetation on the building? Façade greening is particularly interesting because the roofs are usually relatively high and people on the ground floor level don’t receive many benefits from that. Greenery on façades can reach right down to the ground, and thus has a climatic effect and compensatory function where people usually spend most of their time.

Can these effects be quantified?

Yes, there are now a number of measurements of the microclimatic effect that show a relevant reduction in surface temperature. On an exposed building façade with grey exterior plaster, temperatures of up to 80 degrees are reached in summer, whereas a comparable green façade hardly heats up to 30 or 40 degrees more than the surrounding air and this is a relevant difference.

In addition, there is the problem of the increasing frequency of heavy rainfall events in cities and the lack of capacity to retain the water in situ as far as possible. This is one of the most important reasons for green roofs today. It is important to know that even extensive green roofs are able to retain up to 50 percent of annual rainfall, which means a massive relief for the sewage system – a very strong argument for green roofs.

We also know that urban vegetation elements are able to filter pollutants from the air. A lot of research and testing is currently being carried out to determine which greening elements are best and most economical to achieve this. Avoidance, however, is more important than collection.

Despite all the ecological and climatic advantages, there are also disadvantages, for example, for allergy sufferers, from insects or the building fabric?

Let’s start with the last one: again and again, we are confronted with the fear that greenery on buildings could damage buildings or rather their façades. However, this is only true if greening is not carried out professionally, for example, by selecting the wrong plants. Ivy can grow into joints and thus cause damage. Or grapevine, whose suction cups stick to façades. At least on sensitive façade surfaces, these plants should not be used. However, there are constructions with which such problems do not occur.

Roof greening and façade greening should always be carried out professionally. In the meantime, there are appropriate regulations with which one can ensure that no damage occurs.

Allergies are indeed an issue. In the greening of hospital buildings, plant species known to cause possible allergic reactions must be excluded, but there are always alternatives.

This then probably also concerns the question of which insects are attracted to and settle on buildings …

The fear of this is widespread but poorly founded. I do not know of any study that has really proven an increased occurrence of insects in greened buildings. Of course, these are habitats for spiders or insects, but also for those that keep annoying pests off our backs. In general, we should actively do something for more biodiversity in the city.

Which technologies for the greening of façades and roofs are available today, and which plant species are suitable?

A basic distinction is made between intensive and extensive green roofs. This is essentially determined by the respective substrate structure. Plants need substrates in which they can root and grow. If this substrate structure is less than fifteen centimetres thick, this is referred to as extensive greening. This does not require any further supply of water or fertiliser or similar, but lives essentially self-sufficiently, other than necessary care intervention.

Intensive greenery, i.e. any substrate deeper than 15 centimetres, must be supplied with water and nutrients, i.e. it must be looked after more intensively in terms of gardening. Of course, the load-bearing capacity of the structure underneath is always essential. You can’t simply pack tons of additional weight onto a roof without proofing it.

In façade greening, a distinction is made between ground-based and façade-based systems. In the former, the plants root in the existing soil. This presupposes that there is sufficient soil for growth. In densely populated urban areas with corresponding houses, narrow pavements and a variety of pipe systems in the ground, this is often difficult to guarantee, which is why façade-bound systems have been developed which are self-sufficiently attached to the façade but then must be technically supplied with water and nutrients.

As a builder-owner, you do not necessarily have to reinvent the wheel …

Certainly not. Especially in the last ten to 15 years or so, a whole range of manufacturers has come onto the market offering systems for façade greening. Some of these systems can be purchased complete, including warranty.

The greening of bus stop roofs seems to be quite popular at the moment. What are the advantages in your opinion?

On the one hand, I think this has a very significant psychological effect concerning nature in the city. The greenery on bus stops can still be perceived from the pedestrian perspective. In addition, even these small structures have a limited climatic balancing effect. The whole city won’t be immediately cooled down, but its effect is measurable in the immediate vicinity. After all, living space is also being created here. Since we are discussing very intensively the fact that biodiversity is decreasing at every turn, every square centimetre of vegetation ultimately contributes to the creation of habitats.

Buildings and supporting structures are also simultaneously protected against UV radiation …

That’s correct. Provided that the vegetation is professionally applied, vegetation layers can buffer mechanical or radiation influences, giving buildings a longer life span in relation to façades or other structures. The opposite would be the formerly popular bitumen roofs with tar paper, which heat up extremely in summer — they become porous at some point and have to be renovated every few years. Green roofs also protect against heavy hailstorms, which can break roof tiles or shingles.

The greening of buildings, therefore, proves to be very advantageous. How can their spread be further supported, for example, through communication or other measures?

Communication is certainly important. But a lot is already happening through public institutions – federal, state and local authorities. There is an infinite number of brochures, websites and planning tools. We at the TU Darmstadt are also working very intensively on this. The aim is that greenery on buildings should become a normal part of the architects’ planning. This means designing buildings from the outset in such a way that greenery is seen as an integral part of façades or roofs. To this end, improved cooperation between architects and specialist engineers is desirable, as well as between landscape architects and structural engineers.

A very important aspect, I believe, is that we need to think about what cities should actually look like in the future. After all, we have a very diverse range of future fields which are conditioned by digitalisation, new mobility structures, new forms of work and, of course, the form of the city will also change. We must, quite simply, be careful that it does not get too hot and that we achieve better control of rainwater. The greening of buildings is a very important part of this.

The question arises as to how we perceive cities and whether we really continue this old division between city and landscape in such a way that we only allow greenery into the cities to a limited extent or whether we perhaps do not need to think further ahead.

That probably costs money. What does the current funding landscape look like, and what do you, as an expert in open space planning, think makes sense to further promote the greening of buildings?

It is not easy to get an overview of the currently available funding opportunities. Much is regulated differently from municipality to municipality, partly supported by state programmes. There are some very spectacular things, such as in Paris, for example, which has already launched an exorbitant façade greening programme in recent years. In Hamburg, too, there is a current programme to subsidise green roofs.

However, subsidy programmes are not limited to the large metropolises; they are also available in numerous towns. Of course, such programmes help because they make the whole thing more attractive for private individuals and because the additional expenditure that is initially invested is cushioned a little.

Private owners need to know that the longer service life of green roofs or façades, for example, must also be taken into account and that the life-cycle costs repeatedly mentioned today make green structures much more feasible. However, it’s true, this still means additional expenditure, there is no need to beat about the bush.

Plants are living creatures, and one must take care of them. This means: every green roof and every green façade must be cared for from a horticultural point of view. Extensive green roofs less so — one or two inspections per year are enough to ensure that nothing grows there that could cause damage. In the case of extremely elaborate green roofs, which have the show character of hanging gardens, this becomes a horticultural spectacle. Greening is always an additional expense, but you also get an additional yield.

Roof and facade greening seems to be the trend at the moment. Does this stand for a rethink among building owners, local authorities and transport companies?

In the context of global climate change, the issue is very virulent, and I hope that it is not just a trend, but that it will simply become a normal part of how we organise cities in the future.

It is already present almost everywhere in municipalities, and in many places people are thinking about how, in addition to green roofs, green façades can also be legally stipulated, for example in development plans. This would regulate the greening of buildings or make it mandatory by law. An important contribution to the regreening of our cities.

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