A PIONEER: interview with OT HOFFMANN, architect of the TREE HOUSE in DARMSTADT

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BCJ: Dear Ot Hoffmann, for decades you have worked with combining green and nature with the city and its’ architecture. What is your approach and how long you have been working this way?

OH: To put it in somewhat intellectual terms, the approach is the reconciliation of technology and nature.

When I had completed the construction of the tree house, around 1970, there was a public outcry, an uproar ranging from being called an idiot to personal insults that also appeared in the press. And then, quite amazingly, a complete change occurred as the house was slowly covered with green, and has, since then, even been included in the sightseeing tours.

My first project was perhaps the trigger, to deal with it myself. At some point in the sixties, I had built my first house with a planted flat roof, later published in 'Bauwelt'.  At that time nothing was known about green roofs. I was a bit critical of myself and did not apply the greening directly onto the roof sealing. It was a so-called cold roof, an aerated flat roof made of asphalt and gravel covering the asphalt roof. I then made large planters 15cm stilts, so the planters had no contact with the gravel. Then I planted them making for a very nice roof.

After 10 years, the roof began leaking. When I started clear the roof I found that at the extreme point the root of a mountain pine had grown through the drainage hole of the trough and then had continued growing through the air into the gravel and through the bitumen into the roof. Small cause, big effect! That cost quite a lot. Since then I knew this method was not possible. However, for me, there was still such a contradiction, because in Berlin there were indeed many flat green roofs that were planted ​​on this basis in the 1930’s. With much research I found out that these roofs had a tar-based seal. But today, roofing tar is banned for health reasons, and you have to use bitumen or plastic covers. As it turned out bitumen is a wonderful nutrient, within a year or two, dandelion roots come through. Tar has practically an aseptic effect, that is, the roots die when they come in contact with it. This is the reason that in Berlin you even have flat roofs with pioneer trees and many other plants growing which would not be possible with a normal bitumen sealing alone.

Here in the tree house I conducted an experiment using a waterproof concrete roof without seals, and added the corresponding material: thermal insulation, filter layer, fleece, soil. An experiment, mind you, because it was not an acknowledged rule of technology. If you as an architect do something that does not correspond to the rules of the art, you are liable. But here it was my own money, so I tried it. I did this later on, at my own risk. Even later on, it became the acknowledged rule of technology. I would do it the same way today, also for financial reasons, with a waterproof concrete roof with the appropriate precautions that are necessary. I wouldn’t take just any company that is expensive, I would go for a simple concrete infused with inexpensive retardant and that's the whole recipe. The concrete must thicken slowly so there's no shrinkage cracks, and the advantage is that you can flood the tub when it is finished so you be sure it is watertight. And if it is then, it remains really tight. Unless a root comes through, through the concrete...

And then we come to another wonder of nature: For planting I took pines, because on a trip to Scandinavia I noticed that they grow in a relatively thin layer of vegetation. In the archipelago of granite there is often only 10cm topsoil, therefore the pines have a vast network of roots. And then I saw some that had been knocked over by storms, which afforded me a view of the root plate and the rough granite floor beneath. There you could see that the structure of the root mesh was mapped as grooves in the granite floor. I suspect they somehow have worked the rock.

BCJ: Yes, I understood that the roots exude some acids that dissolve the rock-bound nutrients.

Ot Hoffmann: Well, that gives me the idea that at some point the roots grow slowly through the concrete. Then my heirs might have a problem.

BCJ: And ... what is your conclusion for green architecture?

Ot Hoffmann: Well - that was just a joke... The house is now 40 years old - I do not think that the roots will grow through the concrete. But it is of course very interesting to talk about the connection between architecture and vegetation, also because you can see, yes, nature is always the winner. If you let it go, it simply takes over the architecture as a kind of ground, as a growth substrate. Even if they do nothing everything is covered rather quickly with lichens and mosses of all kinds. Especially the concrete, it too, gets green really fast – and we still do not know what that does.

BCJ: Has your approach to reconciliation of nature and technology changed through the decades? Do you regard you aspects of objective and design differently?

Ot Hoffmann: Well, first I learned that in this juxtaposition of nature and technology, nature often wins. And then something happened that I would have never dreamed of. I thought I could do it with greening here, because I have advantages in its’ use, but after about three or four years I saw that my roof garden here has gotten followers.  And actually I found this the most beautiful thing. That the people saw that they could use their roofs, they did that maybe by simple means, with troughs - but they finally used their roofs.  I think that's really good, it is up to an architect to provide a model, an example, not only with things like that but also in general views and ideas. When my roof grew greener and greener and people saw me in the media, the TV had reported about my roof and the weekly “Stern” and other media brought reports, this gave an impulse to concern oneself with greening, even among colleagues. Suddenly the madman changed into a pioneer, like a birch tree... (laughs)

From childhood on nature had a greater meaning for me than usual. Even though I grew up in the city, in Münster in Westphalia, from the earliest days of childhood I was very often in the country. One can call it a romance or love of nature - whatever. Anyway, I lived a long time, up to 8 weeks at a stretch in pure nature and I eked out a living on a former pasture in the Austrian mountains, 1260m high. I did this again and again, for 35 years. Fetching water from the spring, picking food from nature with primitive equipment, no electricity and no running water. Since than I have realized that I cannot live without nature in the city, nor can I always live in nature. I need people, the culture and many other things. But to me it was important to take the car and drive four hours and live out there, in nature, for a few weeks, writing a book or doing something else. It was because of this important connection to nature that the idea of ​​the vegetated roof came up. I told myself, I have to take my age into consideration because then I perhaps couldn’t or wouldn’t be able to take the car and drive many hours. I've got to address my dwelling here - therefore everything has a little "cottage character," improvised and growing nature. Most of what grows here now, was not even planted, but blew in on its own. I then observed and mapped out how these types of vegetation came about, why it happened in this way and so on. This is the connection between technology and nature or urbanity and rural life. That's the change that I've taken to imbue my nature into my home, or better infuse into my house. Laypeople came with this criticism: "Yes, you live in your urban green, but the view is so corrupted by the green, and there is hardly enough sun." It's amazing how people react to something like that, so ... but I feel comfortable in this hedge. But ... that's not the way for all people and we need to recognize that.

BCJ: Through the design of your tree house that you presented, at the end of the 60’s, an opposite model to the almost exclusive rural ideal of cities and living arrangements, a model that shows how the proscribed uncomfortable and unhealthy cities could be turned into” living” places. Do you think now this "negative image" of the city has changed in the minds of people, and if not, do you see enough effort that could cause this change today?

Ot Hoffmann: Well, I do not see any major approaches. There are examples here and there, but you can already see, when browsing architecture magazines, that the direction goes elsewhere today. Oddly enough, it has indeed a direct benefit to the people, to have their roofs green, or greening on a patio or somewhere around or near them. But today the formal approach to design is at the forefront among colleagues and not the functional or ecological approach. At best, green is taken as an embellishment of streets and such.

BCJ: But it's already this way in connection with the shrinking cities. City planners think about and make plans to upgrade the city again.

Ot Hoffmann: You are now speaking about the living environment, not about the building itself. There are, indeed, many changes, especially by urban planning and by the requirements and possibilities being altered. For example, the people are given 15cm greening space on the sidewalk. When you walk through the old quarter, you see they really have changed a lot. And since I’ve been in Darmstadt so many years, ever since my studies, I have a good assessment. On the outside the impression of the living environment has changed a lot - but the actual houses have not changed that much. The people who live inside don't benefit from it that much. Outside, it's also important of course - I do not want to belittle the living environment around us - on the contrary, it contributes to the wellbeing of all. But there is a strange discrepancy. Perhaps it is that one does not want to have anything on the house itself, people often feel the Greening as annoying, annoying when ivy or Virginia creeper is growing along the facade. This could harm the house, or conceal the beauty of the facade. Thus, architecture is still the hostile counterpart to nature. And that's why I handled the somewhat vague notion of reconciliation in order to show clearly, that I do not accept this contradiction, although when ivy is growing through the silicone of my window frame…

BCJ: In your essay "The participation of Nature in building" you speak about the relationship of humans with nature and the human aspiration to control nature in the broader sense. But as we have seen, today, again and again, man cannot definitively guarantee natures’ security. It is just this misunderstanding by which man tries to stand up to or over nature, that leads to the most fatal consequences. Otto Friedrich Bollnow writes in "The City, the Green and the Man": "Living in a house is ultimately insecure and man gains a real sense of security only if he knows his house is inserted into green, i.e. in living and life-supporting nature. Do you think that a stronger involvement of nature i.e. vegetation into architecture would help to boost a rethinking in this direction, or do you think that this, again, would only be understood by the people as a confirmation of their control over nature?

Ot Hoffmann: First of all, this theoretical approach is made ​​by people like us and not made for people who think differently, people who think like, as we just said: “my beautiful white facade, this has to appear first of all !” But the starting point is: Will the standpoint of the people to rule over nature, alter or not when you increase the connection between house and nature? And just as big a problem is the technical effort that is needed to bring nature into the house. In a certain way it is even predicted, so to speak, that man must rise above nature, when he wants to be close to nature. He has to make a pot for nature, a bucket. Nature in buckets, as it were, is a synonym for how man wants to rule over nature.

BCJ: As with bonsai.

Ot Hoffmann: Yes, bonsai are the extreme. The roots may not grow out of it’s enclosure- I can put it wherever I want. (Interruption, someone comes to check the automatic irrigation system). Now here you have - in line with our theme - a living example of technology and nature. This roof garden was essentially tuned to the normal rainfall as sufficient for irrigation. Until three years ago, no irrigation system was necessary. The retention basins were chosen, with those dimensions, so that the normal amount of rainwater was the sufficient amount for irrigation. There are retention basins everywhere and the rainwater, which flows over passes to the next one and another overflow runs into the next one and so on. But six or seven years ago, the climate began changing so rapidly that the amount of water is no longer sufficient.

Yes, it has become much drier. When it rains, it rains very heavily - with the normal roof all the water flows away, here at least it is caught. But the amount insufficient and rain falls at a time when I do not need it. We have gotten dry summers; it's almost a steppe climate here, especially in the area of ​​microclimate, which is important. In addition, the wind rises on the roof and takes a lot of moisture away. Yes, and because of this I had to install this additional automatic irrigation. So you suddenly are attached again to technology with nature, which is so pretty. There, we have a connection, Nature - Technology that goes the other way round, not a very pleasant thing to know that if one leaves out the automated components perhaps it does not work.

BCJ: In the middle of the 80s in the introduction to your "Handbook for green and utilized roofs” you called the "inoperability" of greening as "a chance to bring the dimension of feeling back to architecture again, as a better alternative to 'after modern architecture'. In fact, architecture has always tried to convey certain impressions and feelings in different ways, but aside from the Garden and Landscape Architecture plants often played only a minor role.

Ot Hoffmann: Well, if you see photographs of interiors from the Wilhelminian time you see that the interiors were overloaded, you see that in the 19th Century there was a very long period in which one had a lot of plants. Then the palms entered the house. Look at old photographs and old postcards of interiors. In parallel the greenhouses were cultivated again, this was such an impulse but this became stunted again. At that time the rooms were so big that you could really place palm trees in order to create this exotic style ... Green was the cherry on top! We have had this already, as you see... And now it's just the winter garden, people build an additional room but also use it as a green space and wintering area for Mediterranean plants in the house. So something has already happened. But whether it can be so exaggerated, as I said in the preface, I now have my doubts. That people have presumed to do this, apart from their ideas, that they, in an emotional sense, accept the plant as a partner, so to speak, as a pet in the home.

BCJ: As a potted plant.

Ot Hoffmann: There’s nothing wrong with potted plants. In their flats people have little choice.

BCJ: Potted plants are really partly understood as a partner. Ferns act, as an example, like contacts for a single person.

Ot Hoffmann: Everything that you charge with emotions acts as a contact partner. All things are their partners.

BCJ: Shouldn’t one describe the architect, at least as he appears in history and often still today, as a 'constructive sculptor', who acts at times rather pragmatically and functionally, sometimes very expressively and elaborately, but always regards 'his building' as a clear defined structure, which provides little to no room for the sprawling "proliferation" of plants or "adaptations" of later users.

Ot Hoffmann: Yes, it's the criticism that we have already mentioned, but the criticism must go further: This kind of architect should be abolished. Is untenable in our time. The new role of the architect should be, to create space for the user that offers the possibility to design, apart from wallpaper, curtains and such things that he doesn’t actually really need. This is the future task of the architect. If an architect would plan a terrace like this one here [on his own house, ed.], from the very beginning with trough in front and some plants in it, he would have made so many preconditions that people could not do anything else but add a little plant here and there. No matter what they plant there – whether a Thuja or buxus or whatever else: this does not matter – it is a beginning, the start of a seduction.

BCJ: This reminds me of Louis Kahn, who wrote: "In my opinion, the size of an architect depends more on his ability to fulfill what is home, as his design of a house which is a process related to the circumstances. A home is the house AND its inhabitants. A home changes with each resident "(Louis Kahn," Form and Design "in Vincent Scully, Jr.:" Louis I. Kahn ", Ravensburg 1963). That is the essence of it.

Ot Hoffmann: It goes even further. Man has basic needs. I divide them in the primary ones: food, clothing, housing and in secondary and tertiary ones belong design, the driving force to create. Every person wants to create in some way: with his clothes or today, also with his home, in our eyes, often in an adventurous way however: he wants to create. It's a necessity. And our job is to provide him with the possibilities to fulfill this design impulse and designing with green is one of those ways and one of the last. Because our building process is largely regulated everywhere. Think about this: what are you doing on the roof, it’s free! The roof is a free space! In our world where everything is regulated, also the roof pitch, the roofing, just anything, there is an area that is completely free. Absolute freedom. This is on the one hand the garden on the roof, yes, and on the other hand the green facade.

BCJ: Nevertheless some architects claim, in residential areas, to intervene in the garden design by imposing where fences should be or what kind of awning is used.

Ot Hoffmann: Oh yes, not only architects, but also, quite often in the local building regulations or in the textual requirements within the development plan that it often is included. Then architects cannot do anything else. I would like to talk with Ungers about what he thought when he put down his blocks with holes if he had the people in mind who would live there, or was it completely irrelevant for him.

BCJ: In my eyes Unger executed a very sculptural design approach in his projects that is in great demand in many public buildings as well. This is clear, just have a look at the many unique gems that were and are being built in the cities. This can sometimes lead to reductions in other qualities. If one regards green now as an alternative or necessary complement to the more common and more sculptural design elements (i.e. shape, proportion, color, materials) which is considered to give architecture this particular, shall we say "emotion" and the sense-appealing expression: How do you regard then the qualitative difference between the "emotional" architecture of F. I. Libeskind and, for example, the "emotional" green architecture of the tree house?

Ot Hoffmann: I think don’t that we should be lumped together. You have already distinguished between residential architecture and public buildings. In my view it would be rather absurd to cover a community building or a theater with greening. You can do that - I don’t mind, but what does that really mean? For whom? Maybe so, we even need to increase this contrast that existed between the residential architecture and the architecture used by the public. Just like the contrast, as in the Middle Ages, between the Cathedral, this gigantic structure, and the mud huts around it. For me such a contrast between residential green architecture and a largely non-green public architecture would be conceivable. You cannot have green everywhere.

A good example is the pedestrian street in Darmstadt. Look, with the first draft in 1970 we attempted to identify the character of the street in the city. Keywords: Stone, city, street, the attached elements were cubist and precise. The walls were piled concrete elements, as the water splashed down – that was a very tough image - despite the soft water. Then they were transformed into a contemporary form, benches were installed and trees were planted, the greening served as an embellishment. The trees are absolutely not viable because, every few years they need to be replaced, or they become wispy because they want to grow to the light. But it's green. And today green is taken as a label.

It is the contrast between the stone city street and the green park in the city that would be a strong structuring and design element. But so everything is mixed, green is beautiful, so somewhere it must go green even there where it isn't suitable. One might sometimes think of the poor trees. When one talks of partners, one should ask how your partner is feeling. On the Zeil in Frankfurt there are sycamores and one maple. There's a whole range below the trickle because a subway station is built underneath. They are watered underground and also automatically receive food underground. And in the fall people wonder why everything else is bare, only these trees are still in deep green. Then, when the frost comes all the leaves and shoots are hanging down. Now, that is going to be removed. I think this is right. It's absurd! 
 So, no green at any price, anywhere.

BCJ: Where do you see the greatest resistance to greening buildings? Are there more pragmatic, technical and financial reasons or the desire of the customer or architect for "clean" architecture?

Ot Hoffmann: The architects no longer have to say anything at all once the house is finished. We can forget the architects. Whatever he has built, if the user wants green and is greening the facade, architecture is tossed in the bucket. The architect has nothing to say any more, he's gone. So we must think of the users, what attitudes does the user have?

Well, as I said already: in the residential environment, in these narrow streets, for the most part one can not do more than greening the facade. Look around while you’re walking and you see that the user is thinking about this.... but nevertheless, quite a lot has happened already.

BCJ: It's probably just the fear of the plant or about the effort to maintain it.

Ot Hoffmann: Hesitations of the users? Well, they are already doing quite a lot... Now the question would be: What is the reason for the non-action? It already happens everywhere ... and it is marvelous to see how it is increasing...

BCJ: Could be more.

Ot Hoffmann: You're immodest. You are too young! (laughs) What is the reason for those who don’t? Yes, ... this cleanliness, this hygienic, this clean …

BCJ: I think that the users often do not do it just because of certain stereotypes that result from ignorance. It is also true that you have to bring along some knowledge, you cannot place one particular plant into every type of façade, ivy, for example can also be very destructive. There is a considerable lack of information.

Ot Hoffmann: Yes, but for the layman there is a flood of publications now about green facades. If they want, they would not have difficulties in realizing this. It is now important to figure out why those who do not do it, do not want it. And I just think, from regarding people as they plant their front yards and all, which they really want to have all clean and tidy. And, they feel the greening is messy. And it is indeed, that is the beauty of it! That it is a mess that's gorgeous, it’s chaos. There is an architect who has designed, perhaps even following the golden ratio, a house, designed so beautifully, and then a tree is growing or a plant covering everything. Gorgeous! They don’t care about the golden ratio; they have it inside of them...

BCJ: Thank you very much for this interesting long conversation !