BLACKPRINT: Mr Altzweig, with the Hamburg timber high-rise Roots you delivered a Europe-wide flagship project for sustainable construction in 2024. How did the project come about?

Kasimir Altzweig: I need to give a bit of background here. A few years earlier we had already taken part in another competition for the same client in the immediate vicinity of this site. At that time, the topic of timber hybrid played no role at all — it was only just emerging. The drive came above all from the younger generation in the practice, who were thinking boldly in this direction. As a result, we took the step of proposing a fairly radical timber-hybrid construction on a large scale for the site. We did not win the competition with it, but our proposal triggered an intense discussion in the jury between political representatives and all those involved. And ultimately this led to the City of Hamburg and the client finding a different site that could be procured through an innovation-led tender process.

BLACKPRINT: The master plan already foresaw a high-rise at that location…

Kasimir Altzweig: Yes, exactly. And then the question arose: are you prepared to deliver a timber-hybrid high-rise together with the architects who convinced you in the competition? And the client ultimately had the courage to do it. The story shows how important it is to follow your idealistic drive — even at the risk of failing. Because anyone who really commits to something meaningful often gets their chance on the second attempt.

BLACKPRINT: The result is a real game changer. Until then, eight storeys was the ceiling in Germany. How was this quantum leap possible?

Kasimir Altzweig: You really have to say it was a courageous step by the client. A simple "no" from a case officer at authority level could have stopped the project immediately. The decisive factor was therefore that all parties involved approached the project benevolently from the very beginning. And it is not about bypassing regulations. Because protective objectives such as safety are comprehensible and non-negotiable — after all, we all want to live safely. But if everyone is prepared to examine alternative ways of reaching the same goal, then a timber high-rise of this magnitude is possible.

BLACKPRINT: At the same time, one has to bear in mind that Roots was not a research project but has to survive on the open market…

Kasimir Altzweig: Yes, the project has to function economically, with high-quality owner-occupied apartments and one third publicly subsidised housing. On top of this came the more demanding site conditions of the HafenCity location with ebb and flow, high water in the base and the complicated noise protection due to the nearby railway line. All these factors made implementation complex. But it is precisely this that makes the project particularly valuable and forward-looking today.

BLACKPRINT: What were the biggest challenges with regard to sound and fire protection?

Kasimir Altzweig: According to the master plan, a hotel was originally planned at this location, for which different ventilation and sound standards apply. For permanent residential use, significantly higher requirements apply. The problem here: timber brings with it particular acoustic properties that are valued, for instance, in instrument-making. In our case we had to coordinate the ceiling and wall assemblies very precisely in order to achieve the required acoustic performance instead. As for fire protection, we partly adopted a deliberately conservative approach in order to be able to use timber as a load-bearing material at all. The building has a sprinkler system, for example, just to be on the safe side.

BLACKPRINT: Inside, the effect of the timber construction is rather subtle, while from the outside the building clearly stands out from its surroundings. What role does the front-set glass façade on the high-rise part play here?

Kasimir Altzweig: The second skin was part of the concept from the very beginning and was virtually essential: it protects the timber from weather and UV radiation, creates protected outdoor spaces and provides noise protection. Hamburg and the north of Germany in general are windy; from the fourth floor upwards, balconies would otherwise be hardly usable. The glass can be opened, creates usable winter gardens, offers an additional safety level in fire protection and slows down the colour change of the timber. For residents, the façade brings clear added value: it creates an sheltered outdoor space on which you can set up a table or simply enjoy the view.

BLACKPRINT: The Roots project combines privately financed and 30 percent publicly subsidised apartments. How important is it to you that sustainability in urban planning is implemented in a socially inclusive way?

Kasimir Altzweig: Roots, the strategy lay in combining the high-rise tower with lower-rise volumes. As a result, the owner-occupied apartments in the high-rise finance the subsidised apartments below. This makes affordable housing possible without compromising on quality or sustainability. At the same time, we have developed alternative models, such as low-priced apprentice housing, in close coordination with industrial partners and timber builders — even before the first serious design takes shape. We are generally interested in where the opportunities lie and how affordable housing can be realised realistically, without sacrificing building culture or sustainability.

BLACKPRINT: What lessons do you draw from the project for future projects?

Kasimir Altzweig: The experiences with Roots help us enormously, especially in dealing with regulations that are constantly changing, usually for the better. In all of this, however, we critically examine whether timber really makes sense. In high-rise competitions we now often differentiate: the high-rise part may receive a different construction, while lower-rise elements are consistently realised in timber. But sometimes a CO₂-optimised solid construction can also be the better choice, for example if it offers more flexibility and adaptability in the long term.

BLACKPRINT: The Gascade office building in Kassel is also being realised as a timber-hybrid building. What is the particular challenge there?

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The modules of the solar façade at the Gascade headquarters are arranged like scales that turn towards the sun.

Kasimir Altzweig: The project emerged as a competition for a gas-pipeline operator. And that in a politically sensitive phase. Because the company was at the time 49% in Russian hands; today it is state-owned and an exciting partner in the field of hydrogen infrastructure. Absolutely central in this was the translating the company's identity into an architectural expression. We interpreted the topic of energy through an "energy façade" and created an almost full-surface solar façade whose modules are arranged like scales that turn towards the sun. Once completed, the sculptural effect of the façade will be clearly visible.

BLACKPRINT: How is the building being realised structurally?

Kasimir Altzweig: The building is conceived as a hybrid construction. All timber columns, timber beams and slabs are supplemented by a minimally executed concrete topping, which provides fire separation between floors between the storeys. The serial construction method with standardised, prefabricated components enables flexible coordination, optimised completion and adherence to schedules. The rhythmic arrangement of the timber beams minimises the share of concrete, noticeably reduces the CO₂ footprint and at the same time makes it possible to intelligently combine standardised and individually designable areas. The atrium and the open staircase at the centre of the building are very important here, as they create free spaces for communication. The project is thus an example of how efficient, CO₂-optimised and site-specific attractive timber-hybrid construction can work without becoming a "prefab block 3.0".

BLACKPRINT: Where does the drive to engage more strongly with the topic of sustainability come from? And which tools do you use to integrate CO₂ aspects into the design at an early stage?

Kasimir Altzweig: The drive to engage intensively with sustainability comes strongly from within the practice itself, especially from the younger generation. For years we have had our own sustainability group, led by our associate partner Uta Meins, who organises internal events and drives the topic forward with great commitment. An important role here is also played by our tool ECO₂, which we developed together with partners.

BLACKPRINT: What does the tool do?

Kasimir Altzweig: We have been working digitally with 3D models for a long time, and the next step was to link different topics together. With BIM, information can be prepared at the early design stage so that design options can be directly compared in terms of their CO₂ impact. And that is exactly what the ECO₂ tool enables us to do: we can assess as early as the concept designs which variant has which consequences. At Roots, this allowed us to optimise material use and construction early on, in order to significantly reduce the CO₂ balance without restricting user flexibility or architectural quality. At Gascade, ECO₂ helped to specifically steer the serial hybrid construction method with minimal use of concrete and at the same time fully exploit the advantages of the timber-hybrid system, for example in the rhythmic arrangement of ceiling beams and concrete topping.

BLACKPRINT: Alongside Gascade you are currently also working on an office building on the Behrensufer in Berlin. What exactly are you planning there?

Characteristic of the Behrensufer office building in Berlin is the horizontal layering produced by cantilevered floor slabs.

Kasimir Altzweig: The new building on the Behrensufer is a large-scale transformation project on the former AEG site, where electric vehicles were already being produced more than 100 years ago. The project adjoins a listed former production and office building and is conceived as a circular timber skeleton construction with a stiffening reinforced-concrete core, a large-scale glass façade and a grass landscape on the balconies. Further development is currently still open. Particularly exciting for us here was the partnering model — the close and early dialogue with the construction company, in this case the Züblin Group including its timber-construction subsidiary. The timber contractor was involved from the outset. Together we explored: what can you build particularly well? How does your production chain work? How can this be combined with our architectural structure? In general, you can say that we fundamentally enjoy working closely with industry. In all of this, however, we naturally remain critical, so that architectural quality is not compromised.

BLACKPRINT: Another outstanding project by your practice is the hotel The Fontenay in Hamburg…

Kasimir Altzweig: Our task there was to give an architectural answer to the park-like opening on the Alster at the former Interconti site. The building is organised radially, the rooms are curved, everything is in motion, without dead ends, with flowing movement through atrium and patio. Ecological sustainability was not in the foreground for this project. Decisive was rather the socio-cultural value: the architecture creates high-quality, long-lasting spaces that are strongly accepted by the public. The project shows that high-quality architecture can itself be a contribution to sustainable urban development if it is characterised by durability, experiential quality and design quality.

The Fontenay hotel in Hamburg captivates with its organically rounded building form.

BLACKPRINT: Given urban densification and the challenges of climate and resources: what role should architecture and planning play in the next ten to fifteen years for a real sustainable transformation?

Kasimir Altzweig: Responsibility is more important today than ever before, not in the form of maximum demands such as CO₂ reduction at any cost, but specifically at the opportunities for innovation: where can we work more closely with industry? Which building structures enable cost-effective building without giving up ecological standards? Hasty measures for short-term effects do not get us any further. Architecture has to understand clients and industry better, create optimisations, enable affordable housing and at the same time remain environmentally responsible. Sustainability encompasses more than CO₂: socio-cultural aspects and design quality also count. A building that is demolished after a few years is anything but sustainable. Good architecture itself is a contribution to sustainability. But this also means that radical ideas cannot always be implemented immediately. What is decisive are rather the many incremental improvements that collectively deliver more sustainable outcomes.

BLACKPRINT: Mr Altzweig, thank you for the conversation!

The interview was conducted by Robert Uhde.

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