Cities of timber

Timber is much more than just a building material. The natural forest product excites our senses and gives structures a unique ambience. Some even claim that wood has stress-reducing and calming effects, so it is fitting that cities are focusing more on timber construction for resource- and climate-friendly urban development. But it has taken many decades for a timber construction renaissance to emerge. Lignum, the Swiss Forestry and Timber Industry Organisation, reports that even in the late 1980s, there was little interest in timber as a building material. But that’s changing: according to Alex de Rijke, director of the London-based architecture firm dRMM, “Timber is the new concrete.”

Is this the beginning of the timber age?

As a renewable material, wood is clearly a promising alternative to steel and concrete. The principle of sustainable forestry practised for around 300 years seems to ensure that there will always be enough wood. Architect Michael Green claims that enough wood for a 20-storey building grows every 13 minutes in North America. A single cubic metre of timber grows every second in the forests of Austria. The blog holzistgenial.at reports that one third of the annual timber growth in the forest-rich country would suffice to provide an entire year’s worth of construction material.

Ecological, climate-friendly, technologically advanced

Timber as a building material seems to be plentiful in other countries as well. In an urban context, it protects the climate because each cubic metre of wood absorbs a tonne of carbon dioxide. Thus, timber structures extend the forest’s ability to store carbon and become part of the solution to the global climate problem. And in the production process, timber generates around 50 percent lower CO2 emissions than concrete, and about one third lower than metal. In addition, wood has a high insulation capacity and regulates room climate and humidity naturally.

Timber technology has also advanced considerably in recent years, with the introduction of new wood materials as a result of intensive research and development. Innovations like cross-laminated timber (CLT), a wood panel product made of lumber layers glued together, has virtually revolutionised timber construction. These large, industrially produced wooden panels offer the same compressive strength as reinforced concrete slabs. Computer-controlled calculation and manufacturing methods allow wide-span supports and tall structure heights. It’s no wonder that star architects like Shigeru Ban value the enormous potential of timber construction. In 2013, he created a seven-storey timber structure for the Tamedia media group in Zurich. The structure contains 2,000 square metres of spruce lumber from Austria’s Styria region.

Hybrid timber buildings offer completely new possibilities

Timber construction architect Hermann Kaufmann, the pioneer of modern wood construction in Europe, says: “Modern hybrid timber construction brings together the advantages of concrete, steel and wood construction, which creates entirely new possibilities.” A 2017 study published by the Zukunftsinstitut Österreich entitled “Die Zukunft des Holzbaus” (The Future of Timber Construction) elaborates: “Hybrid constructions with reinforced concrete are often built with a concrete circulation core containing stairs and lifts. Timber-concrete composite floors enable greater spans and thinner floor thicknesses in office buildings. These are coupled with good soundproofing and fireproofing properties.”

Compared to traditional structures, building with wood takes place more in a factory than on the construction site. Prefabrication of timber modules and serial wood elements is not weather-dependent and makes good business sense because it shortens construction time and therefore reduce building costs somewhat. Less than ten weeks of construction time have been slated for the 20,000-square-metrem T3 (Timber, Transit, Technology) office building in Minneapolis, USA. Furthermore, precision work in the shop provides a high degree of building accuracy. The Woodie student residence hall in Hamburg is a successful example of this modular construction style with prefabricated units. And finally, the reusability of wood has a positive impact on the project. Interboden, a project development firm based in Ratingen, is planning a hybrid timber office building in Düsseldorf called “The Cradle” in reference to the reuse of building materials in keeping with the cradle-to-cradle principle.

New building codes are providing momentum

Numerous institutions and initiatives are working to promote timber construction and raise awareness. Fire protection is one of many focuses. These days we know that timber supports of sufficient size hold longer than heat-sensitive steel. Beneath the charred layer, wood remains undamaged, which allows it to maintain its load capacity for a long time. Ultimately, however, barriers in building codes need to be eliminated so timber buildings can gain a foothold in cities.

In Hamburg, for example, timber buildings up to 22 metres high have only been allowed since 2017. The Hanseatic city had to change its building codes – following in Baden-Württemberg’s footsteps. “Germany’s state construction regulations are changing,” confirms Arnim Seidel, Managing Director of Berlin-based Informationsverein Holz e.V. Six to seven-storey buildings with loadbearing timber construction can now be built in Hamburg, compared to the three storeys that were previously allowed. However, Hamburg’s new 22-metre height limit for timber buildings is not an impossible hurdle. In fact, Garbe Immobilien-Projekte GmbH is currently planning Germany’s tallest timber building in the harbour city. The 18-storey Wildspitze building should be visible from far and wide starting in 2021. Garbe’s Managing Director Fabian von Köppen explains: “Changes to Hamburg’s building codes makes getting an exemption for constructing a 60-metre-tall building much easier.” Thus, the Wildspitze is a worthy competitor in the race for the world’s tallest timber building (see sidebar).

Investors look at construction costs and image

Cost-effective solutions are in demand, so building with timber has to prove itself against steel and mass wall construction. People have changed their thinking here, says Armin Seidel. “In the past, building with wood was considered light-frame construction ­– cheap and low quality. These days no one would seriously claim that timber construction is cheaper than working with mineral building materials.” Therefore, construction cost is a key focus. For the Wildspitze, Fabian von Köppen draws a comparison: “Timber construction currently costs 10 percent more than traditional construction. The precision provided by extensive prefabrication and significantly shorter building time partially compensates for the higher cost.”

However, Interboden CEO Vanja Schneider says that for The Cradle, “the issue of higher costs is secondary because I believe that we will find  owners and users for this environmentally sustainable building who value these principles.” His company believes in the positive image effect, and they may very well be right. A study published this year by Linz University confirms that companies with timber buildings are much better perceived by the public. “The reputation effect is a total of 12 percent higher than company buildings made from mineral construction materials,” quotes holzistgenial.at from the study.

Potential of urban timber construction

Arnim Seidel sees the best opportunities for timber construction in Germany in multistorey residential buildings (see sidebar). “Demand for office and administrative buildings is still relatively low in Germany,” says Seidel. It will take many years to see noticeable interest in timber buildings from commercial investors. There are many reasons for this, as the Ruhr-University Bochum found in its 2017 study on the influence of timber construction in a climate change context. 19 barriers were identified in Germany alone. Although the authors believe a massive increase in timber construction is necessary, it will take a while – at least in Germany – until timber buildings become the new ideal of modern architecture.

However, the growing number of international projects show that the increasing importance of timber buildings in cities cannot be overlooked. For example, a 125,000-square-metre office campus with mass timber construction is being built in Paris: the Arboretum. So it is entirely possible that London’s timber building pioneer, Andrew Waugh, is right when he says: “This is the beginning of the timber age.”

By Elke Hildebrandt

The joker in residential construction

Timber offers a key advantage when it comes to growing density in cities. Thanks to its high load capacity coupled with a lower dead load, adding storeys is lighter with timber construction – a major plus for urban residential construction considering the space shortage. But timber is also scoring points in the new urban building sector. A high degree of prefabrication and short building time for timber structures can provide quick relief for local housing needs. Municipal housing association Howoge is making use of these advantages and building three multistorey individually designed prefabricated buildings in Berlin-Adlerhof. The association is planning around 1,000 hybrid-timber homes with the Kaden + Lager architecture firm, half of which will be social housing – a rarity to date. The reason for the low introductory rent from €6.50 per square metre is the use of timber as a building material. According to architect Markus Lager, timber frame construction costs as much as traditional construction while at the same time enabling more gross floor area.

TRADA, the Timber Research and Development Association, confirms that housing built out of wood does not need to be more costly. It recently published a detailed cost model for residential timber, which demonstrates that the costs of CLT (cross-laminated timber) are “comparable” to reinforced concrete for housing construction.