What do engineers really think of timber buildings?

Timber is one of the world’s oldest building materials, and in recent years it has come back in style as one of the preferred sustainable materials for designers and architects – and with good reason.

“Timber is often more sustainable than other building materials like concrete and steel,” says Frank Schwartz, Director for high rise buildings at Ramboll. And timber becomes even more sustainable if we ensure what we build is built to last, and we keep the materials in use as long as possible.

“Then there are the less tangible benefits: timber has a pleasant materiality, which I think we can all relate to. It is generally easier to bond with a timber building than one in hard concrete or steel,” he adds.

The buildings sector accounts for nearly 40% of global energy-related carbon emissions. But while timber is part of the solution, it is not a panacea.

Kaj 16, a multi-storey mixed-use residential and commercial building in the Swedish city of Gothenburg, offers a case study in how far we have come with timber buildings. The developer originally wanted the entire building to be made from timber, but Ramboll engineers demonstrated how a hybrid solution would be better.

“For Kaj 16, the first thing we did was to create ten different models based on different materials to assess the carbon impact of each: a traditional concrete building, one in steel, and so on. We found what the developer had originally thought would be the most sustainable – a building solely made from timber – did not perform as well as a hybrid building,” says Riccardo Pedroni, Associate and timber specialist on the Kaj 16 project.

The team found that the better solution was to use concrete for the cores and podium levels, and timber for the remaining superstructure – beams, columns, and slabs. If the cores had been made from timber they would have been too large, leading to an inefficient use of materials.

Read more: Explore Kaj 16 in pictures

The findings corroborated those of a recent internal Ramboll study that screened all timber projects in the world, and concluded that above a certain height, hybrid buildings perform better.

“Timber excels in buildings up to 15-20 stories, but once we reach heights above, other materials are likely to be better. So it’s not a one-size-fits-all,” Frank Schwartz explains.

At Kaj 16, Ramboll’s engineers were able to reuse materials from the site’s original structure, reducing carbon impact and mitigating inherent weak points of building with timber.

“We took the existing concrete structure, which was a somewhat out-dated 1960s build, and crushed the concrete down, separated it from the rebars and reused it in the new concrete mix for the basement and cores,” explains Frank Schwartz.

“We are also planning to place recycled small dry aggregates on top of the concrete deck to improve the acoustics and vibration response of the deck. In doing so, we also avoid placing wet concrete on top of the timber, mitigating one of timber’s weak points.”

“We can make buildings in timber, steel, concrete, or 200 different hybrids. But by using data, and modelling different solutions, we can ensure the finished building is actually better for the specific location,” he adds.

Globally, less than 9% of the economy across sectors follow circular principles, with severe climate and environmental consequences.

Some experts worry that using more timber will have a negative impact on biodiversity. According to Riccardo Pedroni, the question to consider is ‘what is the alternative?’

“The alternative to building in timber is to use concrete and steel, which emit more CO2, that will hurt biodiversity more,” he says. “One of the major questions with

regards to timber today is whether we have enough. What the data shows us is that, even in Europe, where the demand for timber has increased tremendously in the last 30 years, the forests are still expanding.”

“In sustainably managed forests, you ensure a variety of trees which is good for biodiversity, and you never log trees solely from one area of the forest. When done right, logging some trees can actually improve biodiversity, as it ensures trees have the best conditions to grow and support animal life, explains Riccardo Pedroni.

Ease of access to materials further strengthens the use case for timber, Frank Schwartz adds: “We shouldn’t use timber just for the sake of it, but where it’s abundant, close by, and sustainably managed, it can be an obvious choice.”

Europe produces roughly 25% of the world’s timber supply, and these forests have grown steadily in the last three decades because most are sustainably managed.

Frank Schwartz predicts that, in ten years, timber will become widely used in the building sector, as legislation compels the use of materials with lower carbon intensity in new buildings.

“That’s why it is important we build up competencies today to integrate timber into our engineering mindset and standardised production,” he says.

New legislation to reduce the carbon footprint of new buildings is being introduced in many countries. Check out how our experts believe we can get ahead of this trend.

To contact the editor of this article, please email Anders Brønd Christensen.