Cleaner, greener, quicker, stronger: Is wood the building material of the future?

Is it time to swap the concrete jungle for timber? Harry Cockburn investigates

Cross-laminated timber can be stronger than concrete or steel (Getty)

We live in a world of concrete. After water, it’s the most widely used substance on our planet, and its usage around the world, ton for ton, is twice that of steel, wood, plastics and aluminium combined.

Its invention in 1824, by a bricklayer in Leeds who first produced the Portland cement used to bind the aggregates used in concrete, quite literally paved the way for the creation of the modern world, enabling humans to mould high-strength, stone-like structures of almost any shape.

Concrete’s ubiquity comes at a high price though: if the cement industry were a country, it would be the third-largest emitter of carbon dioxide in the world, after the US and China, as it releases over 2.8 billion tonnes into the atmosphere each year.

A 2018 report suggested concrete contributes up to 8 per cent of the world’s CO2 emissions.

It also requires phenomenal amounts of water, sucking up around a tenth of all water used in industry – often in areas with critical water shortages.

Meanwhile, steel, the other mainstay of the construction industry, also takes a heavy toll on the environment. The huge amounts of energy needed to make a blast furnace operational still depend on coking coal, and no large-scale alternatives are yet available.

In the UK, controversy surrounds the Woodhouse Colliery in Cumbria, a coal mine which the government has vacillated over supporting, which is primarily to be used – if it goes ahead – for the extraction of coking coal.

As the future of our planet hangs in the balance, our dependence on these polluting materials is in the spotlight.

Architects and developers are increasingly aware of the role played by steel and concrete manufacturing in the worsening climate crisis, and there is already a move in some quarters towards much more sustainable building materials.

One such material is already being widely used in construction, but recent advances mean it is stronger and more durable than it has ever been before, and has a far, far lower environmental impact than concrete or steel. That material is wood.

Though timber-framed buildings are hardly new, instead of sawing enormous beams from ancient trees, new techniques focus on using fast-growing soft woods, stuck together in a form which provides massive strength, durability, and flexibility of design.

One example of this, cross-laminated timber (CLT), is manufactured using a technique developed in the 1990s in Austria, in which sheets of kiln-dried wood are glued on top of each other, with the grain of each layer running perpendicular to the next.

This method can create huge boards, up to a foot thick, and as long and as wide as the manufacturer’s premises allow.

What’s more, the strength of these coagulated timber slabs can match or exceed steel or concrete.

The use of CLT as a modern construction material has already been definitively proven.

The world’s largest CLT structure is Dalston Works, a 10-storey residential building of 101 flats, in Hackney, London, which was completed in 2017, and won the “eco living award” at the Evening Standard’s 2018 New Homes Awards.

Meanwhile, the world’s tallest timber building has also been built using CLT – the 85.4-metre, 18-storey Mjostarnet building in Norway, which was completed in 2019 and is also the country’s third-tallest building. The mixed-use building contains apartments, a hotel, a swimming pool, office space and a restaurant.

The 18-storey Mjostarnet building in Norway is the world’s tallest timber building (Nina Rundsveen/CC)

As the UK government seeks to “build back better”, from the impacts of the coronavirus crisis, should the use of timber in construction play a greater role?

Dr Aurore Julien, a senior lecturer in environmental design at the University of East London’s School of Architecture and Design, says we should view our use of buildings in a similar way to how we assess other consumable goods, and CLT can be an effective means of making significant reductions in emissions.

Though it is usually more environmentally friendly to improve insulation and heating mechanisms for existing buildings, Dr Julien says CLT should replace some existing construction materials in new builds.

She tells The Independent: “Every time a new building is built this results in carbon emissions. And we consume buildings in the same way that we consume everything else.

“For new buildings, the energy regulations are pushing energy consumption and carbon emissions down to the point that the main carbon emissions from new buildings through their life cycle come from their construction and materials.

“The manufacture of steel and concrete results in high carbon emissions. In some cases steel can be imported over long distances, even from China, which adds considerably to its carbon impact.”  

But she says CLT buildings may be able to store more carbon in the wood than their entire construction generates.

“As trees absorb CO2 when they grow, CLT is considered to have a negative embodied carbon – meaning that the CO2 absorbed by the tree during its growth can be more than that emitted in the manufacture of the CLT product and its transportation to the site.

“So a building made out of CLT in effect stores the sequestered carbon during growth and becomes a carbon sink.”

Asked if it could replace steel in the wider construction industry, Dr Julien says “It certainly should.”

At the end of its useful life CLT can be repurposed – something tricky to achieve with other building materials.

“A large majority of timber – around 80 per cent – can be reprocessed at the end of the life of the building, when it is demolished, into lower-grade materials such as particleboard.”

But Dr Julien warns it is “absolutely essential” that the timber used should come from managed forests which have been properly certified as being sustainable sources of wood.

However, “sustainable forestry” is a contentious subject, with different meanings in different countries. While vast forests of fast-growing conifers may be able to rapidly fulfil timber orders, a growing understanding of the impact of monoculture cropping on biodiversity, and what it means for carbon sequestration, is also a key consideration for those seeking to herald CLT as a straightforward environmentally friendly choice.

The wood used in the 10-storey Dalston Works building in London was grown in forests in Austria and Germany which have been certified as sustainable.

It was then manufactured into CLT in Austria and brought by road to the UK.

According to the developers, the building used 4,500 cubic metres of timber, which equates to about 2,300 trees. With more than 800 people living in the building, they say it worked out at about three trees per person.

Timber from the UK is widely used in construction, but it is not usually considered to be of the highest quality. This is for a variety of reasons.

A 2016 study noted that the main tree grown for construction in the UK is the sitka spruce, an imported conifer from the Pacific northwest of North America.

In their home region these trees can reach 40-70 metres in height, but in the UK, where conditions are milder, their growth rate is faster but the resulting density of the wood is lower, making it weaker.

As a result, higher strength timber grown in Europe is normally used for key structural purposes.

One particular concern about the extensive use of wood in construction is the potential for flammability.

In order to be used as a commercial construction material, CLT has been extensively fire-tested, and is designed to accommodate substantial fire resistance.

Furthermore, unlike steel, CLT remains structurally stable when subjected to high temperatures.

However, last year, CLT manufacturers invested £500,000 in developing a new fire-safety compliance framework to ensure the material meets the latest building regulations in England.

According to Dr Julien, overall “there is no doubt that it is an excellent construction method.

“CLT is green, cost-effective, fast to install, requires less foundation, and results in less waste than traditional construction.”