Our dependency on concrete and steel to build everything from homes to sports stadiums, comes at a severe environmental cost. Concrete is responsible for 4-8% of the world's carbon dioxide (CO2) emissions. Second only to water, it is the most widely used substance on Earth, accounting for around 85% of all mining and linked to an alarming depletion of the world's sand. Globally, enough concrete is poured each year to cover the whole of England.
Some architects such as Waugh are therefore arguing for – and pressing ahead with – a return to wood as our primary building material. Wood from managed forestry actually stores carbon as opposed to emitting it: as trees grow, they absorb CO2 from the atmosphere. As a rule of thumb, a cubic metre of wood contains around a tonne of CO2 (more or less, depending on the species of tree) – which is similar to 350 litres of gasoline.
Not only does wood remove more CO2 from the atmosphere than it adds through manufacture, but by replacing carbon-intensive materials such as concrete or steel it doubles its contribution to lowering CO2. A recent advisory report to the UK government on the uses of "Biomass in a low-carbon economy" found that, "the greatest levels of [greenhouse gas] abatement from biomass currently occur when wood is used as a construction material… to both store carbon and displace high carbon cement, brick and steel."
Between 15% and 28% of new homes built in the UK annually use timber frame construction, capturing over one million tonnes of CO2 a year as a result. Increasing the use of timber in construction could triple that amount, the report concluded. "Savings of a similar magnitude may also be possible in the commercial and industrial sectors by utilising new engineered wood systems such as cross-laminated timber."
Cross-laminated timber, or CLT, is the primary material on the construction site Andrew Waugh shows me around in east London. Because it's described as an "engineered wood", I expect to see something similar to chipboard or plywood. But CLT just looks like ordinary 3m (10ft) planks of wood, one inch thick, replete with knot-holes and splinters. The ingenuity is that the planks are made stronger by gluing them in layers of three, with each layer perpendicular to the other. This means that the CLT "doesn't bow or bend, it has integral strength in two directions", says Waugh. "[A CLT] wall supports the floor above, with a horizontal strength to carry a load above it, acting like a long beam". That, he says, "changes architecture".
This part about trees and carbon capture is interesting.
Recently there have been calls for tree planting on a colossal scaleto capture CO2 and curb climate change. However, whilst young trees are efficient and effective carbon sinks, the same is not so true for mature trees. The Earth maintains a balanced carbon cycle – trees (along with all other plants and animals) grow using carbon, they fall and die, and release that carbon again. That balance was knocked out of kilter when humans discovered ancient stores of carbon in the form of coal and oil, which had been captured during previous carbon cycles, and began burning them, releasing the resulting CO2 into our atmosphere far faster than the current cycle can deal with.
Many pine trees in managed forests, such as the European spruce, take roughly 80 years to reach maturity, being net absorbers of carbon during those years of growth – but once they reach maturity, they shed roughly as much carbon through the decomposition of needles and fallen branches as they absorb. As was the case in Austria in the 1990s, plummeting demand for paper and wood saw huge swathes of managed forests globally fall into disuse. Rather than return to pristine wilderness, these monocrops cover forest floors in acidic pine needles and dead branches. Canada's great forests for example have actually emitted more carbon than they absorb since 2001, thanks to mature trees no longer being actively felled.
Arguably, the best form of carbon sequestration is to chop down trees: to restore our sustainable, managed forests, and use the resulting wood as a building material. Managed forests certified by the Forest Stewardship Council (FSC) typically plant two to three trees for every tree felled – meaning the more demand there is for wood, the greater the growth in both forest cover and CO2-hungry young trees.
Rewilding and protecting virgin forests is essential. But unmanaged monocrops help no-one, and floors full of dry pine needles are also the primary cause of wildfires – something that North America and many parts of the world experience on a now annual basis. Managed harvesting greatly reduces that risk.
These benefits have not been lost on the US authorities. Melissa Jenkins, of the US Federal Forest Service, explained at a recent meeting of the Environmental and Energy Study Institute (EESI), that "we have a situation of overstocked forests: if a wildfire blows through, these fires burn hotter, they burn faster and they take a lot more effort to put out… If we can build markets for these wood products, landowners will be more likely to sustainably manage or sustainably thin their land." She highlights that CLT in particular as having the potential to reduce "wildfire risk [and] support rural economic development and jobs".
The market seems to agree. Less than five years after its arrival on US shores, there are now CLT projects underway in almost every mainland US state. More importantly, unlike the UK – which currently imports all of its CLT – the US is investing in domestic CLT manufacturing, with factories in Montana and Oregon, and more planned in Maine, Utah, Illinoi, Texas, Washington State, Alabama and Arkansas. Amazon's new "tech-hub" building in Minneapolis is made from nail laminated timber (like CLT, but using nails rather than glue). The 2018 Timber Innovation Act also included provisions for research and development into mass timber.
Not everyone believes that the future is CLT, however. When I ask Chris Cheeseman, professor of materials resources engineering at Imperial College London, whether wood could usurp concrete as our primary building material, his response is blunt. "No. That isn't going to happen. It might happen locally with some small schemes. But you've got to appreciate the massive use of concrete, and the massive importance of concrete to infrastructure and society. It is an exceptionally good material because of its functionality and its robustness."
There is also the "end of life" question. Carbon only remains trapped in the wood for as long as the building remains standing or is reused in another building – if it rots or is burned for energy, then all the stored carbon is released. Doug King, a chartered engineer and building sustainability advisor, tells me, "unless we attend to the disposal of timber materials at their end of life there is no guarantee that the overall cycle is making a positive benefit to society." Previous research work by Arup in 2014 estimated that half of all construction timber ends up in landfill, 36% is recycled and the remaining 14% burnt for biomass energy.
Despite these issues, Waugh remains ambitious. The average lifetime of a building is 50-60 years – that, he believes, is more than enough time for architects and engineers to work out the re-use and recycling issues. Turning it into biochar could be one possibility. Waugh's buildings are made to be easy to take apart for re-use by future generations.
Fundamentally he – along with a growing group of international architects – is convinced that mass adoption of CLT is an important weapon in the fight against climate change. "It's not a fad or a fashion," he tells me as we finish the tour of his east London build, and I take my final, incongruous breath of the forest air. "The largest commercial developer in the UK have just bought this building. For me, that's where you want to be… I want this to be mainstream. Everybody should be building with this."
