Concrete and steel are an integral part of the built environment but are responsible for an estimated 13% of global carbon emissions.
As concern grows about climate change and the need for greater sustainability, interest in wood as a climate-friendly alternative has increased.
This interest has further accelerated with the development of cross-laminated timber (CLT), which has rapidly gained popularity as a construction material over the past few years and is now being used all over the world in ever taller and more complex wooden buildings.
CLT was ﬁrst developed and used in Austria and Germany in the early 1990s. It is a group of engineered wood products comprising sheets, panels, posts and beams made by gluing layers of solid sawn timber together alternately at rightangles. The physical properties of wood change depending on the direction of the force applied. Gluing layers at right angles therefore achieves rigidity in both directions, improving structural integrity and dimensional stability. Kiln dried timber is used with a low enough moisture content to eliminate the risk of pest or fungal attack. When manufactured from suitably cut, dried, machined and graded timber, the structural capabilities of CLT are comparable to concrete and it can be used for all elements of a building’s superstructure including walls, ﬂoor and roof.
Austria published the ﬁrst national guidelines for using CLT in 2002 which made possible its wider acceptance as a material in multi-storey buildings. Because of its favourable structural properties, ease of prefabrication and comparative lightness compared to concrete and steel, CLT is now being used more widely in the design of mid and high-rise buildings and there are now many examples of large wooden structures in the UK, Europe, USA and further aﬁeld.
This rising interest has led to an increase in factories producing CLT. Until recently, most CLT was made in Austria, Germany, Switzerland and Scandinavia but within the past ﬁve years factories have opened in Japan, France, Latvia, Australia, USA and Canada. Despite the involvement of many UK ﬁrms as leading innovators in its use, there is currently no largescale producer of CLT in the UK.
CLT panels are usually constructed from spruce, although it can be manufactured from larch, Douglas ﬁr, pine and other timbers. While higher strength grade timber is typically used by European manufacturers, trials by Napier University and industry partners have established that the lower grades of structural timber grown in UK forests are also suitable for CLT production if correctly graded.
However, there are technical challenges surrounding the viability of CLT from UK material compared to CLT from European timber, principally the number of rejections from defects caused by kiln drying to a signiﬁcantly lower moisture content, typically 12% instead of the usual 20% for other sawn timber. The signiﬁcant additional costs associated with CLT production and easy accessibility of high quality European products are disincentives to establishing large scale manufacturing facilities in the UK.
Although still in its infancy as a viable construction material in high-rises, the development and uptake of CLT has been rapid and there are already numerous examples showcasing its considerable potential.
Dalston Lane residential development in Hackney, east London is a pioneering CLT project completed in early 2017. Although only 33 metres high, it is the largest contiguous load-bearing timber building in the world with 4,649 cubic metres of CLT and only seven tonnes of steel beams used throughout. The completed building achieved a net carbon footprint of –2,600 tCO2e, compared to +2,000 tCO2e for an equivalent development using concrete.
The 8,000 tonne diﬀerence in weight between CLT and concrete allowed another three storeys to be added.
The 10-storey 25 King oﬃce block in Brisbane was completed in 2018 and is Australia's largest all-timber building at 52 metres high. Compared to using conventional materials, environmental beneﬁts included a 74% reduction in embodied carbon (carbon dioxide emitted during the manufacture, transport and construction of building materials). The 20% weight saving allowed more area to be allocated as functional space and construction time was signiﬁcantly reduced by oﬀ-site prefabrication.
The tallest timber building in the world is mjøstårnet in Brumunddal, Norway at 85.4 metres, an 18-storey mixed-use building completed in march 2019.
Following the trend for timber high-rises, PLP Architecture and the University of Cambridge are researching a design concept for London's ﬁrst wooden skyscraper – the 300-metre 80-storey Oakwood Timber Tower at the Barbican. meanwhile, the Japanese company Sumitomo Forestry has revealed plans for what would be the world's tallest wooden building in Tokyo at 350 metres, containing 90% or 185,000 cubic metres of wood.
The most obvious concern about high-rise timber buildings is the ﬁre risk. CLT is designed to perform well in ﬁres, building up a layer of char which can provide a substantial margin of ﬁre resistance depending on the size and number of panels. PLP believe that their building will eventually meet or exceed existing ﬁre regulations for steel and concrete buildings. However, the Hacket review of building regulations and ﬁre safety that followed the Grenfell tragedy, has had a major inﬂuence on the construction industry, restricting for now the use of structural timber walls in buildings taller than 18 metres.
The growth of CLT has been driven by practical and ﬁnancial beneﬁts as well as by clear environmental advantages. The embodied carbon of CLT is low compared to other materials because, apart from using comparatively little external energy in its manufacture, carbon is stored during tree growth and for the duration of its use. At the end of its life as a building material, carbon is released through natural decomposition or through the generation of energy from burning, both highly eﬃcient waste disposal solutions compared to other materials.
As the only sustainable alternative to concrete and steel, wood is once again establishing itself as a versatile, cutting-edge building material perfectly suited to the times. For timber growers and managers, it is another indication of how important well managed forests, and the products they produce, will continue to be in the future.
A LONG HISTORY OF BUILDING WITH WOOD
Wood has been used as a primary building material in the British Isles since Neolithic times, when communal longhouses accommodating 30 people were among the largest structures in the world.
As technology progressed with the discovery of copper, bronze and iron, it was possible to process and use wood with increasing eﬃciency. By the beginning of the Iron Age, Britain’s population had reached more than a million and it is estimated that 50% of woodland had already been cleared to satisfy the demand for timber.
By the Middle Ages, advances in timber frame construction culminated in structures like the hammer-beam roof of Westminster Hall, the largest Medieval timber roof in Northern Europe, commissioned in 1393 by Richard II.
From this period onwards, the pre-eminence of wood was gradually eroded by innovations such as brick and structural iron work. When reinforced concrete ﬁrst appeared in the mid-1800s, its strength and versatility as a construction material were quickly recognised, signalling a new approach to building that has dominated ever since. One of the ﬁrst skyscrapers made with reinforced concrete was the 16-storey Ingalls Building in Cincinnati, Ohio, in 1904. The rest, as they say, is history.
Concrete and steel are a long way from being superseded by wood, but the emergence of CLT as a serious alternative in large structures demonstrates that, far from being a redundant or outdated material, the unique qualities of wood are still as relevant as ever.