CROSS LAMINATED TIMBER
With the current global focus on the climate crisis, which has been brought to the forefront by the Paris Agreement and subsequent COP meetings, it is as important as ever to consider the carbon impact of the built environment.
Consequently, there has been an increasing amount of debate about the use of timber in construction and the sustainability benefits of using Cross-Laminated Timber (CLT), particularly where traditional builds of taller buildings have necessitated the use of steel and concrete. But does an increase in height preclude the use of timber in taller buildings and what are the other key opinions of the debate, especially considering the concerns following the Grenfell Tower Fire? Read more to determine our view.
What is CLT?
Cross laminated timber (CLT) is produced from softwood timber, which is made up of sections that are laid across each other at right angles before the finished material is then industrially laminated. The number of layers incorporated depends on thickness and structural requirements and as such, can be made according to the exact structural loads needed in each piece of CLT. The layers are sawn, planed and then glued together under pressure using a polyurethane adhesive. Panels are fabricated to precise dimensions in architectural plans, including window, door, plumbing and ventilation openings with any channels for electrical wiring or services being cut into the panels in the workshop.
At the construction site, the panels are hoisted into position and bolted together with metal brackets to build up the structure floor by floor. Construction is therefore quicker and cleaner than traditional builds with virtually no waste or lost waiting times.
What are the main Benefits?
There are many benefits of CLT. Compared with steel and concrete construction it is a low impact material with a much lower embodied carbon footprint. Using a material such as CLT offers carbon benefits through the sequestered carbon throughout the lifetime of the timber and unlike concrete and steel, is a regenerative material. CLT has a quicker construction time over traditional methods with some estimates claiming it is six times faster than a standard build due to its pre-fabricated construction and ease of subsequent fixing and remediation. CLT is much lighter than typical concrete and steel construction, which allows for reduced slabs and foundations. In some cases, there is an estimated up to 15% saving in material cost as a result of this. This in turn passes on the savings to the building’s whole life carbon performance. Furthermore, it is clean with much less waste produced on site, limited wet trades or brick/block work creating dust, while handling is vastly reduced so it is much better in terms of health and safety.
There are some Concerns…
CLT showed promise of becoming a widely used construction material in the mid-2010s but despite it being desirable due to the environmental benefits, in light of the Grenfell Tower fire in 2017, there has been a reluctance to use combustible materials across the built environment, especially within high-rise buildings. Due to this the government has brought in legislation in relation to the height of buildings with combustible facades between 11-18m, this has also placed restraint on the use of CLT. However, CLT does have some inherent fire-resistant properties, providing up to 120 minutes fire resistance rated (source – Structuralam)
One of the main barriers to date relates to cost. The cost of CLT per square meter is higher when compared to steel and concrete. However, when reduced programmes and onsite waste reduction are taken into account, the cost of building with CLT is competitive with concrete structures over six storeys high.
Another perceived obstacle is that the CLT construction method makes demands on contractors different to those of a traditional build. Contractors have therefore needed to respond to these differences by ensuring their teams are appropriately skilled and familiar with the material and its attributes. The technology results in different interactions with other trades on site, and contractors will need to gear up to take advantage of the potential speed gains or to demands on higher team numbers over shorter periods.
Furthermore, due to the precision elements produced off site, on site tolerances have resulted in a learning curve for some. For instance, floor slabs accuracy levels for CLT can be under a millimetre and such tolerance levels can slow proceedings on site, initially at least with lead-in times. This then can have a knock-on effect on the construction programme. Within the UK, however we should look to timber buildings in Europe as case studies to overcome any issues within construction we may face. For example, HAUT in Amsterdam is a 21-storey timber-hybrid residential tower, providing 55 apartments. This building follows the design principle of using timber where possible, with concrete and steel where necessary. This results in a concrete foundation, basement and core with timber composite floor plates. This has offered enhanced performance in footfall, acoustics, construction sequencing and reduces material demand with the low weight of CLT. HAUT demonstrates how timber can be used in conjunction with concrete to satisfy the requirements of a building, whilst implementing a highly sustainable material.
Forecasts expect global CLT markets to grow over the coming years, with Europe having the largest market share. In order to ensure continued supply from sustainable sources, properly managed plantations are paramount. Across the EU, Austria currently produces the majority of CLT with the largest plant producing an estimated 130,000m3 of CLT in 2020. As demand grows, new sources will need to be brought forward, and this requires forward planning to limit interruptions to supply and minimise price fluctuations – the latter further influenced by the external forces of the Euro-Sterling exchange rate.
CLT also requires a properly integrated design process to mitigate potential issues. The design team need to ensure they specify cladding or even shading to mitigate sun discoloration or incorporate acoustic panelling to improve noise attenuation. Importantly, detailed design needs to be completed accurately and before factory work commences, in order to ensure that M&E is fully and properly built in. Retrospective design changes, as in any build, result in wasted material and cost increases.
Thus in Conclusion…
From an environmental point of view, CLT is preferable to steel and concrete. The above mentioned process and cost issues can be overcome, especially when reduced programmes are promoted. However, with the impact worldwide of the Grenfell Tower fire as a result of combustible cladding, there has been a clampdown on combustible materials used in construction, which has set back the use of CLT across the industry. This has also been enforced by legislation. However, CLT can offer incredible advantages to projects with its inherent fire resistant properties and low carbon intensity.
While some areas of the industry are adapting through research and development of new products, the entire industry needs to ensure that efficiencies are maximised at all stages of the design, production and construction stages. This is also true for the timber and CLT production. Designing out waste at every stage is necessary to ensure reduction of wood use and costs. Such efforts will no doubt encourage its uptake and will help the construction industry to embrace sustainable and low impact CLT long into the future.
Use of CLT is one method that can contribute to the design of low carbon buildings. For more details on how Greengage can help with this, please contact firstname.lastname@example.org.