CROSS LAMINATED TIMBER
There has been an increasing amount of debate about the use of timber in construction and the sustainability benefits of using 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? Read more to determine our view.
What is CLT?
Cross laminated timber (CLT) is produced from softwood timber, that 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 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.
CLT is increasingly being used in the construction of high-rise residential buildings. Bridport House, on the Colville Estate, in Hackney incorporated this technology into its eight and five storey residential buildings. Lend Lease are utilising CLT in a number of their flagship residential buildings currently under construction at Trafalgar Place as part of the Elephant and Castle Master plan. Lend Lease also developed an 11 storey residential block in Melbourne.
What are the main Benefits?
There are many benefits of CLT. Compared with steel and concrete construction it is obviously a low impact material with a much lower embodied carbon footprint. CLT has a quicker construction time over traditional methods with some estimates claiming it is six times faster than a standard build due to panel construction and ease of subsequent fixing and remediation. It is much lighter which allows for reduced slab – a positive impact for project wide embodied carbon totals. 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…
Due to successful developments such as those mentioned above, industry awareness and knowledge on the attributes and capabilities of CLT is improving. However, there are a number of perceived barriers and concerns by the construction industry which have meant that compared to our European neighbours, the UK market has been reluctant to incorporate extensive use of CLT.
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 similar to traditional construction up to 7-8 levels.
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.
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, results 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 easily be overcome, especially when reduced programmes are promoted. However in order to ensure continued supply from sustainable sources, properly managed plantations are paramount. At the moment the EU only harvests two thirds of growth annually with Austria currently producing 80 percent of the world’s CLT panels. As demand grows, new sources will need to be brought forward, and this obviously 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.
Forecasts state a doubling of materials demand by 2050. While some areas of the industry is adapting through research and development of new products (such as carbon negative concrete which uses magnesium silicates in lieu of cement), 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 of 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.