On a sunny but cold morning in January a few of us had the opportunity to visit Dalston Lane, a 121 unit residential and commercial development in Hackney, London. Designed by Waugh Thistleton Architects, this building is reputed to become the tallest cross laminated timber (CLT) structure of its kind in the UK.
Back in 2009 we completed John Hope Gateway at the Royal Botanic Garden Edinburgh (RBGE) using CLT and glulam (the header photo shows one of the CLT partition walls - laser cut holes inspired by microscopic diatoms). At that time CLT was still fairly new in the UK construction industry and RBGE was one of the first public buildings to use CLT in the way that it did. A few years on and it seems that there is now a vast increase in projects wanting to use it. As knowledge of the material increases, many more will start to see its benefits as a solid structural material. CLT is made of cross laminated layers of timber (lamellas) that are glued perpendicular to their adjacent layers thus giving the CLT panels strength in two directions. This allows them to be used in floor, wall and roof construction and are most closely comparable to concrete as a structural material. CLT has been used in precisely this way on the Dalston Lane project.
There are many advantages to using CLT as a structural material, one of which is its lower embodied carbon in comparison to concrete. It uses far less water, is a dry assembly process and is incredibly quick to erect on site, yielding programme advantages. There are major environmental advantages too. On the Dalston Lane project a tight site made incorporation of renewables unachievable. For this reason the specification of CLT allowed the project to meet its carbon reduction targets. The CLT sequesters the carbon within the material essentially making it ‘carbon negative’. At RBGE using CLT panels allowed a removal of approximately 539 tonnes of CO2 from the atmosphere.
The benefits of using CLT are fully obtained by using the material for its strengths and for streamlining the production process accordingly. For example, although a building constructed fully of CLT is achievable, sometimes this is not necessarily the best solution. Is it better to have a 1 metre deep CLT beam or instead to use a 300mm deep steel? Materials should be used where they perform efficiently. Equally, CLT should not be left exposed in acoustically sensitive areas simply to achieve an aesthetic appearance. Instead the exposed grade CLT should be specified where it doesn’t compromise on performance. With regards to creating appropriate production processes, from an architect’s perspective there are things we need to take into consideration. For example, we need to realise that due to the requirement to coordinate services etc. prior to production of timber, there is a need for resource early in the project to ensure incorporation prior to manufacture. The beauty of CLT and programme benefits from a quick erection on site come from achieving this coordination in the studio, on the computer. As soon as service holes need to be cut on site, the programme benefits are lost. In this sense although architecture may be ready for manufacture, there is a need for M&E to also keep pace to keep time on site to a minimum.
Advantages of CLT:
- Carbon negative building material with low embodied energy
- Fast erection process
- Dry trade
- Lightweight material, allowing for smaller foundations
- Can be used for walls, floors and roof
- Prefabrication leading to better quality and accuracy
- Single point of contact – a single contractor will usually supply and fit the CLT, glulam and sometimes the steel too.
Disadvantages of CLT:
- Does not perform well in acoustically sensitive environments
- Requires more input in the early stages to allow full coordination of services prior to manufacture of CLT
- External walls are unable to be left exposed externally and must be clad
- Unsuitable to be used below ground