David Nutsford shares his knowledge on using timber in the construction of our buildings
One of the greatest attributes of timber is that it absorbs carbon during growth. By using more timber in the construction of our buildings we can remove increased amounts of carbon from the atmosphere. By producing buildings that act as carbon stores, we can make a significant contribution to carbon reduction and help to mitigate the impact of climate change. (1)
When it comes to dealing with climate change we often think of how to reduce consumption, reduce waste or reduce size; however, with engineered timber we need to start thinking how can we use as much timber as possible in order to sequester carbon from the atmosphere.
So, what are the downsides to using mass timber?
Is it too complicated?
The Kajstaden Tall Timber Building by CF MØller Architects was completed in 2019. The eight-story building’s structure is entirely CLT, fixed together with large screws, allowing easy dismantling at end of life. A feature of this building is that each floor was constructed in just three days on-site with each interior fit out able to begin immediately after the floor above was complete. (2)
It is too expensive?
Yes, CLT is currently more expensive per cubic metre than concrete, and on par with steel, mostly due to the scale of economy those two industries have enjoyed and the freedom to emit carbon at no cost.
However, in the example of the Barretts Grove flats, by Nick Grant, it was found that ‘using CLT as structure and finish removed the need for plasterboard walls, suspended ceilings, cornices, skirtings, tiling and paint; reducing by 15% the embodied carbon of the building, and saving approximately £30,000 per flat.’
Using CLT does not mean eliminating all steel and concrete – these materials have their place – something called Hybrid CLT, which was used in London’s Wenlock Cross Flats.
The real cost of concrete comes in the labour and on-site time, while the true cost of steel is in complex engineering and fabrication required – alongside their long-term cost of high carbon emissions. The super structure of the Barretts Grove flats was erected in just two weeks – the value of CLT is not in the cost, but the absence of weight, time, labour, and emissions.
This has real world effects with the Dalston Works building achieving 35 additional apartments by using CLT, rather than concrete and steel, due to weight savings.
If we all use CLT will there be any forests left?
Yes. There may even be more forests as we incentivise more plantation planting.
A study found that ‘In Austrian and German forests alone, enough timber is grown within one hour to produce the CLT required for Dalston Works, currently the largest timber building in the world. On this measure, an average dwelling requiring 30m3 would be grown every 20 seconds…’ (1. Page 18)
What about Fire, Acoustic and Earthquake Design?
All these issues have been solved both overseas and in New Zealand. CLT chars in a highly predictable manner making it simple to design for required Fire Ratings, while the flexible connections and light weight make it ideal for EQ resistance.
Acoustics is perhaps the most difficult aspect of CLT design; however, a collection of approved acoustic build ups has recently been released by Wood Solutions compiling acoustic testing carried out at Auckland University Acoustic laboratory (3)
Another issue is timber treatment and dealing with NZs outdated building code. The problem here is not with the timber but with the tacit acceptance that buildings will be leaky and have moisture containment issues. Properly protected and ventilated timber will last for centuries.
Of course, there are other issues at play including skills and training, material sourcing, scale of economy and engineering design capacity.
So, what’s the problem?
Despite all the benefits inherent in Engineered Mass Timber, it appears to be stuck in a niche of ‘special projects’ where the exposed timber is celebrated and treated as a feature.
While this is great for architectural expression – to shift engineered mass timber to the next level we need to take it down from the pedestal and start using it for the amazing structural material it is.
The fact it is beautiful is just a bonus.
1. 100 Projects UK CLT UK by Waugh Thistleton Architects on behalf of the Softwood Lumber Board & Forestry Innovation Investment. 2018
The book can be downloaded here for free and is well worth a look:
2. Kajstaden Tall Timber Building, online
3. CLT Acoustic Performance by Wood Solutions (Forest and Wood Products Australia), 2018, online
The report can be downloaded from Woodsolutions.com.au by registering your email address