THE WHY & THE HOW:
Designing timber buildings for disassembly

Timber buildings sequester and store carbon — until they’re demolished.
So why don’t we design them for a second life?

July 2022

Words by Thomas Musson

“Why, given the pressing need to reduce and stabilize atmospheric CO2 levels, are we content to design these durable, highly engineered elements to be single-use?”

THE WHY


Wood is becoming a popular solution for many buildings where steel or concrete used to be the only options. This has been made possible by the development of engineered timber, a relatively modern collection of products made by binding timber laminations, strands, particles or fibres using adhesives or mechanical means to create composite elements that are far stronger and more robust than traditional wooden components.

Timber’s appeal lies in its low embodied carbon. Well designed and executed engineered timber structures, using wood from sustainable sources, are inherently sustainable due to the natural process of sequestration. As trees grow, they absorb carbon dioxide from the environment and lock it away in the structure of the wood. Depending on its density, 1m3 of wood may store more than 1 tonne of CO2. This remains locked in during the production of engineered timber elements and over the life of the building.

But when the structure is burned or sent to landfill, this carbon is released back into the environment. The way that most timber buildings are currently designed, with no thought for how the components might be recovered and reused, makes this a certainty. This means that we are missing a major opportunity to keep it locked away for longer, and to avoid the CO2 emissions associated with producing new building materials. Why, given the pressing need to reduce and stabilize atmospheric CO2 levels, are we content to design these durable, highly engineered elements to be single-use?

Accurate figures are hard to come by, but some estimates put the volume of recycled wood products as low as 30%. In comparison, over 95% of steel recovered from demolition waste in Europe is reused and recycled into new steel products, and a similar proportion of concrete waste is crushed and repurposed for other uses such as recycled aggregate. To maximize the long-term sustainability of engineered timber buildings, the timber recovery and reuse market must reach the same level of maturity.

This is partly a question of policy and economics, but it’s also an engineering challenge that we need to start solving now. The potential stock of reusable timber components increases with every new timber building — but only if we design them so that they can be dismantled.

De Geusselt swimming pool in Maastricht is based on “cradle-to-cradle” principles, constructed from sustainable materials that can be reused at the end of their lives. The timber-frame structure with engineered timber panels is designed so that all of the elements can be dismantled and used again. It sits on a concrete base that contains a high proportion of granulate from demolition waste, in place of coarse aggregate. WSP was the structural engineer and energy advisor on the project. Photo: Marcel van der Burg

THE HOW


Keep systems separate

The life expectancy of building systems varies. For example, cladding may be designed to last 20-25 years, compared to 40-50 years for the load-bearing structure within or behind it. Keeping systems separate ensures that those with a shorter service life can be repaired or upgraded with low risk of damage to other building elements. Cladding should be modular and connected to the structure at discrete points using easily accessible brackets or fixings; services should run in dedicated zones, avoiding integration with structural elements or screeds. This extends the lifespan of the building while ensuring that the structure is protected.

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Plan for dismantling as well as construction

Just as designers consider how a building can be safely constructed, they should also think about how it can be dismantled at the end of its life. This may mean avoiding “cliff edge” structural designs where the removal of an element, or series of elements, would lead to the loss of stability and robustness, or sizing structural elements so that they can be lifted using commonly available mobile cranes or equipment. An outline plan for dismantling the building should be included in the design documentation, describing the assumed sequence and method, as well as any temporary works that will be needed. Provisions for dismantling the structure should be integrated into the elements wherever possible to reduce the additional works required. This could include sockets for lifting eyes, built-in connection points for temporary works, or fixings at floor edges for the necessary edge protection.

"Just as designers consider how a building can be safely constructed, they should also think about how it can be dismantled at the end of its life"

Hand over accurate, as-built information

A digital twin of the as-built structure, updated over the building’s life, is an invaluable resource for future designers, with the ability to contain almost limitless information about the arrangement and position of elements, materials used, design and connection loads, proprietary product data and aspects of sustainability such as embodied carbon. Emerging technologies allow elements to be marked with QR codes or embedded with chips linked to cloud-based databases, to improve data collection and accessibility.

Designers should also provide a clear maintenance and repair plan including guidance on the frequency and types of inspection, anticipated or likely repairs, and any elements of the structure that are likely to need replacing or repairing before the design working life is reached, such as structural bearings and coatings. If a structure is properly maintained, there is a much higher chance that the elements will be in a good condition when the building is no longer needed.

Specify to allow the use of recovered components

Structural specifications should be written flexibly to allow contractors to use recovered engineered timber where it is available, subject to all necessary provisions for quality, performance and safety. In all instances, they should call for manufacturers to have independently verified policies and systems on quality, environmental management, and health and safety, both for their own operations and those of their supply chain. All new manufactured timber products should be sourced from forests that are responsibly managed, socially beneficial, environmentally conscious and economically viable, and it is recommended that all timber be 100% Forest Stewardship Council (FSC) certified.

Thomas Musson is a structural engineer and consultant with WSP in the Netherlands

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