Mass timber construction: Building for the better

February 26, 2021

Sarah Hicks and Scott Jackson

[1]
Photos courtesy ZAS/BMCEA

Construction, maintenance, and demolition of the built environment are a leading source of the world’s carbon dioxide (CO2) emissions. In the face of climate change, exponential population growth, and urban densification, it is imperative society develops sustainable building solutions to meet growing housing and infrastructure needs without accelerating climate change.

The building industry must be empowered to tackle these challenges with every possible tool. The industry cannot improve by doing things the way they have always been done, and small incremental changes over time will not be enough. New products, systems, and ways of thinking must prevail. This understanding of the need for a radical shift in our thinking is fueling a mass timber revolution in the construction industry. Environmentally responsible development requires high-performance buildings that pay greater attention to both embodied energy and operational impact. Mass timber construction provides an avenue for delivering sustainable buildings of the future.

We now know for a certainty mass timber is a strong and fire-safe construction option. There is ample empirical evidence proving this from building material research scientists in Canada and around the world. There are also thousands of mass timber buildings already in service that demonstrate this to be true. Nevertheless, mass timber construction has, until relatively recently, been a niche market.

That is about to change, thanks to the industrialization of the construction process. The shift from site-built developments to offsite manufacturing, enabled by building information modelling (BIM) and computer numerical control (CNC) fabrication, is gaining momentum. Mass timber is perfectly suited to this form of construction.

Prefabricated mass timber components have the potential to transform the construction process and the built environment while simultaneously reducing its carbon footprint. Canada’s forest management practices are recognized among the best in the world with more third-party certified lands than any other country. The country has the natural resources and intellectual capital needed to be a world leader in green construction technology and sustainable development if the industry can be transformed to unlock the potential of mass timber builds.

Forests as carbon sinks

[2]
The Toronto Region Conservation Authority’s (TRCA’s) new four-storey administrative office building in Toronto incorporates numerous sustainability features, including a mass timber structure.

Although the creation of artificial carbon sinks has been explored, no major artificial system has been developed to remove carbon from the atmosphere as effectively or at the scale forests do.

Wood is not simply our only major renewable building material, but also the most environmentally friendly option with respect to the construction and development sectors. Wood uses less energy to produce than alternatives, results in less water and atmospheric pollution, and perhaps, most importantly, represents the best opportunity we have to mitigate the impacts of climate change.

As trees grow, they remove CO2 from the air. On average, 50 per cent of an 80-year-old tree’s dry weight is made up of carbon.[3]

However, when it comes to carbon sequestration, all trees do not perform equally. The amount of CO2 removed from the atmosphere is directly related to the incremental growth of the tree (i.e. the more weight a tree puts on in a given year the more carbon dioxide it removes from the air). Small trees may double in size quickly, but their overall growth is relatively small. Conversely, older (or larger) trees will eventually stop growing. As a result, while larger trees may represent big storehouses of carbon[4], they do not typically remove additional carbon dioxide from the air (i.e. no net gain). In fact, elderly trees can act as carbon sources as they senesce and decay. Older forests are also more susceptible to some natural disturbances, such as fire, which can result in large stores of carbon being returned[5] to the atmosphere in a relatively short period of time. The frequency and severity of fires[6] are expected to increase in the face of climate change, potentially exacerbating this concern. In Ontario, Crown-managed forests[7] (the publicly owned area that is actively and sustainably managed for forestry operations) are currently net carbon sources, in part due to an older than natural age structure. Ultimately, middle-aged trees provide the greatest rate of carbon sequestration.

Carbon sequestration

[8]
Offsite construction provides many benefits, such as enhanced sustainability, controlled expenses, reduced construction noise and other neighbourhood disruptions, more efficient use of material and labour resources, and improved worker health and safety.Image © Sarah Hicks

These are all important considerations when looking at wood as a building material since wood products—especially long-lived types—can extend the shelf-life of the carbon stored in a tree. When a tree is harvested and processed, the carbon in the wood is retained in the resulting product, often well beyond the natural life of the tree. In many cases, this results in extended carbon storage on a scale of decades or even centuries. As new trees are planted or take root, they begin removing carbon dioxide, resulting in an additional net benefit.

Peter Moonen, national sustainability manager at the Canadian Wood Council (CwC), speaks frequently on the topic of sustainable wood construction. He often addresses the sustainable materials question like this: “If I were to invent an organic, solar-powered, renewable, reusable, and recyclable material that cleans the air and water, gives off oxygen, provides recreation and habitat well as a strong and lightweight building product, I would be a very wealthy person! But we already have this material. We have wood. We have trees. So when people ask me why we should build with wood, I always say that is the wrong question. The real question is if we are going to build, what are we going to build with if not wood?”

Sceptics, however, question the true sustainability of using forest resources for development, fearing what may work at a smaller scale might not be true at a larger level. “This is a question we run into sometimes,” confirms Patrick Chouinard, founder of a mass timber fabricating company. “It is an important one. We know our operations are sustainable, and we are committed to using Forest Stewardship Council (FSC)-certified Ontario wood inputs for our factory, but the magnitude of our forest resource was challenging to explain until a professional forester with the Canadian Institute of Forestry, who was presenting an educational session to our team, helped us look at the numbers in a way that relates directly to our own operation.”

In Ontario, for example, where Chouinard’s firm sources wood fibre, the sustainable allowable cut in 2018 was about 20 million m3 (705 million cf), of which approximately 15 million m3 (530 million cf) were actually harvested. Chouinard’s firm has a factory in St. Thomas, Ont. It has the capacity to produce 44,000 m3 (1.5 million cf) of cross-laminated timber (CLT) and glue-laminated (glulam) timber annually. The fact is the 5 million m3 (177 million cf) of wood identified in the sustainable allowable cut that went unharvested in 2018 could have kept more than 100 factories the size of the St. Thomas one operating at capacity for a year.

Forest management practices in Canada are among the best in the world. In Ontario, for example, trees that are eligible for harvest are determined through a forest management plan, which outlines all activities that will take place over a 10-year period. The plan is developed in accordance with provincial legislation and policies. The areas to be harvested are driven by a need to emulate natural disturbance patterns and to maintain or re-establish natural forest conditions (i.e. what would the forest look like with no anthropogenic disturbance). The natural forest condition relates to both species composition and age class (how old the trees are). A natural forest is typically a mix of various species and age classes. Sustainable harvesting is not driven by what is in demand (product wise), but is a function of emulating the ecology of the forest.

The role long-lived wood products play in fighting climate change is well documented[9] and has been known for some time. For example[10], the amount of carbon stored in the structure of a typical light wood-framed home is roughly equivalent to the amount of carbon emitted by running the family car for five years. When it comes to larger buildings constructed in mass timber, the impact is greater.

A material take-off of the Toronto Region Conservation Authority’s (TRCA’s) new four-storey, 7432-m2 (80,000-sf) administrative office building, currently under construction, calculates the wood volume of the structure to be 3259 m3 (115,091 cf) of CLT and glulam. The building is intended to be one of the most energy-efficient office facilities in North America, and incorporates numerous sustainability features, including a mass timber structure.

The CWC’s Carbon Calculator[11], a free online tool, calculates the volume of wood in the TRCA building stores the equivalent of 2893 metric tons of carbon dioxide. However, this is not the entire story. The carbon benefit of the structure is greater because wood not only sequesters carbon, it also avoids the greenhouse gas (GHG) emissions that would have been generated by using other, more carbon-intensive materials instead of wood. In this case, the total potential carbon benefit of the wood structure is 4012 metric tons of carbon dioxide, which is roughly equivalent to taking 848 cars off the road for a year.

Prefabricated construction

Chouniard believes prefabricated construction is the future of the industry. “When it comes to meeting the major construction challenges of our time, how we build is as important as what we build with. Skilled labour shortages, compressed schedules, and the potential for greater quality control are all factors contributing to the popularity of prefabricated mass timber construction. Factory-built solutions harnessing the power of modularization are reinventing the construction industry,” he said.

The science is clear—building with sustainably sourced wood products can make a difference in the fight against global warming, but it can also change the quality of our buildings. When advanced mass timber products are combined with offsite construction processes, performance and efficiency are greatly improved. Mass timber is well-suited to prefabrication because it is easy to manufacture and light to transport and assemble.

The skills, knowledge, and behaviours of offsite construction are different from those of traditional site build. The move to a controlled indoor setting reduces risk and delivers leaner production and higher quality results. There are no adverse weather conditions to contend with during production; lighting and temperature are perfect all the time, schedules are predictable, and the operation can run 24 hours a day if needed.

Depending on the size and complexity of manufactured components, offsite prefabrication has the potential to considerably reduce a construction schedule, which can generate significant cost savings over a strictly site-built project. Offsite construction provides many other benefits, too, including enhanced sustainability, controlled expenses, reduced construction noise and other neighbourhood disruptions, more efficient use of material and labour resources, and improved worker health and safety.

Of course, to realize a successful project, professionals must take into consideration the unique characteristics of wood. As a natural, organic material with hygroscopic properties (able to take on and give off moisture), it can be vulnerable to the elements if exposed to rain and snow during transportation and storage onsite. To prevent unwanted water exposure, components are wrapped and the trucks are tarped at the factory prior to delivery. The protective coverings used during transit are removed only as necessary, keeping the timber protected from the elements. By using the just-in-time delivery model, the potential for moisture exposure is greatly reduced because components are not stored onsite for long periods of time. Nevertheless, if site storage is required, a detailed moisture management plan clearly outlining storage requirements and conditions is employed and other protective measures, such as factory-applied sheathing or coatings, can also be used depending on project requirements.

[12]
The volume of wood in the TRCA building is able to store the equivalent of 2893 metric tons of carbon dioxide. Image courtesy Canadian Wood Council.

The significant gains that can now be made through offsite manufacturing, including just-in-time delivery, are fuelled by BIM technology. BIM provides visualization capabilities for an integrated schedule, enabling simultaneous monitoring of offsite and field activities, empowering the project manager to more effectively manage the construction schedule. The ability to integrate offsite and field construction allows a project team to work in ways that synchronize the progress of these operations. Site preparation, for example, can be done while the building components are being fabricated in the shop.

The DfMA model

The degree of prefabrication is determined through a collaborative assessment phase that leads to a design optimized for manufacturing and assembly (DfMA). The model successfully integrates architectural, structural, mechanical, and logistical constraints. In that sense, design and manufacturing are intrinsically linked when it comes to modular mass timber building construction.

DfMA represents a shift in the delivery model of the standard engineer-to-order procurement method. The process of adding value is front-loaded, with earlier decision-making and purchasing required. Design integration activities are defined at the outset of the project to understand constraints and boundaries. The integration takes place in a collaborative BIM environment where dimensional information can be co-ordinated.

Asset information, such as specifications, part numbers, suppliers, and loading sequences, can be exchanged during this process. Panels and components are identified through a codification system that corresponds to the digital twin (BIM model). This enables accurate traceability of the panels and supports planning, logistics, and assembly activities, notably in the form of 4D modelling. This process provides an opportunity to make ongoing improvements by recording performance against planning and budget to integrate lessons learned.

The role of the mass timber supplier is to ensure the constraints relative to the mass timber shell are thoroughly considered at all stages of the design and strategic decision-making process, from early design to assembly. In this way, the fabricator can help guide the project team toward improvements that can be realized at scale through a commitment to detailing out complexity in form and sequence, thus generating savings.

Conclusion

The construction industry is changing to respond to the challenges of climate change, urban intensification, skilled labour shortages, demanding project timelines, and economic constraints. In the last decade, engineering innovation and the commercial development of mass timber components improved the industry’s ability to meet these constraints, and the result is the mass timber revolution we are witnessing today.

Going forward, the future of construction, of course, is high-performance buildings that pay attention to embodied carbon and operational impact while striving to achieve maximum social and economic value. These outcomes require a level of efficiency and co-ordination that can only be achieved through computer modelling and offsite manufacturing, an area where the wood sector is already operating and positioned to grow.

[13]Sarah Hicks brings 15 years of communications experience in the forest products industry to her role as manager of the marketing and communications department at Element5. Whether through articles, project case studies, or the wood education events she organizes for design and construction professionals, her mission is to tear down misperceptions about wood construction and share relevant information about mass timber that helps practitioners expand their capacity for wood design. She can be reached
at sarah@elementfive.co.

[14]Scott Jackson is the director of Indigenous and stakeholder relations at Forests Ontario. He oversees the It Takes a Forest initiative that delivers fact-based information to the public about forests and forest products, and the role they play in sustaining the economy, mitigating climate change, and creating healthy environments. Jackson has more than 20 years of experience in the field of natural resource policy and conservation science. He can be reached at sjackson@forestsontario.ca.

Endnotes:
  1. [Image]: https://www.constructioncanada.net/wp-content/uploads/2021/08/TRCA-aerial-_ZAS_BMCEA.jpg
  2. [Image]: https://www.constructioncanada.net/wp-content/uploads/2021/08/TRCA-Interior-Render-3-OFFICE-SPACE.jpg
  3. made up of carbon.: https://forestlearning.edu.au/images/resources/How%20carbon%20is%20stored%20in%20trees%20and%20wood%20products.pdf
  4. larger trees may represent big storehouses of carbon: https://www.ontario.ca/page/managed-forests-and-climate-change
  5. large stores of carbon being returned: https://cfs.nrcan.gc.ca/publications?id=38304
  6. frequency and severity of fires: https://www.nrcan.gc.ca/our-natural-resources/forests-forestry/wildland-fires-insects-disturban/climate-change-fire/13155
  7. Crown-managed forests: https://www.constructioncanada.net/waterfront-toronto-announces-quayside-development-opportunity-shortlist/
  8. [Image]: https://www.constructioncanada.net/wp-content/uploads/2021/08/Off-Site-Advantages-Graphic.jpg
  9. well documented: https://www.ipcc.ch/site/assets/uploads/2018/02/ar4-wg3-chapter9-1.pdf,%20www.nrcan.gc.ca/our-natural-resources/forests-forestry/indicator-carbon-emissions-removals/16552,%20www.ontario.ca/page/managed-forests-and-climate-change
  10. For example: https://www.safetycodes.ab.ca/
  11. CWC’s Carbon Calculator: http://www.cwc.ca/design-tools/carbon-calculator.
  12. [Image]: https://www.constructioncanada.net/wp-content/uploads/2021/08/TRCA-Preliminary-Carbon-Calculation.jpg
  13. [Image]: https://www.constructioncanada.net/wp-content/uploads/2021/08/Sarah-Hicks-Headshotcrop.jpg
  14. [Image]: https://www.constructioncanada.net/wp-content/uploads/2021/08/SJackson-Head-Shotcrop.jpg

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