Mass timber construction: Building for the better

Carbon sequestration

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 and has been known for some time. For example, 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, 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.

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