Toronto's TRCA Building: Unpacking Moisture in Mass Timber Roofs During Construction

Mass timber is revolutionizing the Toronto construction scene, championed by projects like the Toronto and Region Conservation Authority’s (TRCA) new administrative headquarters in North York. This four-storey, 7300 m² building showcases the sustainability and aesthetic appeal of CLT, but as a recent 11-month field study reveals, it also highlights the critical importance of moisture management in the unique Toronto climate.

The TRCA: A Real-World Mass Timber Laboratory

The TRCA building isn't just a construction project; it's a valuable case study. Researchers meticulously monitored the CLT roof assemblies during construction, providing crucisal insights into the real-world moisture behavior of this innovative material in Toronto. CLT panels are hygroscopic, meaning they readily absorb and release moisture. While this contributes to the natural breathability of wood, it also makes CLT panels vulnerable to moisture intrusion, especially in Toronto's variable weather conditions.

Key Insights for Toronto's Mass Timber Projects:

Edge Protection is Crucial: The study at the TRCA site confirmed high moisture vulnerability at CLT panel edges, especially before membrane application, a key consideration for Toronto's wet seasons.
Dry-Out Delays: Panels exceeding 15% moisture content experienced significantly prolonged drying, particularly at the exterior layer, emphasizing the need for robust drying protocols in Toronto's climate.
Unpredictable Moisture Movement: The study revealed unexpected moisture sorption within CLT panels, highlighting the complexity of moisture dynamics, even in planned projects.
Membrane Timing Matters: Applying impermeable membranes before proper drying significantly slowed the process, impacting construction schedules.
Measurement Precision: Accurate moisture measurement in CLT remains challenging, even with advanced monitoring, requiring careful data interpretation.

Why This Matters for Toronto's Mass Timber Future:

Understanding these moisture behaviors is vital to protect Toronto's mass timber buildings from:

Aesthetic damage like staining and mold.
Health risks associated with mold growth.
Potential structural decay.

Essential Practices for Toronto's Mass Timber Construction:

Prioritize edge protection against moisture.
Ensure thorough CLT drying before membrane application.
Implement continuous moisture monitoring and mitigation.

Further Research Should Focus On:

Long-term CLT roof performance in Toronto's specific climate.
Improving moisture monitoring techniques for our conditions.
Evaluating the effectiveness of various moisture control strategies.

The Future of Mass Timber in Toronto:

As mass timber construction continues to gain popularity in Toronto, the lessons learned from the TRCA building is going to be invaluable. By understanding the hygroscopic nature of wood and implementing effective moisture control strategies specific to Toronto, we can ensure the longevity and sustainability of mass timber buildings. For more detailed information, refer to the full case study provided by Dorothy Johns and Russel Richman.