New Code Updates Unlock Greater Aesthetic Potential
On April 10th, 2024, the Ministry of Housing announced that the 2024 BC Building Code was being updated to allow for an expanded use of encapsulated mass timber construction (EMTC). At the same time, the province of Ontario announced their intent to amend the Building Code in the coming months to similarly permit encapsulated mass timber construction up to 18 storeys.
While at the time there was much discussion around the new 18 storey height limit, a more interesting nuance in the new code provisions for EMTC was the allowance for certain occupancies to have fully exposed mass timber construction, and for a broader range of occupancies that were permitted to be mass timber.
Prior to this recent April update, EMTC was permitted in Ontario and BC up to 12 storeys in building height, with an area limit of 6,000m² for residential occupancy and 7,200m² for commercial occupancy. There were allowances for certain occupancies to be in the lower 2 to 4 stories, including assembly, mercantile, and low-hazard industrial.
Crucially, mass timber was required to be encapsulated with a material providing at least a 50-minute encapsulation rating, most typically achieved with two layers of 12.7mm Type-X gypsum boards. Prescriptive allowances existed for exposing mass timber, though this was capped at approximately 25% of the ceiling in a suite or 35% of the wall area. This could be increased through the use of alternative solutions to maximize how much timber is exposed, something Entuitive has experience in helping clients to achieve, but the prescriptive options for exposing timber were limited.
What’s New in the 2024 Building Code Updates?
Now, a new construction classification has been added to subsection 3.2.2.93 of the BC Building Code and 2024 OBC Amendment Document that greatly expands the options available to design teams.
This new construction classification is titled “Encapsulated Mass Timber Construction, Various Occupancies, Heights and Areas, Sprinklered.” As the name of the construction classification implies, it contains a table of various permitted building sizes for each occupancy, replicated below with highlights added by Entuitive.
One of the most exciting parts of this new code update is the range of buildings and occupancies permitted with a 0-minute encapsulation rating in 3.2.2.93. Since the required encapsulation rating is zero minutes, this means the mass timber in these buildings is fully exposed to view with some minor exceptions like public corridors and exits.
Notably, this means design teams can prescriptively pursue fully exposed mass timber buildings for 6-storey Group A-2 assembly, 4-storey Group B-3 care (no treatment), 8 storey residential, 9 storey commercial, and 6 storey retail buildings. While this was possible before through the use of alternative solutions, a prescriptive pathway greatly reduces approvals risks and uncertainties for these projects.
Encapsulation Rating vs. Fire Resistance Rating
The encapsulation rating is different from the fire-resistance rating, and not to be confused. While the buildings described above now permit fully exposed mass timber construction, they still require a two-hour fire-resistance rating. This can be achieved either through char calculations, protection with encapsulation materials, or UL tested assemblies.
On past projects, we’ve found there is an optimal scenario where timber exposed to view is sized based on charring to achieve the required fire-resistance rating, while timber that may be encapsulated for other reasons can take advantage of this drywall protection for part of the fire-resistance rating to reduce fiber.
As an example, the timber design standard O86 permits 60 minutes of the fire-resistance rating to be provided by two layers of 12.7mm gypsum board, with the balance achieved through charring. This optimization is also specific to the timber system used, as reducing the depth of timber only results in savings if the entire panel is thinned and not just a portion of it. It demonstrates the value of designing timber buildings holistically, accounting for the required fire-performance of the buildings as part of the structural design, something Entuitive has deep experience in by offering structural and building code consulting services on these types of projects.
Reduced Approval Risks: Prescriptive Pathways for Mass Timber
These new allowances for exposed mass timber are already adopted in the BC Building Code. The OBC Amendment Document was also recently updated to include these new allowances, which will be adopted on January 1, 2025, when the 2024 OBC comes into effect and harmonizes with the National Building Code.
By prescriptively providing a path for 6- to 9-storey fully exposed mass timber buildings, these new code allowances are helping midrise mass timber projects to become more cost effective with less uncertainty on the approvals side. We see this as a huge step forward for the scale of projects where mass timber was already a very viable construction typology providing sustainability, biophilic, and architectural benefits.
To further explore mass timber, please reach out! We would love to discuss how our structural, code, building envelope, sustainability, and bridge services can leverage this material to contribute to a better built environment.