Proposed Change 1872

Article 5.6.4.3. of Division B of the 2020 edition of the National Fire Code of Canada (NFC) provides requirements related to the type and degree of encapsulation protection of mass timber elements necessary in encapsulated mass timber construction (EMTC) while the building is under construction. In particular, these requirements mandate that at least 80% of the underside of mass timber floor assemblies be protected by some minimum level of encapsulation (e.g., using one layer of 12.7 mm thick Type X gypsum board). 

This proposed change is related to PCFs 1870 and 1963 from the national public review of proposed changes to the 2020 National Model Codes (which ran from October 24, 2023 until December 18, 2023), which described proposed changes to Articles 3.1.6.4. and 3.1.6.6., respectively, of Division B of the 2020 edition of the National Building Code of Canada (NBC). These NBC Articles address the requirements for and exceptions to the encapsulation of mass timber elements in finished buildings of EMTC. In particular, the Articles permit from 10% to 25% of the surface of mass timber ceilings to be left exposed within a suite.

PCFs 1870 and 1963 included proposed changes, based on several research studies involving mass timber compartment fire scenarios, to allow 100% of the surface of mass timber ceilings within a suite to be exposed.

After considering the comments from the public on PCFs 1870 and 1963, it was recommended that these proposed changes, with some minor modifications, be included in the NBC 2025.

Consequently, there is a need to propose the alignment of the requirements for mass timber protection for ceilings during construction in NFC Article 5.6.4.3. and allow ceilings to be left exposed where no such encapsulation is required in the finished building.

Further, during current EMTC projects, difficulties have been encountered when reconciling the requirements of NFC Article 5.6.4.3. during construction with measures to control moisture and mould affecting the encapsulated wood elements, as well as the materials used for encapsulation.

In aligning the provisions of the NBC and NFC to permit 100% exposed mass timber ceilings during construction, the following impacts of not encapsulating the ceilings must be considered:

• the risk of fire spread to more than one storey during a fire, and
• the need to ensure emergency responders and firefighters can access the fire floor.

Critical elements for reducing and limiting these impacts include:

• development of and compliance with a construction fire safety plan, which may include:
  • managing combustible materials on site,
  • maintaining access to exit routes from and ingress routes to the fire floor, and
  • providing site security (e.g., to reduce the threat of arson) and monitoring of fire hazards. 
• reducing or protecting the physical pathways along which fire spreads within a storey or between storeys.

Experience has shown that light wood-frame buildings under construction constructed are highly susceptible to significant loss or collapse in a fire, especially when the passive and active fire protection measures that have been planned for the finished building are not in place. Recent fire research on mass timber buildings using both compartment and construction fire scenarios indicates that mass timber buildings are more structurally resistant to the effect of fire and more difficult to ignite and sustain combustion, especially given the reduced fuel load on a construction site.

PCFs 1870 and 1963 include proposed changes, based on the most recent fire research, to allow 100% of the underside of each mass timber floor assembly within a suite to be left exposed in the finished building, expanding the NBC provisions related to encapsulation of mass timber elements within buildings permitted to be of EMTC.

The rationales supporting both PCFs 1870 and 1963 explain how recent fire research was performed that supports the proposed changes to the permitted percentage exposure of mass timber elements. With that, this proposed change revises the degree of protection of mass timber ceiling elements in EMTC while a building is under construction. This proposed change would harmonize the NFC requirements with those of the NBC for finished buildings of EMTC.

The NRC report titled, “Large-Scale Fire Tests of a Mass Timber Building Structure for MTDFTP" [1], describes a total of five mass timber compartment fire tests in a large two-storey structure that included varying amounts of exposed mass timber wall, ceiling, floor, beam and column surfaces. Two of the five fire tests used construction site fire scenarios with different levels of fuel loads. Test 3 was conducted in a small (3.2 m × 7.0 m × 3.0 m) compartment on the second storey with a metal garbage can containing a wood crib fuel load, while Test 4 was run in a larger compartment (7.1 m × 7.5 m × 3.0 m), again on the second storey with a more severe moveable fuel load of wood cribs and unprotected light wood-frame partition walls. These relatively small compartments represented a severe fire scenario from the point of view of heat re-radiation between mass timber surfaces and other combustible surfaces, which is a critical aspect for the continued burn of the mass timber surfaces once the movable fuel load is consumed. The window openings of the compartments, located within the protected light frame steel exterior walls, provided a ventilation condition in which a longer fire duration and confinement led to higher plumes being ejected through the openings. This ventilation condition represented a more severe fire scenario compared to an open floor area undergoing construction. Table 1 provides a summary of the tests.

Table 1. Summary of Mass Timber Demonstration Fire Test Program (MTDFTP) Construction Site Fire Compartment Test Configurations

Both fire test scenarios had 100% of the ceilings exposed, yet both design fires reached the decay phase within 60 min with no fire department intervention which, in a real construction scenario, would occur following site personnel detection. These results could suggest that other fire safety measures may be an option to encapsulation during construction while appropriately reducing the risk of fire spread to more than one storey.

Consideration of the Fire Safety Concepts Tree

The new fire safety strategies proposed in PCF 1872 for EMTC buildings are built upon, supplement, or provide options to the existing fire safety strategies in NFC Subsections 5.6.1., 5.6.3. and 5.6.4. Overall, these combined fire protection strategies for construction and demolition sites provide a holistic and reliability-structured approach to limit the escalation of fire and have generally been developed in the context of the Fire Safety Concepts Tree (FSCT) described in NFPA 550, "Guide to the Fire Safety Concepts Tree." The FSCT provides a structured means to examine fire safety strategies in a risk reduction context, and in consideration of the reliability of sets of measures (for more details, see Figure 4.3 in NFPA 550).

Two key objectives of the FSCT are:

1. Prevent fire ignition: reduce the probability of fire occurring
2. Manage fire impact: reduce the consequence of the fire

Table 2 categorizes and annotates select NFC provisions relative to the objectives and sub-objectives of the FSCT.

Table 2. NFC Provisions (Existing and Proposed) Relative to the Fire Safety Concepts Tree

Note to Table 2:
(1) The abbreviations used in the table have the following meanings:
MR+MT = midrise and mass timber
MT-e = mass timber — existing 
MT-p = mass timber — proposed

Organization of mitigation strategies based on the FSCT allows for consideration of the holistic approach to control fire initiation (ignition), growth and spread. These strategies can also be considered in the context of degrees of reliability in limiting fire escalation. For example, certain strategies limit the probability of fire initiation; however, where ignition occurs, additional measures then limit early fire growth. The probability of fire escalation is reduced at each stage of fire development by individual or groups of strategies. The proposed requirements build upon or provide options to the existing strategies to more specifically address the fire hazards associated with mass timber building construction and demolition sites.

Proposed Article 5.6.4.3.-2025 on Measures to Reduce the Risk of Fire Spread Between Storeys

This proposed Article addresses the reduction of risk of fire spread to more than one storey and includes a reference to both prescriptive  and performance-based approaches applicable to all EMTC buildings under construction.

This proposed Article is analogous to the current NFC Article 5.6.1.2. for measures to mitigate fire spread to adjacent buildings, but includes an explicit prescriptive path as well. This proposed Article allows for the protection and hazard management approaches in Articles 5.6.4.4. to 5.6.4.7.-2025, while also allowing for the option of a fire risk assessment being conducted to determine the level of fire protection required on the site.

The proposed accompanying Note provides recommendations related to the professionals conducting the risk assessment.

Proposed Revision to Sentence 5.6.4.3.(2) (Now 5.6.4.4.(2)-2025) on Contiguous Storeys

A minor editorial revision is also proposed to clarify the intent of the existing requirement in Sentence 5.6.4.4.(2)-2025 regarding the four uppermost storeys.

Because this proposed change to the NFC aligns the requirements with the proposed changes to the NBC in PCFs 1870 and 1963, builders would not need to encapsulate the underside of floors during construction, only to remove the encapsulation when the building is finished, as would be permitted in the NBC as a result of PCFs 1870 and 1963.

Proposed Revision to Sentence 5.6.4.3.(3) (Now 5.6.4.4.(3)-2025) on Encapsulation Material

This minor revision aligns the existing provision on expanding the application methods for gypsum board encapsulation materials with the proposed changes to the NBC as a result of PCF 1963.

Proposed Article 5.6.4.5.-2025 on Alternative Protection Measures

Proposed Article 5.6.4.5.-2025 introduces an alternative to the requirements described in Clause 5.6.4.4.(1)(a)-2025, for protective encapsulation, and in Article 5.6.4.6.-2025, for progressive sprinklering. Article 5.6.4.5.-2025 introduces new fire protection measures to address safety during construction. These new measures are included in the following provisions:

• Sentence (2): the proposed measures for the protection of openings through floor assemblies intend to limit the potential for fire spread through the openings to upper storeys for a minimum of 30 min. This 30-min value aligns with the 15-min fire performance required of fire block materials (see NBC Article 3.1.11.7.), wherein a 12.7 mm thick gypsum board and a 12.5 mm thick phenolic bonded plywood board are each deemed to remain in place and prevent the passage of flames for at least 15 min under standard fire exposure. The accompanying Note also explains that large openings may require additional fire protection.
• Sentence (3): the protection of perimeter joints between the edge of floor assemblies and exterior walls to limit the potential for fire spread through the joint to upper storeys.
• Sentences (4) and (5): the installation of hose stations with hose lines on standpipe systems for use in extinguishing or controlling any fires that may occur.
• Sentences (6) and (7): a person to conduct a fire watch on all storeys at certain times during the workday and afterwards to ensure the exits are kept clear and ensure the fire safety plan is followed. An explanatory Note provides recommendations regarding the knowledge and skill level of the person conducting the watch and considerations to be addressed by the construction fire safety plan, if necessary, such as the scheduled intervals of supervision of the site and the overall construction works during nonworking hours.

Proposed Sentence 5.6.4.5.(8)-2025 on Sequence of Implementation of Alternative Fire Protection Measures

Proposed Sentence 5.6.4.5.(8)-2025 is intended to be applied in conjunction with the other proposed requirements for protection described in that Article.

This involves a different approach than the current permission that allows the upper four unprotected contiguous storeys, in Article 5.6.4.4.-2025 and proposed Article 5.6.4.6.-2025. With that, once the fifth storey ceiling is installed, all storeys, including the first four storeys would then be required to use the alternative protection measures.

Proposed Article 5.6.4.6.-2025 on Progressive Sprinklering

Proposed Article 5.6.4.6.-2025 introduces an alternative to the requirements described in Clauses 5.6.4.4.(1)(a), (c) and (d)-2025 for protective encapsulation and in Article 5.6.4.5.-2025 for alternative protection measures. Proposed Article 5.6.4.6.-2025 introduces an alternative approach to the passive fire protection requirements by providing for an operational automatic sprinkler system that is installed progressively during construction in buildings of EMTC.

Proposed Article 5.6.4.7.-2025 on Indoor Storage of Combustible Materials

This proposed Article aims to manage the amount of combustible materials in discrete piles of limited sizes, as well as place restrictions on the relative locations of these piles. This provision aims to reduce the potential for fire spreading to multiple piles or from other ignition sources, without affecting the safe egress of occupants and while allowing for safe firefighting staging tactics.

Sentence (1): In line with current practices, the proposed limit on footprint area of an individual pile of combustible materials is representative of a standard wooden crate size (1.2 m × 2.4 m) used to transport materials.

Clause (2)(a): The proposed 3 m clearance between storage of combustible materials on a storey and the outer edge of the floor assembly has been established based on a study involving a radiant heat transfer analysis from the flame extension of the fire plume from a fire on the storey directly below the storey containing the storage.

The analysis assumed a vertical exposing plane on the upper storey, perpendicular to the slab edge, 20 m in length and extending the full 4 m storey height to represent the plume flame extension from below, reaching to and beyond the next storey above. Testing and studies of flame extension from openings in a compartment enclosure has shown a reduction of flame temperature away from the opening as a function of the horizontal distance, as well as a reduced emissivity as a function of the flame depth measured from the plane of the wall. A sensitivity study was conducted examining the radiant heat emissive power of the 20 m × 4 m plane as a function of temperature and emissivity, and the results from this study were then used to determine acceptable distances at which a critical heat flux of 30 kW/m² (approximate autoignition heat flux for wood) would not be reached.

In establishing an acceptable distance, the study considered heat flux measurements of 30 kW/m² in one of the construction site tests as part of the NRC report [1], as well as the acceptance and calibration criteria (35 kW/m² and 45 kW/m², respectively) used with respect to the NBC referenced standard CAN/ULC-S134, "Standard Method of Fire Test of Exterior Wall Assemblies," for exterior flame exposure.

This information is supportive of an upper emissive heat flux of approximately 50 kW/m² to establish acceptable corresponding distances from the sensitivity study. These distances ranged from 0 m to 2.6 m. Therefore, based on these results, 3 m has been identified as an acceptable distance for clearance between storage of combustible materials and the outer edge of the floor assembly.

Clauses (2)(b), (c) and (d): The proposed 3 m separation distance between individual piles is somewhat analogous to a similar provision for controlling the effects of fuels in proposed Sentence 3.1.6.4.(4)-2025 (PCF 1870), which would permit two facing or adjacent exposed mass timber walls within a suite provided a 4.5 m horizontal distance is maintained between them. A distance of 3 m between storage piles as well as between piles and exits is considered to provide a reasonable level of protection in this situation and maintains access to exits to facilitate the safe egress of occupants as well as staging for firefighting operations. The 3 m dimension is also applied elsewhere in the NFC and NBC where limits are placed on the distance between storage and other elements (e.g., wall openings, flammable liquid storage and exits) in occupied buildings.

Sentence (3): A clearance of 1.5 m is proposed to be maintained between the top of storage and the underside of the lowest plane of any beam, floor or roof assembly above the storage to permit hose streams to be directed onto the top of storage, which is 50% greater than the NFC provision (Sentence 3.2.2.3.(2)) for 1 m clearance above the indoor storage in a building that is not sprinklered.

Explanatory Note: Proposed Note A-5.6.4.6.-2025 provides further insight into the accumulation of short-term transient storage which may be necessary for daily construction activities and connects the management of combustible materials to the existing NFC provisions.

Reference

[1] J. Su, E. Gibbs, M. Weinfurter, P.-S. Lafrance, K. Gratton, A. Frade, and P. Leroux. “Large-scale fire tests of a mass timber building structure for MTDFTP”. National Research Council of Canada. Report No: A1-018329.1/A1-018487.1 (Ottawa, Canada; May 2023).

This proposed change would introduce an option to allow 100% of the surface of the underside of each mass timber floor assembly to be exposed during construction. As noted above, Article 5.6.4.3. of Division B of the NFC 2020 mandates that at least 80% of the underside of mass timber floor assemblies be protected by a minimum level of encapsulation (e.g., using one layer of 12.7 mm thick Type X gypsum board).

As a result of the proposed increase in exposed ceiling construction, risk of fire growth and extended duration of flaming combustion also increases, which can contribute to spread within the storey and between storeys. 

A more holistic approach is warranted and is being proposed in these changes with respect to addressing such risks from both a performance- and prescriptive-based perspective. This approach is somewhat analogous to what is already intended by NFC Article 5.6.1.2. with respect to addressing fire spread from construction sites to adjacent properties.

Table 3 provides a list of the proposed changes to the NFC, including the intent of the changes as related to managing/reducing the risk of fire during construction.

Table 3. Proposed Changes Related to Course of Construction Fires in EMTC

This proposed change provides additional options for fire protection during building construction, which may reduce costs in some instances as well as the need for extensive oversight from the authority having jurisdiction. As the existing compliance options (protective encapsulation) remain in the NFC, this proposed change would not entail any additional costs should that option be used.

By aligning the NFC requirements for the protection of mass timber elements during construction with those proposed for the NBC 2025 for the finished building, the expectation is that costs would either remain the same or be reduced due to the harmonization of requirements.

As the installation of encapsulation materials during the construction process can be physically challenging, having the option not to install encapsulation materials could help avoid issues related to:

• moisture and mould,
• costs of fixing moisture-related damage, and
• creation of waste,

while maintaining an acceptable level of fire safety.

There are no anticipated enforcement implications as this proposed change could be enforced by the existing Code enforcement infrastructure.

Architects, designers, developers, owners, and engineers would benefit from the increased flexibility provided by this proposed change.

Authorities having jurisdiction, including fire departments, would need to continue to evaluate their operating procedures in response to the requirements for EMTC.