NBC 2025 Standards for Combustible Elements in Encapsulated Mass Timber Construction

Navigating the National Building Code of Canada 2025 requires a deep understanding of fire safety and material science. At empcontracting.ca Inc., your construction contractor in London, Ontario, we ensure every mass timber project meets these rigorous safety standards for combustible elements.

Combustible Stairs and Floor Systems in Mass Timber

According to Division B 3.1.6.13, wood stairs and landings are permitted in exit stairwells within buildings of encapsulated mass timber construction (EMTC), provided they conform to specific floor assembly requirements. For stairs located within a suite, the regulations are more flexible, exempting them from certain articles required for main exit paths.

Raised Platforms and Subfloors

Per Article 3.1.6.16, floor systems for raised platforms are permitted to include combustible subfloors and finished flooring, allowing for aesthetic and functional versatility while maintaining structural integrity.

Regulations for Interior Wall and Ceiling Finishes

Article 3.1.6.14 outlines strict flame-spread ratings for interior finishes to ensure occupant safety:

• Thin Finishes: Combustible finishes not more than 1 mm thick are generally permitted on walls and ceilings.
• Wall Finishes: Materials up to 25 mm thick are allowed if they have a flame-spread rating of 150 or less.
• Ceiling Finishes: These face stricter limits, requiring a flame-spread rating of 25 or less, though 10% of the area may reach a rating of 150.
• Fire-Retardant-Treated Wood: Treated wood finishes up to 25 mm thick are permitted under specific conditions.

Combustible Elements in Partitions and Concealed Spaces

Partitions in EMTC buildings, including those using solid lumber (minimum 38 mm thick) or wood framing, are permitted under Article 3.1.6.15 if they are properly protected. Requirements include:

Protection Methods

1. A single layer of 12.7 mm Type X gypsum board (taped and filled).
2. A 19 mm layer of fire-retardant-treated wood for solid lumber partitions.
3. For wood-framed partitions, 19 mm fire-retardant-treated wood combined with noncombustible insulation in the cavities.

Concealed Space Safety

Under Article 3.1.6.16, only materials permitted in noncombustible construction are generally allowed to have exposed surfaces in concealed spaces within floor, roof, and wall assemblies. This prevents the unseen spread of fire within the building's core structure.

For professional implementation of these complex building codes, contact empcontracting.ca Inc., your construction contractor in London, Ontario, to discuss your next encapsulated mass timber project.