Typology
3-Storey Apartment Building

Occupancy
Group C, 6 Dwelling Units

Classification
2024 Ontario Building Code, Part 9

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Status: Submitted for Building Permit, City of Toronto (March 2025)

A building permit application and alternative solution has been submitted to the City of Toronto for a three-storey apartment building to be served by a single exit stair. Two semi-detached houses are proposed to be replaced with context-appropriate “missing middle” housing. The proposed building is located just south of Bloor Street, and a 5-minute walk away from two TTC subway stations; an ideal location for gentle density in urban areas.

The proposed building contains six dwelling units of varying sizes. One basement dwelling unit, one two-storey dwelling unit split between the basement and ground floor, one ground floor unit, two second floor units, and a large third floor unit (for a total of 6 dwelling units and total of 15 bedrooms). An entrance lobby and mail room are located on the ground floor; a mechanical room and bicycle room are located in the basement. The building is also designed to support aging-in-place, as all units are provided with elevator access.

Single-stair is ideal for this infill housing project, allowing for optimization of floor plates, better unit layouts, and more leasable and habitable floor area. The municipal zoning by-law permits multi-unit residential buildings of up to 3 storeys and the design team explored many two-stair options for the site (with both interior and unenclosed exterior stairs). However, to maximize floor area efficiency and exterior landscape areas, as well as optimize access to light, views and privacy for each apartment, the preferred solution is a single stair design.

The proposed development consists of dwelling units on each of the four levels (basement to third storey). Egress from dwelling units is subject to the provisions in OBC 9.9.7.2. and OBC 9.9.9.

In accordance with OBC 9.9.9.2.(1), in an apartment style dwelling unit building, where an egress door from a dwelling unit opens onto a public corridor, it is required to be possible to travel in opposite directions to two separate exits in the corridor unless if the dwelling unit is provided with a second and separate means of egress. Each floor area in the building is provided with dwelling units. As such OBC 9.9.9.2.(1) requires access to a minimum of two exits from inside the public corridor that serves each storey.

The main level has access to an exterior exit door through the exit stair enclosure. The two-storey dwelling unit on the main level has two means of egress: one via an interior stairway to the level below and then to the exit stair enclosure and another through an exit door. Therefore, egress from this unit complies with the prescriptive requirements of the Ontario Building Code. However, the single-storey unit on the main level is served by only one exit and does not meet acceptable solution requirements in the Code.

The alternative solution report will therefore address exiting from the main level, second and third level.

The building will be designed in accordance with the provisions in Part 9 of the OBC.

The proposed alternative solution incorporates active and passive fire protection measures designed to achieve timely, unimpeded egress without undue delays during emergencies. These measures are informed by well-established research and standards, including NFPA 101, Life Safety Code, and NFPA 13R, Standard for Sprinkler Systems in Low-Rise Residential Occupancies. Key compensatory measures include: 

• Sprinkler Protection: The installation of a sprinkler system compliant with NFPA 13R will provide an additional layer of fire suppression, significantly reducing fire growth and improving tenability for occupants.

• Shorter Travel Distances: Travel distances to the single exit are minimized, ensuring occupants can reach the exit enclosure within a time frame consistent with life safety standards.

• Enhanced Fire Resistance Ratings: The building structure, exit enclosure, and suite separation walls and doors will have fire resistance ratings exceeding the Code's minimum requirements.

• Flame Spread Limitation: Interior finishes in the stair will be limited to a flame spread rating  of 25 or less, per CAN/ULC S102 testing, reducing the risk of fire propagation in critical egress paths.

• Occupant Load Management: The number of suites on each floor will be limited to ensure manageable occupant loads during evacuation, supporting faster and safer egress.

• Fire Safety Plan: A fire safety plan will be developed for the building, with emergency planning procedures for evacuation and maintenance procedures for life safety systems in the building.

1. Risk Assessment (Comparative OBC Part 9 Analysis)
2. Evacuation Timing (Timed Egress Analysis)
3. Fire Protection System Success Probability for Sprinkler Systems (Reliability Analysis)
   
4. Comparative Safety Analysis (Single-Exit Sprinklered vs. Two-Exit Non-Sprinklered Designs)

A comprehensive risk assessment demonstrates that the proposed design achieves equivalent safety outcomes when compared to a prescriptive Code-compliant solution with two exits. The analysis considers fire scenarios, occupant egress timelines, and potential interactions with emergency responders. Notably, evacuation modeling under various occupant load scenarios confirms that:

• Timely Egress: All occupants can safely evacuate the building within acceptable timelines, with movement unimpeded by delays or bottlenecks.

• Firefighter Access: The design minimizes interactions between evacuating occupants and responding firefighters, ensuring operational efficiency during fire suppression efforts.

The proposed design achieves equivalence to OBC requirements by combining robust active and passive fire protection measures with thoughtful egress design. The alternative solution with a single exit, supplemented by these compensatory measures, aligns with the performance objectives of the OBC. It ensures that occupant safety, fire resistance, and egress efficiency are not compromised while addressing the practical constraints of the building site.