Typology
3-Storey Apartment Building

Occupancy
Group C, 6 Dwelling Units

Classification
2024 Ontario Building Code, Part 9

---

Status: Submitted to City of Toronto (March 2025), Accepted by City of Toronto (November 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.

The following compensating measures have been provided: 

• Sprinkler Protection of the Building: an automatic sprinkler system constructed and installed in accordance with NFPA 13-R, monitored and electrically supervised through the building’s fire alarm system. (Note: OBC does not require sprinklers at 3 storeys)

• Additional Sprinkler Protection at Dwelling Unit Entrances: installation of a sprinkler head within dwelling units located in proximity to the suite entry doors. (Note: NFPA 13-R does not require additional sprinkler coverage at this specific location) 

• Occupant Notification: a single-stage fire alarm system, designed and installed to the provisions of the Ontario Building Code. (Note: OBC does not require fire alarm system where not more than 4 suites share an exit, but does require a fire alarm system in any sprinklered building)

• Mechanical Smoke Control in Exit Stair: positive pressurization fan designed and installed to maintain a pressure differential of approximately 12 Pascals betweeen the exit stair and adjacent floor areas, to operate automatically upon activation of the building fire alarm. (Note: OBC does not require pressurization in buildings not classified as high-rise buildings per 3.2.6. and MMAH SB-4)

• Compartmentation: enhanced fire-resistance rating of floor assemblies, fire separation of exit and fire separations between suites of not less than 1 hour. (Note: OBC Part 9 requires a 45-min rating)

• Compartmentation and Smoke Control: enhanced fire-protection rating of suite entry doors, not less than 1 hour and to be leakage-rated and installed in accordance with NFPA 105 and NFPA 80. Monthly testing on all door closers to be documented and available for inspection. (OBC 9.10.13. permits 20-min doors constructed as solid core wood doors in unrated wood frames)

• Counterflow Concerns: enhanced exit stair landings to 1650mm width. (OBC requires minimum 900mm in 3 storey buildings and 1100mm in buildings of more than 3 storeys)

• Areas of Refuge and Alternative Fire Department Access: fire-rated balconies having a fire-resistance rating of not less than that required for the floor assembly, with adjacent ladder set-up areas to reach the balconies of minimum 3.0m x 3.0m, clear of obstructions, snow and ice. (OBC Part 9 provides balconies conditions per 9.9.9.9.1. and OBC Part 3 provides balcony conditions per 3.3.4.4. but does not prescribe minimum ladder set-up areas)

• Electric Bicycle Charging Mitigation: bicycle storage room to be provided with six (6) receptacles installed to provide safe charging of electric bicycles.

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.