The 15-Minute Fire Separation

When discussing new accessory apartment conversions within existing residential buildings in existence for more than five years, I am always asked the same question: How do you come to the conclusion that a 15-minute horizontal fire separation is required between dwelling units? Many are surprised that this is a possible compliance path as it appears too good to be true. It is an acceptable compliance path, but it’s not without its additional requirements.

When constructing a new dwelling unit within the basement of an existing single-family dwelling unit, the installation of the basement ceiling is a critical element to review for the plans examiner. Generally, we need to have fire separations between different suites to ensure adjacent occupants have time to react and exit the building.

Part 11 of the Ontario Building Code provides direction in extensive renovation regarding the removal and installation of a new ceiling. Division B, 11.3.3.2.(1) tells us that new ceilings must meet the provisions provided in other parts with regards to the fire-resistance elements. This leads us to Division B, 9.10.9.14.(4), which requires a fire separation with a fire-resistance rating of 45 minutes for a house. Many stop at this provision, but the path to compliance is not yet complete. Don’t forget about the compliance alternatives.

Division B, 11.5.1.1.(2) permits the design and construction to meet a compliance alternative in lieu of the Part 9 prescribed requirement without satisfying the chief building official. Compliance alternative tables are provided for many Code provisions to allow flexibility within existing buildings to make it easier to comply. When we review the compliance alternative for the 45-minute requirement for a fire-separation between dwelling units in a house, we find compliance alternative C152 for Division 9.10.9.14.(1). This alternative allows a 15-minute horizontal fire separation in lieu of a 45-minute fire separation.

However, both dwelling units, including common areas, are required to have smoke alarms upgraded to meet the provisions as if it were a new building (i.e., all bedrooms, hallways serving bedrooms, all levels, visual components, interconnected, etc.).

In order to show compliance with the required fire-resistance rating, we first review Division B, 9.10.3.1.(1), and it tells us that we need to use SB-2, SB-3 or Part 3. When reviewing these sections, you will come to the conclusion that there is no reference to the 15-minute barrier. This is until you remember that compliance alternatives are also available for this Code provision. Compliance alternative C143 allows us to use the HUD Rehabilitation Guidelines, “Guideline on Fire Ratings of Archaic Materials and Assemblies.” Table 1.5.1., item FR-1-16 confirms a 15-minute fire-resistance rating for a ½” regular drywall.

So, in the end, a 15-minute horizontal fire separation is permitted as long as you upgrade the smoke alarm system within the building. Although builders will save the cost of drywall installation, they will incur the costs to upgrade the smoke alarms within the entire building.

Gerald is the president of RSM Building Consultants, which provides support services to municipal building departments. His background includes over 25 years’ experience working with building departments. RSM provides Building Code training, plans examination services, alternative solution review, inspection and CBO transition support.