Fire Resistance Rating of Supporting Construction

Date of Publication:  Nov 26, 2025 [2025.10.V-01.C]

Subject:  Fire Resistance Rating of Supporting Construction.

2024 Ontario Building Code Reference(s):

Article 3.1.3.1. of Div. B.

Article 3.1.7.5. of Div. B.

Subsection 3.2.2. of Div. B.

Article 3.3.1.1. of Div B.

Incoming Request: 

A newly constructed two-storey office building (classified under Articles 3.2.2.26 and 3.2.2.65) is undergoing a tenant fit-out. A proposed A2 major occupancy (daycare) will occupy part of the main floor and second floor. An existing D major occupancy is located on the second floor of the building, above a portion of the proposed A2 occupancy. 

Table 3.1.3.1. requires a one-hour fire separation between these two major occupancies. The existing precast second-floor slab assembly meets a one-hour FS rating. The floor is supported on unprotected structural steel beams and columns. Article 3.1.7.5. appears to allow the floor to be supported on unprotected non-combustible construction. Does this supporting structure need to be upgraded to a one-hour fire-resistance rating (FRR)?

Executive Summary:

When a floor assembly is required to have a fire‑resistance rating under Articles 3.1.3.1. or 3.3.1.1., its supporting construction must also have an FRR at least equal to the required rating. The permission for a 45-minute fire-rated floor to be supported on unrated, but non-combustible construction, is limited to satisfying the requirements of Articles 3.2.2.20 to 3.2.2.93.

Discussion & Considerations:

Sentence 3.1.7.5.(1) requires fire-resistance rated floor assemblies to be supported by construction having an FRR not less than that of the supported floor, except where permitted by the 3.2.2. classification. In this scenario, Articles 3.2.2.26. and 3.2.2.65. require floors to be fire separations with a 45-minute FRR, however supporting construction is permitted to be 45-minute rated or non-combustible.

Since our building contains multiple major occupancies and separate suites, the provisions of Article 3.1.3.1 (Separation of Major Occupancies) and Article 3.3.1.1. (Separation of Suites) must also be considered. Table 3.1.3.1. requires a one-hour fire separation between an A2 and a D major occupancy. Sentence 3.3.1.1.(2) requires a 45-minute separation between suites based on the 3.2.2. classification. 

In summary, this building is subject to these three separate requirements:

• Subsection 3.2.2 – 45-minute floor separation, with non combustible support permitted;
• Article 3.1.3.1 – One-hour fire separation between major occupancies; and
• Sentence 3.3.1.1.(2) – 45-minute fire separation between suites.

It has been suggested that Articles 3.2.2.26 and 3.2.2.65. (which permit 45-minute rated floors to be supported by non-combustible supporting elements) could govern. This is not permissible as the floor is required to provide a one-hour fire separation between major occupancies.

Risk/Benefit Analysis:

In cases where a building element is subject to multiple Code requirements, and there are no specific exemptions between them, the most restrictive requirement would logically govern. 

It has been suggested that the exemption in 3.2.2.26 and 3.2.2.65. (which permits 45-minute rated floors to be supported by unrated but non-combustible supporting elements) could govern. This is not permissible as the floor is required to provide a one-hour separation between major occupancies.

In a case where Article 3.1.3.1. does not require the major occupancies to be separated, and Article 3.3.1.1. (Separation of Suites) governs, a 45-minute fire separation between suites may be required. Within subsection 3.2.2. non-combustible supporting construction is often permitted to be substituted for 45-minute fire-rated construction. However, the Code does not provide a path for this substitution with respect to Articles 3.1.3.1. or Article 3.3.1.1. 

Final Recommendation: 

In this example, the one-hour rated fire separation (floor) between suites must be supported by construction having a fire-resistance rating (FRR) of not less than one hour.

Recommendation to Ministry of Municipal Affairs and Housing:

None at this time. 

Referenced Documents:

2024 Ontario Building Code:

Disclaimer:

This guidance document is intended to assist building officials by gathering relevant information to interpret the OBC Act and the prescriptive requirements of the Ontario Building Code, and is intended to be a best practice aid for building officials.

The views expressed within this guidance document should not be considered as the official interpretation of legislated requirements based on the Ontario Building Code, as the final responsibility for interpretation rests with the local Authority Having Jurisdiction. 

The views of this advisory committee should not be construed as legal advice.