Date of Publication: Dec 4, 2025 [2023.01.V-02.C]
Subject: Permitted reduction in required ceiling height 2024 OBC UPDATE
Summary of Updates: Incoming request and response revised to reflect new definitions and Sentences.
Building Code Reference(s):
Article 1.4.1.2. of Div A. Definition of secondary suite
Article 9.5.3.1. of Div. B.
Part 11 Compliance Alternative C105
Incoming Request:
If a secondary suite is being added to a house, Sentences 9.5.3.1.(2) and (3) permit a ceiling height of 1.95m (76-3/4”) with a minimum clear height of 1.85m (72-3/4”) under beams and ducts. Part 11 Compliance Alternative C105 also allows a reduced ceiling height, but it does not apply to secondary suites.
Question:
- If a third dwelling unit were added, would it be permitted the reduced ceiling height allowed in Sentences 9.5.3.1.(2) and (3)?
- If a third dwelling unit is added, would it be subject to C 105?
- How much construction tolerance is reasonable in application of the clearance under beams and ducts in this scenario?
Executive Summary:
Sentences 9.5.3.1.(2) and (3) of the 2024 OBC allow significant reductions in ceiling height in secondary suites, but these reductions are not applicable to other residential suites or areas. Part 11 (C105) allows reduced ceiling heights in dwelling units other than secondary suites.
Considerations:
Application of the 2024 OBC: Article 9.5.3.1. has been changed for the 2024 OBC to specifically allow a lowered ceiling height for secondary suites, which is a defined term. Although many secondary suites are located in basements, the location of the suite does not form part of the definition. Therefore, if a secondary suite is being constructed it is permitted the reduced ceiling height of Sentences 9.5.3.1.(2) and (3) regardless of its location in the building. The designer would have to specify what area of the building will constitute the secondary suite. It is worth noting that this change harmonizes Sentences 9.5.3.1. (1) to (4) with the 2020 NBC. The 2020 NBC intent statements note that the reduced ceiling height is to allow the reduction where it would be cost-prohibitive to provide the ceiling height specified elsewhere in the Article.
Part 11 (where applicable) also provides relief to dwelling unit ceiling heights. Compliance Alternative C105 allows a general ceiling height of 1950mm, and allows 2030mm over 50% of the required area of a room, but makes no mention of a reduction under beams or ducts, and it is not applicable to a secondary suite.
In a case where a secondary suite is proposed, the secondary suite could use the reduced ceiling heights found in Sentences 9.5.3.1.(2) and (3), and if Part 11 is applicable, other dwelling units could use the relief provided by compliance alternative C105. In a case where 3 suites are proposed in a building, none would fit the definition of secondary suite, and therefore the ceiling reduction found in Sentences 9.5.3.1.(2) and (3) would not be applicable. However if Part 11 were applicable, all new dwelling units in the existing building would be entitled to the relief provided by compliance alternative C105. Generally, only the newly constructed suites, common spaces and their access to exit would be evaluated. Previously constructed (and approved) secondary suites would not be subject to new code requirements.
Construction tolerance: The term “construction tolerance” is generally not encountered in the Code except in referenced standards. Very few dimensions referenced in the Code allow a tolerance (stair risers is the obvious exception). A better way to assess whether or not a small variance in ceiling height or clear height is acceptable is to look at the functional statements and objectives. For the ceiling height requirements of Article 9.5.3.1., these can be summarized as ensuring occupants are not delayed in egressing during an emergency (F10) and ensuring they are not injured by tripping, falling or contacting parts of the building (F30). With this in mind, areas where clear height is lower than that stipulated by the Code would seem to meet the functional statements if they are located out of egress paths or areas where people would normally be standing or moving about.
Risk/Benefit Analysis:
The permitted reduction in ceiling height for secondary suites is not based on a risk assessment of secondary suites vs. other residential suites, but is based on avoiding cost prohibitive scenarios specifically in a secondary suite. The Ontario Building Code Commission (BCC) Ruling 18-07-1503 has permitted a 40 mm (1-1/2”) reduction in clear height under beams (in a basement in this instance) stating that this falls “within acceptable construction tolerances”. BCC Ruling 98-16-623 also allowed a ceiling height variance of approximately 25mm (1”), citing construction tolerances.
Stated simply the reason for minimum ceiling heights is to facilitate safe egress. Within the objectives and functional statements, reference is made not only to the ceiling height but to fixtures and other items that might obstruct the clear height. In applying the code-mandated ceiling heights to scenarios not envisioned by the code, an AHJ may want to consider the following:
- Construction details, i.e. a sloping floor may affect ceiling height or clear height, and carpet (with various underpad weights and pile thicknesses) may also affect clear heights.
- How much variance is there between the required height and the proposed or actual height?
- Have all efforts to comply with the minimum ceiling height been exhausted, i.e. oval ducts and smaller framing members?
- Is the lowered ceiling height completely inside an individual dwelling unit?
- Is the lowered ceiling height in a means of egress or where people would normally stand?
- Are there any fixtures, registers or other obstructions that may further lower the ceiling height or pose a hazard?
- Have efforts been made to mitigate the risk of someone injuring themselves, i.e. round corner bead, contrasting paint?
- Has the decision-making process been documented?
Final Recommendation(s):
As currently written, the reduced ceiling heights of Sentences 9.5.3.1.(2) and (3) are applicable only to secondary suites. Where additional residential suites are proposed, the definition of secondary suite is not met, and the above sentences do not apply. Compliance Article C105 is not applicable to secondary suites, but does apply to other residential areas.
Recommendation to the Ministry of Municipal Affairs and Housing:
None at this time, however some organizations have submitted a code change request to allow the reduced ceiling heights to be extended to all residential suites, not only secondary suites.
Referenced Documents:
2024 Ontario Building Code [O.Reg 163/24 Amended to 5/25]:
National Building Code of Canada, 2020
2020 National Building Code of Canada intent statements
BCC Ruling 18-07-1503
BCC Ruling 98-16-623
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.