Date of Publication: July 7, 2025 [2025.06.V-01.C]
Subject: Determination of Limiting Distance for Multiple buildings on One Property
Ontario Building Code Reference(s):
Article 3.1.10.6. of Div. B.
Article 3.2.3.13. of Div. B
Article 3.2.3.14. of Div B
Article 9.9.4.6. of Div. B.
Article 9.10.12.3. of Div. B.
Subsection 9.10.14 of Div. B.
Incoming Request:
Two buildings are proposed on the same property. The buildings are positioned such that the exposing building faces (EBF) do not directly face or overlap each other. Is it permissible to use the property line as the distance for measuring limiting distance (LD), or must an imaginary line be drawn between the two buildings? See Sketch 1, below.
Sketch 1
Executive Summary:
Creation of an imaginary line for measuring LD is recommended in most cases where two buildings are proposed on one property. However, where the orientation and proximity of the buildings to one another results in a high risk of fire spread or unnecessarily restrictive design, more detailed analysis may be required.
Discussion & Considerations:
The definitions in Article 1.4.1.2. state that “limiting distance means the distance from an exposing building face to a property line, the centre line of a street, lane or public thoroughfare, or to an imaginary line between 2 buildings or fire compartments on the same property, measured at right angles to the exposing building face.” There is no further direction in the Code as to what scenarios dictate which of the three possible datum lines the LD should be measured to.
Exposing building face means “that part of the exterior wall of a building that faces one direction and is located between ground level and the ceiling of its top storey or, where a building is divided into fire compartments, the exterior wall of a fire compartment that faces one direction.”
Based on the strict wording above, the Code appears to permit the LD to be measured directly to the property line, and disregards other buildings not within the right angle (90 degree) projection of the EBF.
The reason spatial separation calculations are measured to a property line is to limit the likelihood of fire spread from one building to an adjacent property or building. Limits on allowable openings reduce fire spread from fires originating within a building. Cladding and fire-resistance rating (FRR) requirements of exterior walls serve to limit ignition of the cladding, and limit the fire spread on the outside of the building adjacent to openings. Given this, it is likely that the Code introduced the idea of an imaginary line as a way to ensure the same protection is provided between 2 buildings on one property, as 2 buildings on separate properties.
Looking at our scenario, if the limiting distance is permitted to be measured directly to the property line, 100% unprotected openings (UPO’s) would be permitted in both buildings on the EBFs in question. See Sketch 2, based on two Part 9 Group D occupancy buildings.
Sketch 2
Alternatively, if LD were measured to an imaginary line between the buildings (placed and identified as part of the building design), it would result in some restrictions being placed on the building’s spatial separation requirements. Sketch 3 and Sketch 4 below show two possibilities using the provisions of 9.10.14. as an example.
Sketch 3
Sketch 4
Each of these approaches permits different amounts of unprotected openings, and cascading effects on cladding and wall construction.
Permitting LD to be measured directly to the property line does not take into account the diagonal distance between exterior walls of buildings arranged as in our example. However, if the buildings were separated by a property line, spatial separation requirements would limit the FRRs, UPOs and cladding, based on the distance from the property line. If the walls were of the same building or buildings separated by a firewall, Articles 9.9.4.6. / 3.2.3.13. and 9.10.12.3. / 3.2.3.14. would also apply.
Risk/Benefit Analysis:
Since radiative heat energy may expand beyond the 90 degree confines of the EBF in question (as recognized by the Articles quoted above), this risk should not be ignored where two buildings are proposed on one property.
Sketch 2 (where LD is measured to the property line) results in much less protection from fire spread than any of the others, as it permits two exposing building faces with 100% UPO. Even though they are not oriented directly opposite each other, there may be significant heat transfer between the two EBFs in a fire scenario. Using this methodology, Building 1 and Building 2 could have their closest corners almost touching each other and still be permitted 100% UPOs (See Sketch 5). This clearly is not consistent with the Code objective to limit fire spread between buildings.
Sketch 5
Sketch 3 and Sketch 4 (where LD is measured to an imaginary line) reduce the design options, but if the building is comprised of fire compartments or is divided into portions as per Clause 9.10.14.2.(3)(a), some flexibility is available.
One further scenario is where the 90 degree projection of the EBFs do intersect, but only by a small amount. If LD is measured to an imaginary line placed between these buildings, the entire opposing EBFs would be subject to the most restrictive UPO limits.
Additionally, while Clause 9.10.14.2.(3)(a) allows Part 9 buildings to divide the EBF into portions for the purposes of applying cladding and construction requirements, the same relaxation does not exist for Part 3 buildings. As a result, the entire EBF must comply with the cladding and construction requirements based on the closest limiting distance even though the risk of fire spread from one building to another may be limited. (See Sketch 6). This may be unnecessarily restrictive, especially for very long buildings.
Sketch 6
In cases where proposed buildings are located close to each other, it may be beneficial to compare provisions that would apply when two buildings on the same property are connected by a fire wall. In such cases, the OBC provides clearer provisions on how to address fire exposure between the two buildings. Article 3.1.10.6. of Division B specifies that exposure protection shall comply with Article 3.2.3.14., and that unprotected openings in the 2 buildings be separated by the Do distance (See Sketch 7). Although there is no direct Code path to use Article 3.2.3.14., when two buildings are close but detached, adopting a similar approach through internal policy or as an alternative solution could be a reasonable consideration for an AHJ.
Sketch 7
When evaluating designs as described above, sound judgement is required to ensure fire spread between buildings is limited and the design requirements are not overly restrictive. The following points may be helpful:
- Measuring LD to an imaginary line between the buildings provides a simple means of ensuring the likelihood of fire spread between 2 buildings on one property is not higher than the same risk of 2 buildings on separate properties;
- Where buildings are only partially overlapping, creation of fire compartments may allow some flexibility;
- Segmenting an irregularly shaped or skewed EBF is permitted under Sentence 9.10.14.2.(3);
- Articles 9.9.4.6., 3.2.3.13., 9.10.12.3., and 3.2.3.14., while not explicitly applicable to these examples, offer a useful framework for evaluating risk of fire spread between exterior walls of buildings;
- Comparison of the spatial separation requirements of closely spaced buildings with the requirements for buildings separated by a firewall, found in Subsection 3.1.10. and Article 3.2.3.14. may also be informative; and
- Heat flux analysis by a qualified individual may also be done to evaluate the risk of fire spread between two buildings.
In all cases the LD assumptions, calculations and location of the datum line should be recorded to inform any future development or building alterations.
BC Building Code interpretation committee decision 12-0057 provides another viewpoint.
Final Recommendation:
When two buildings are proposed on the same property, regardless of their orientation, measuring LD to an imaginary line between the buildings is generally recommended. In cases where buildings are closely spaced but not overlapping or overlapping by only a small amount, further in-depth analysis may be required.
Recommendation to Ministry of Municipal Affairs and Housing:
Add an Appendix Note to clarify the requirement to measure LD to the property line versus an imaginary line, as well as any considerations if the buildings are within close proximity to each other or partially overlapping.
Referenced Documents:
2024 Ontario Building Code O.Reg 163/24 Amended to 5/25
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.