Date of Publication: June 21, 2026 [2026.07.V-01.C]
Subject: Sewage system design flows for multiple occupancies
2024 Ontario Building Code Reference(s):
Article 7.4.9.3. of Div. B.
Section 8.1. and 8.2.of Div B.
Incoming Request:
An application has been submitted for a building on agricultural lands which is identified as a horse barn. Within this building there are stables, offices, a viewing area, a kitchen and washrooms. In reviewing Article 8.2.1.3., there is nothing in the tables that references a farm building. How should the total daily design sanitary sewage flow (Q) be calculated?
Executive Summary:
Total daily design sanitary sewage flow (q) for buildings with multiple uses or major occupancies must take those specific uses and occupancies into account. Table 8.2.1.3.B is a good starting point, but sound engineering judgement and potentially reference to metered flow data from other establishments are also required in order to calculate a realistic Q.
Discussion & Considerations:
As noted in the question, Table 8.2.1.3.B does not list farm buildings, but that’s because Article 8.2.1.3 doesn’t classify total daily design sanitary sewage flow (Q) by building type at all. Instead, it provides a list of "establishments" to assist in calculating Q. The values are based on average or typical use patterns, with reference the actual or designed occupant load for additional accuracy.
In your case, the applicant is calling this a “horse barn”, but the inclusion of offices, viewing areas, a kitchen, and washrooms means the building contains uses that fall outside an agricultural occupancy. (Note that the 2024 OBC has a new definition of farm building, and has introduced a new major occupancy type – agricultural occupancies.) Sentence 8.2.1.3.(3) and (4) give clear direction that where there are multiple “establishments” Q should be based on the sum of all the establishments or by actual metered flow from similar establishments.
For this building the relevant establishments would be the offices, viewing/auditorium‑type areas, kitchen, and washrooms. Table 8.2.1.3.B provides enough similar flow data to interpolate a reasonable and defensible Q for these uses. I would also draw the designer’s attention to the discharge limitations in 8.1.3.1. The notes in A‑8.1.3.1.(1) and (3) are particularly important when a building includes both plumbing fixtures and non-domestic waste, i.e. waste related to the agricultural portion of the building. It should also be noted that the values in Tables 8.2.1.3.B are factored daily volumes of waste, measured in litres/day and should not be confused with fixture units values from Table 7.4.9.3., which are instantaneous volumes used for hydraulic calculations.
In cases like this of multiple major occupancy types (a more familiar building code term) or uses, Q will be interpolative and based on professional judgment. The values should align with the requirements of 8.2.1.3. for the selected establishments and flow assumptions should be appropriate to the actual uses within the building. The methodology should reflect accepted engineering practice and established interpretation policy. In other words, correctness is demonstrated not by a single prescribed formula, but by a defensible, Code‑consistent rationale supported by competent design. This reasoning can be extended to all buildings of multiple major occupancy types or “establishments” and is not limited to agricultural occupancies or farm buildings.
Risk/Benefit Analysis:
Clear descriptions of the fixtures, the “establishments”, the major occupancies and the overall intended use of the spaces in the building will ensure that a reasonable and defensible Q can be arrived at. Where sanitary sewage of a non-human origin is likely to be generated this must be identified and accounted for in the design and calculations, per appendix note A-8.1.3.1.(3) and (4).
Final Recommendation:
Total design sanitary sewage flow (Q) for a building with multiple major occupancies or uses should be calculated for each use, using the values in Table 8.2.1.3.B as a baseline, and following established engineering practices. Actual metered flow from similar establishments may also be used as a reference.
Recommendation to Ministry of Municipal Affairs and Housing:
None
Referenced Documents:
2024 Ontario Building Code O.Reg 163/24 Amended to 05/25
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.