The Ontario Building Code | Additional Circuit Vents and Relief Vents
7.5.7.3. Additional circuit Vents and Relief vents
(1) Except as provided in Article 7.5.7.1. and in Sentence 7.5.3.1.(7), the minimum size of an additional circuit vent or relief vent installed in conjunction with a circuit vent is permitted to be one size smaller than the required size of the circuit vent, but need not be larger than 2 in.
(2) The size of the soilor waste pipe acting as a relief vent in accordance with Sentence 7.5.3.1.(4) shall be in conformance with Table 7.4.10.6., 7.4.10.7. or 7.5.8.1. or Article 7.5.7.1., whichever size is the largest considering the hydraulic load drained into the soil or waste pipe.
In Plain Language
Additional circuit vents and relief vents may be required beyond the minimum, addressing situations where the basic venting arrangement alone wouldn't adequately protect trap seals under expected use.
This is a plain-language summary of the 2012/2017 Ontario Building Code text above, for general understanding only and not a substitute for the Code. Always confirm current requirements against the 2024 edition (O.Reg. 163/24) or a qualified professional before construction.