The Ontario Building Code | Offset Relief Vents
7.5.4.4. Offset Relief vents
(1) A soil or waste stack that has a nominally horizontal offset more than 1 500 mm long and above which the upper vertical portion of the stack passes through more than 2 storeys and receives a hydraulic load of more than 100 fixture units shall be vented by an offset relief vent connected to the vertical section immediately above the offset, and by another offset relief vent,
(a) connected to the lower vertical section at or above the highest soil or waste pipe connection, or
(b) extended as a vertical continuation of the lower section.
In Plain Language
A soil or waste stack with a horizontal offset longer than 1500 mm needs an offset relief vent above that offset, since long horizontal runs in an otherwise vertical stack disrupt normal air flow and pressure balance.
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.