The Ontario Building Code | No Reduction in Size
7.4.9.1. No Reduction in Size
(1) Except as permitted in Sentence (3), no drainage pipe that is of minimum size required by this Part for the purpose for which it is installed shall be so connected as to drain to other drainage pipe of lesser size.
(2) Where a building drain connects to a stack through a wall or floor, the drain shall retain its full size through the wall or floor.
(3) A sanitary drainage pipe may be connected to a pre-engineered waste water heat recovery system that incorporates piping of a lesser size than required by Sentence (1) provided that it does not convey sewage,
(a) from a sanitary unit, or
(b) that contains solids.
In Plain Language
Except in one permitted case, a drainage pipe can't be reduced below the minimum size required by this Part for its purpose, preventing a bottleneck that would restrict flow capacity.
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.