The Ontario Building Code | Wall Thickness
9.21.4.8. Wall Thickness
(1) The walls of a masonry chimney shall be built of solid units not less than 70 mm thick.
In Plain Language
This clause sets a minimum wall thickness for masonry chimneys: at least 70 mm of solid masonry units, ensuring the chimney structure itself has adequate mass and fire resistance independent of any liner inside it.
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.