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.