The Ontario Building Code | Thickness of Walls
9.22.3.1. Thickness of walls
(1) Except as provided in Sentence (2), the thickness of the back and sides of a fireplace, including the thickness of any firebrick liner, shall be not less than 190 mm where a metal liner or a firebrick liner less than 51 mm thick is used.
(2) When a steel fireplace liner is used with an air circulating chamber surrounding the firebox, the back and sides of the fireplace shall consist of,
(a) solid masonry units not less than 90 mm thick, or
(b) hollow masonry units not less than 190 mm thick.
9.22.4. Fire Chamber
In Plain Language
Fireplace walls must meet a minimum thickness requirement set out in this article, ensuring adequate heat resistance and structural strength.
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.