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