The Ontario Building Code | Thickness of Walls
220.127.116.11. 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.
(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