The Ontario Building Code | Support of Hearth
9.22.5.2. support of Hearth
(1) Except as permitted in Sentence (2), the fire chamber floor and hearth shall be supported on a reinforced concrete slab not less than a 100 mm thick at its supports and, if cantilevered, not less than 50 mm thick at its unsupported edge.
(2) A hearth for a fireplace with an opening raised not less than 200 mm from a combustible floor is permitted to be supported on that floor provided the requirements of Clauses 5.3.6.5. to 5.3.6.7. of CAN/CSA-A405-M, "design and Construction of masonry Chimneys and fireplaces", are followed.
9.22.6. Damper
In Plain Language
The hearth must be adequately supported as set out in this article, ensuring this heavy masonry element doesn't sag or crack over time.
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.