The Ontario Building Code | Support of Hearth 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 to of CAN/CSA-A405-M, "design and Construction of masonry Chimneys and fireplaces", are followed.

9.22.6. Damper