The Ontario Building Code | Hearth Extension
220.127.116.11. Hearth Extension
(1) Except as required in Sentence (2), fireplaces shall have anoncombustible hearth extending not less than 400 mm in front of the fireplace opening measured from the facing, and not less than 200 mm beyond each side of the fireplace opening.
(2) Where the fire chamber floor is elevated more than 150 mm above the hearth, the dimension of the hearth measured perpendicular to the plane of the fireplace opening shall be increased by not less than,
(a) 50 mm for an elevation above 150 mm and not more than 300 mm, and
(b) an additional 25 mm for every 50 mm in elevation above 300 mm.