(1)  Not less than a 100 mm clearance shall be provided between the back and sides of a solid fuel-burning fireplace and combustible framing, except that a 50 mm clearance is permitted where the fireplace is located in an exterior wall.       

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(2)  Not less than a 50 mm clearance shall be provided between the back and sides of the smoke chamber of a solid fuel-burning fireplace and combustible framing, except that a 25 mm clearance is permitted where the fireplace is located in an exterior wall.

Article 9.22.9.3. Ontario Regulation 332/12 Building Code,   Information published by oncodes.ca for educational purposes only.