Fireplace Construction Requirements

WETT Inspections in Alliston has provided some of the information available in the Ontario Building Code regarding Fireplaces.   This information is provided just to indicate the type of regulations that apply to different wood burning appliances and should not be used as an installation guide.  If you require specific information about your wood burning appliance call the Alliston WETT Inspector at 705-795-8255 for current WETT requirements.


OBC  (1)  Every solid fuel-fired fireplace, including factory-built fireplace, shall have a supply of combustion air from outdoors, etc.

Hearth Extension

(1) Except as required in Sentence (2), fireplaces shall have a noncombustible hearth extending not less than 400 mm (15 3/4 in) in front of the fireplace opening measured from the
facing, and not less than 200 mm (7 7/8 in) beyond each side of the fireplace opening.

(2) Where the fire chamber floor is elevated more than 150 mm (5 7/8 in) 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 (2 in) for an elevation above 150 mm (5 7/8 in) and not more than 300 mm
(11 3/4 in), and
(b) an additional 25 mm (1 in) for every 50 mm in elevation above 300 mm (11 3/4 in).

Sample Fireplace ClearancesClearance to Fireplace Opening

(1) Combustible material shall not be placed on or near the face of a fireplace within 150 mm (5 7/8 in) of the fireplace opening, except that where the combustible material projects more than 38 mm (1 1/2 in) out from the face of the fireplace above the opening, such material shall be at least 300 mm (11 3/4 in) above the top of the opening. 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 (4 in) thick at its supports and, if
cantilevered, not less than 50 mm (2 in) thick at its unsupported edge.
(2) A hearth for a fireplace with an opening raised not less than 200 mm (7 7/8 in) 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. Steel Liners

(1) Steel liners for fireplaces shall conform to CAN/ULC-S639M, “Standard for Steel Liner Assemblies for Solid-Fuel Burning Masonry Fireplaces”, and shall be installed in accordance
with the installation instructions in that Standard.

Wood Fireplace:
These require a non-combustible hearth extension. Below are non-combustible materials and the required thickness that meets the specs.(min. R value of 1.16) for the fireplaces non-combustible hearth extension.

Required Damper and Size

(1) The throat of every fireplace shall be equipped with a metal damper sufficiently large to
cover the full area of the throat opening.

Hearth Extension Insulation Alternatives, R Value = 1.16

Hearth & Home HX3, HX4 – 1/2 inch
USG Micore 300 – 1/2 inch
USG Durockâ„¢ Cement Board – 2-1/4 inches
Cement Mortar – 5-7/8 inches
Common Brick – 5-7/8 inches
Ceramic Tile – 14-5/8 inches
Marble – 16-5/8 in. to 23-3/8 in.

These are the materials that can separate combustible material, lets say the wood flooring or wood framed hearth extension from the fireplace hearth.

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