What are the flue lining requirements for masonry chimneys in Ontario?
Masonry and concrete chimneys must have a continuous flue liner, such as fired clay tile, firebrick or a listed metal or concrete liner, installed with sealed joints and separated from the surrounding masonry so it can expand. As a rule, each appliance generally needs its own flue, with limited exceptions.
Paraphrased for clarity — open this provision in the BUILDAiD Library for the exact wording.
The liner is the actual gas-carrying vessel; the brick is structure and cladding. Cracked or missing clay tiles are the most common defect found in older Ontario chimneys and a typical WETT inspection failure when woodstoves are installed. Liner sizing must match the appliance: oversized flues cool exhaust, cause condensation and creosote, which is why gas retrofits into old fireplace chimneys generally require an appropriately sized listed liner. Offsets are limited, joints must not project into the flue, and the space between liner and masonry should not be grouted solid.
Inside every brick chimney there must be a smooth, sealed liner, usually clay tile, that actually carries the smoke or exhaust. Cracked liners are the number one problem in older chimneys, and each stove or furnace normally needs its own liner sized for it.
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This page is provided for general information. Building code requirements depend on your specific project, occupancy and municipality, and code editions change. Always verify requirements against the current Ontario Building Code and confirm with your local building department.