How high does a chimney have to extend above the roof in Ontario?
A chimney flue must terminate at least 900 mm (3 ft) above the highest point where it passes through the roof, and at least 600 mm (2 ft) above any roof surface, ridge or obstruction within 3 m (10 ft) horizontally — the classic 3-2-10 rule.
Paraphrased for clarity — open this provision in the BUILDAiD Library for the exact wording.
The rule exists for draft and safety: wind washing over a nearby ridge can pressurize a short chimney and push smoke or combustion products back down. On steep roofs or chimneys emerging near the eaves, the within-3-m test, not the 900 mm penetration test, usually governs and drives surprising heights, which then triggers bracing considerations for slender masonry stacks. Additions and dormers built after the fact frequently place new roof surfaces inside the 3 m radius, quietly making an existing chimney non-compliant. Factory-built chimneys follow the same geometry under their listings.
Measure straight up from where the chimney pokes through the roof: it needs at least 900 mm there. Then look around within 3 m sideways; the chimney top must clear anything in that circle, ridge included, by at least 600 mm so smoke drafts properly.
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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.