What are the fire code requirements for chimney masonry in Ontario residential homes?

Ontario's fire code requirements for chimney masonry in residential homes are governed by the Ontario Building Code Part 9 (Section 9.21 — Chimneys and Venting) and the Ontario Fire Code, and they exist because a deteriorated masonry chimney is one of the most common sources of house fires in the GTA. Understanding these requirements is essential whether you are repointing an existing chimney, rebuilding a chimney crown, or assessing whether your chimney masonry is safe for continued use.

The Ontario Building Code Section 9.21 specifies that masonry chimneys serving solid fuel appliances (wood-burning fireplaces and stoves) must have a minimum wall thickness of 90 millimetres (approximately 3.5 inches) of solid masonry around the flue liner. The flue liner itself must be a listed clay tile liner or a stainless steel chimney liner that meets CSA standards. The critical fire safety concern with chimney masonry is clearance to combustibles — the OBC requires a minimum 50 millimetre (2 inch) air space between the outside face of a masonry chimney and any combustible material, including wood framing, sheathing, insulation, and trim. This clearance must be maintained from the foundation through the roof and above. In many older GTA homes — particularly pre-war homes in Toronto, East York, and the inner suburbs — original chimneys were built before modern clearance requirements, and combustible framing may be in direct contact with the chimney masonry, creating a fire hazard that should be addressed during any chimney renovation.

Chimney height requirements are also a fire safety issue. The OBC requires that a chimney extend at least 900 millimetres (3 feet) above the point where it passes through the roof and at least 600 millimetres (2 feet) above any part of the roof or structure within 3 metres (10 feet) horizontally. These height requirements ensure proper draft and prevent sparks and embers from igniting roofing materials. If your chimney masonry has deteriorated and the chimney height has been reduced through crumbling or removal of courses, it may no longer meet height requirements and must be rebuilt to the correct height.

The chimney crown — the mortar cap at the top of the chimney — is a critical fire and weather protection component. A properly constructed chimney crown overhangs the chimney walls by at least 50 millimetres on all sides, is sloped to shed water away from the flue, and is made from a durable Portland cement or polymer-modified mortar mix. A cracked or missing chimney crown allows water into the chimney structure, which accelerates mortar joint deterioration through the GTA's 50-plus annual freeze-thaw cycles. Crown repair or replacement in the GTA costs $500 to $2,000 depending on chimney size and accessibility.

Chimney repointing — replacing deteriorated mortar joints — is essential for both fire safety and structural integrity. Mortar joints that are cracked, recessed, or missing allow heat to transfer to combustible framing and permit water entry that accelerates further deterioration. Repointing a chimney in the GTA costs $1,000 to $5,000 depending on chimney size and height, with scaffolding adding $500 to $1,500. The mortar used for repointing must be Type S or Type N for modern brick chimneys. For heritage chimneys with soft brick, lime-based mortar is required to prevent brick spalling.

The Ontario Fire Code requires homeowners to maintain chimneys in safe working condition and to have chimneys serving wood-burning appliances inspected and cleaned annually by a WETT-certified (Wood Energy Technology Transfer) technician. A WETT inspection assesses the structural integrity of the chimney masonry, the condition of the flue liner, clearances to combustibles, and the chimney crown and cap. If your chimney masonry shows signs of deterioration — spalling bricks, crumbling mortar, leaning, white efflorescence staining, or visible cracks — have it assessed by a qualified mason before using the fireplace. Chimney fires in the GTA cause millions of dollars in property damage annually, and most are preventable through proper masonry maintenance.